Control Flow & Decision Making

01What is Control Flow?

Simple Definition

Control flow lets your program make decisions. Instead of running every line from top to bottom, your program can choose which code to run based on conditions.

Real World Example

️

Is it raining?

→

YES

☂️ Take umbrella

Wear sunglasses

This is exactly what if-else does in programming!

Type	Purpose	Keywords
Decision Making	Choose which code to run	if, else, switch
Looping	Repeat code multiple times	for, while, do-while
Jump	Skip or exit code blocks	break, continue, goto

This Tutorial Covers Decision Making

This tutorial focuses on if-else and switch statements. For loops (for, while, do-while), see the Loops Tutorial.

02Understanding True and False in C

Before using if statements, you must understand how C interprets true and false. Unlike some languages, C doesn't have a built-in bool type (before C99).

The Golden Rule in C

= FALSE

The condition is NOT met

Code block is SKIPPED

≠ 0

= TRUE

Any non-zero value

Code block EXECUTES

What Values Are True or False?

Expression / Value	Numeric Value	Result	if() Behavior
0	0	FALSE	Block skipped
1	1	TRUE	Block executes
-1, -5, -100	non-zero	TRUE	Block executes
42, 100, 999	non-zero	TRUE	Block executes
0.0 (float)	0	FALSE	Block skipped
3.14 (float)	non-zero	TRUE	Block executes
'\0' (null char)	0	FALSE	Block skipped
'A' (any char)	65	TRUE	Block executes
NULL (pointer)	0	FALSE	Block skipped
&variable (address)	non-zero	TRUE	Block executes

Comparison Expressions Return 0 or 1

When you use comparison operators (>, <,==, etc.), the result is always 1 (true) or 0 (false).

Expression	Evaluates To	Meaning
5 > 3	1	TRUE (5 is greater than 3)
5 < 3	0	FALSE (5 is not less than 3)
5 == 5	1	TRUE (5 equals 5)
5 != 5	0	FALSE (5 is not different from 5)
10 >= 10	1	TRUE (10 is >= 10)
(5 > 3) && (2 < 1)	0	FALSE (true AND false = false)
(5 > 3) \|\| (2 < 1)	1	TRUE (true OR false = true)

▶ Try it: Demonstrates how C evaluates different values as true or false.

true_false.c

1#include <stdio.h>
2
3int main() {
4    // Demonstrating true/false values
5    printf("=== True/False in C ===\n\n");
6    
7    // Zero is FALSE
8    if (0) {
9        printf("0: This won't print\n");
10    } else {
11        printf("0: FALSE (block skipped)\n");
12    }
13    
14    // Non-zero is TRUE
15    if (1) printf("1: TRUE\n");
16    if (-5) printf("-5: TRUE (negative numbers are non-zero)\n");
17    if (100) printf("100: TRUE\n");
18    
19    // Comparison results
20    printf("\n=== Comparisons Return 0 or 1 ===\n");
21    printf("5 > 3  = %d (TRUE)\n", 5 > 3);
22    printf("5 < 3  = %d (FALSE)\n", 5 < 3);
23    printf("5 == 5 = %d (TRUE)\n", 5 == 5);
24    printf("5 != 5 = %d (FALSE)\n", 5 != 5);
25    
26    // You can store comparison results
27    int isAdult = (25 >= 18);  // isAdult = 1
28    printf("\n25 >= 18 stored in variable: %d\n", isAdult);
29    
30    return 0;
31}

Output

=== True/False in C ===

0: FALSE (block skipped)

1: TRUE

-5: TRUE (negative numbers are non-zero)

100: TRUE

=== Comparisons Return 0 or 1 ===

5 > 3 = 1 (TRUE)

5 < 3 = 0 (FALSE)

5 == 5 = 1 (TRUE)

5 != 5 = 0 (FALSE)

25 >= 18 stored in variable: 1

Common Mistake: = vs ==

= is assignment, == is comparison. Writing if (x = 5) assigns 5 to x and is always TRUE (5 ≠ 0)!

if (x = 5)   // WRONG! Assigns 5 to x, always TRUE
if (x == 5)  // CORRECT! Compares x to 5

03Comparison Operators

Before writing conditions, you need to know how to compare values. These operators compare two values and return 1 (true) or 0 (false).

Operator	Meaning	Example	Result
==	Equal to	5 == 5	1 (true)
!=	Not equal to	5 != 3	1 (true)
>	Greater than	5 > 3	1 (true)
<	Less than	5 < 3	0 (false)
>=	Greater than or equal	5 >= 5	1 (true)
<=	Less than or equal	3 <= 5	1 (true)

Quick Example

main.c

#include <stdio.h>
int main() {
    int a = 10, b = 5;
    
    printf("a = %d, b = %d\n", a, b);
    printf("a == b: %d\n", a == b);  // 0 (false)
    printf("a != b: %d\n", a != b);  // 1 (true)
    printf("a > b:  %d\n", a > b);   // 1 (true)
    printf("a < b:  %d\n", a < b);   // 0 (false)
    
    return 0;
}

a = 10, b = 5

a == b: 0

a != b: 1

a > b: 1

a < b: 0

Common Mistake: = vs ==

= (Assignment)

x = 5; // Sets x to 5

== (Comparison)

x == 5; // Is x equal to 5?

04Logical Operators

Logical operators let you combine multiple conditions.

Operator	Name	Meaning	Example
&&	AND	Both must be true	a > 0 && b > 0
\|\|	OR	At least one must be true	a > 0 \|\| b > 0
!	NOT	Reverses true/false	!(a > 0)

&& (AND)

Both must be TRUE

true && true= true ✓

true && false= false ✗

false && true= false ✗

false && false= false ✗

|| (OR)

At least one TRUE

true || true= true ✓

true || false= true ✓

false || true= true ✓

false || false= false ✗

! (NOT)

Reverses the value

!true= false

!false= true

In C:

!0 = 1, !1 = 0

Real World Examples

&& (AND) - Both conditions required

Can drive = age >= 18 && hasLicense

Must be 18+ AND have license

|| (OR) - Either condition works

Free entry = age < 5 || age > 65

Either under 5 OR over 65

Example: Combining Conditions

main.c

#include <stdio.h>
int main() {
    int age = 25;
    int hasLicense = 1;  // 1 = true
    
    // AND: Both conditions must be true
    if (age >= 18 && hasLicense) {
        printf("You can drive!\n");
    }
    
    // OR: At least one condition must be true
    int isWeekend = 1;
    int isHoliday = 0;
    if (isWeekend || isHoliday) {
        printf("No work today!\n");
    }
    
    // NOT: Reverses the condition
    int isRaining = 0;
    if (!isRaining) {
        printf("No umbrella needed.\n");
    }
    
    return 0;
}

You can drive!

No work today!

No umbrella needed.

05If Statement

The if statement executes code only when the condition is true.

How if Works

Check condition

↓

if (condition)

↓ TRUE

Run code block

↓ FALSE

Skip code block

Syntax

if (condition) {

// Code runs if condition is TRUE

}

1.Simple Example: Check Temperature

main.c

#include <stdio.h>
int main() {
    int temp = 35;
    
    if (temp > 30) {
        printf("It's hot today!\n");
    }
    
    printf("Done.\n");
    return 0;
}

It's hot today!

Done.

Since 35 > 30 is true, the message prints.

2.Another Example: Check if Positive

main.c

#include <stdio.h>
int main() {
    int num = 10;
    
    if (num > 0) {
        printf("%d is positive\n", num);
    }
    
    return 0;
}

10 is positive

06If-Else Statement

Add else to handle the case when the condition is FALSE. Now you have two paths - one for true, one for false.

How if-else Works

if (condition)

↓ TRUE

Run if block

↓ FALSE

Run else block

↓

Continue...

1.Check Even or Odd

main.c

#include <stdio.h>
int main() {
    int num = 7;
    
    if (num % 2 == 0) {
        printf("%d is EVEN\n", num);
    } else {
        printf("%d is ODD\n", num);
    }
    
    return 0;
}

7 is ODD

% is the modulo operator (gives remainder). 7 % 2 = 1, not 0, so it's odd.

2.Check Pass or Fail

main.c

#include <stdio.h>
int main() {
    int marks = 55;
    
    if (marks >= 40) {
        printf("Result: PASS\n");
    } else {
        printf("Result: FAIL\n");
    }
    
    return 0;
}

Result: PASS

3.Check Voting Eligibility

main.c

#include <stdio.h>
int main() {
    int age = 16;
    
    if (age >= 18) {
        printf("You can vote!\n");
    } else {
        printf("Too young to vote.\n");
        printf("Wait %d more years.\n", 18 - age);
    }
    
    return 0;
}

Too young to vote.

Wait 2 more years.

07Else-If Ladder

When you have multiple conditions to check, use an else-if ladder. Conditions are evaluated top-to-bottom; only the first true condition executes.

Syntax

if (condition1) {
    // Runs if condition1 is TRUE
}
else if (condition2) {
    // Runs if condition1 is FALSE and condition2 is TRUE
}
else if (condition3) {
    // Runs if condition1, condition2 are FALSE and condition3 is TRUE
}
else {
    // Runs if ALL conditions are FALSE
}

▶ Try it: Converts marks to letter grades using multiple conditions.

grade_calculator.c

1#include <stdio.h>
2
3int main() {
4    int marks = 73;
5    char grade;
6    
7    printf("Marks: %d\n", marks);
8    
9    if (marks >= 90) {
10        grade = 'A';
11        printf("Excellent work!\n");
12    }
13    else if (marks >= 80) {
14        grade = 'B';
15        printf("Great job!\n");
16    }
17    else if (marks >= 70) {
18        grade = 'C';
19        printf("Good effort!\n");
20    }
21    else if (marks >= 60) {
22        grade = 'D';
23        printf("You passed.\n");
24    }
25    else {
26        grade = 'F';
27        printf("Need improvement.\n");
28    }
29    
30    printf("Grade: %c\n", grade);
31    
32    return 0;
33}

Output

Marks: 73

Good effort!

Grade: C

Order Matters!

With 73 marks: We check marks >= 90 (false), then marks >= 80 (false), then marks >= 70 (TRUE!). The rest are skipped.

08Nested If Statements

You can place an if inside another if to check multiple related conditions. This is called nesting.

▶ Try it: Checks if a person is eligible to vote (must be a citizen AND at least 18).

nested_if.c

1#include <stdio.h>
2
3int main() {
4    int age = 20;
5    int isCitizen = 1;  // 1 = true, 0 = false
6    
7    printf("Age: %d, Citizen: %s\n", age, isCitizen ? "Yes" : "No");
8    
9    if (isCitizen) {
10        // Only check age if they're a citizen
11        if (age >= 18) {
12            printf("✓ You can vote!\n");
13        }
14        else {
15            printf("✗ Too young to vote (must be 18+).\n");
16        }
17    }
18    else {
19        printf("✗ Only citizens can vote.\n");
20    }
21    
22    return 0;
23}

Output

Age: 20, Citizen: Yes

✓ You can vote!

Simplify with Logical AND

Nested ifs can often be simplified using &&:

// Instead of nesting:
if (isCitizen && age >= 18) {
    printf("You can vote!\n");
}

09Ternary Operator (? :)

The ternary operator is a shorthand for simple if-else statements. It's called "ternary" because it takes three operands.

Syntax

condition ? value_if_true : value_if_false

If condition is true → returns first value, else → returns second value

Using if-else (4 lines)

main.c

int max;
if (a > b) {
    max = a;
} else {
    max = b;
}

Using ternary (1 line)

main.c

int max = (a > b) ? a : b;

Same result, much shorter!

Example: Find Maximum

main.c

#include <stdio.h>
int main() {
    int a = 10, b = 20;
    
    int max = (a > b) ? a : b;
    
    printf("Max of %d and %d is %d\n", a, b, max);
    return 0;
}

Max of 10 and 20 is 20

Example: Even/Odd String

main.c

#include <stdio.h>
int main() {
    int num = 7;
    
    // Ternary in printf
    printf("%d is %s\n", num, (num % 2 == 0) ? "even" : "odd");
    
    return 0;
}

7 is odd

Example: Absolute Value

main.c

#include <stdio.h>
int main() {
    int num = -5;
    
    int abs_val = (num >= 0) ? num : -num;
    
    printf("Absolute value of %d is %d\n", num, abs_val);
    return 0;
}

Absolute value of -5 is 5

When to Use Ternary

• Use for simple one-liner assignments
• Use inside printf for dynamic strings
• Avoid for complex logic (hard to read)
• Avoid nesting ternary operators

10Switch Statement

The switch statement selects one of many code blocks based on the value of an expression. It is cleaner than multiple if-else when comparing one variable to many constant values.

Syntax

switch (expression) {
    case value1:
        // code for value1
        break;        // Exit switch
    case value2:
        // code for value2
        break;
    default:
        // code if no case matches
}

Component	Description
switch(expr)	Expression must be int or char (integer types)
case value:	Must be a constant (not variable), unique values only
break;	Exits switch. Without it, execution "falls through" to next case
default:	Optional. Runs if no case matches (like else)

▶ Try it: Displays the day name based on a number (1-7).

switch_day.c

1#include <stdio.h>
2
3int main() {
4    int day = 3;
5    
6    printf("Day number: %d\n", day);
7    printf("Day name: ");
8    
9    switch (day) {
10        case 1:
11            printf("Monday\n");
12            break;
13        case 2:
14            printf("Tuesday\n");
15            break;
16        case 3:
17            printf("Wednesday\n");
18            break;
19        case 4:
20            printf("Thursday\n");
21            break;
22        case 5:
23            printf("Friday\n");
24            break;
25        case 6:
26            printf("Saturday\n");
27            break;
28        case 7:
29            printf("Sunday\n");
30            break;
31        default:
32            printf("Invalid day (use 1-7)\n");
33    }
34    
35    return 0;
36}

Output

Day number: 3

Day name: Wednesday

Don't Forget break!

Without break, execution falls through to the next case. This is sometimes intentional, but usually a bug.

Intentional Fall-Through

Sometimes fall-through is useful to group multiple cases with the same code:

▶ Try it: Uses fall-through to group weekdays and weekends.

switch_fallthrough.c

1#include <stdio.h>
2
3int main() {
4    int day = 6;
5    
6    printf("Day %d is a ", day);
7    
8    switch (day) {
9        case 1:
10        case 2:
11        case 3:
12        case 4:
13        case 5:
14            printf("Weekday\n");
15            break;
16        case 6:
17        case 7:
18            printf("Weekend!\n");
19            break;
20        default:
21            printf("Invalid day\n");
22    }
23    
24    return 0;
25}

Output

Day 6 is a Weekend!

11Switch vs If-Else: When to Use Which

Criteria	switch	if-else
Comparison Type	Exact value match only	Any condition (>, <, &&, etc.)
Data Types	int, char (integer types)	Any type (int, float, pointers)
Case Values	Must be constants	Can use variables
Range Checking	Not supported	Supported (x > 10 && x < 20)
Readability	Better for many exact values	Better for complex conditions
Performance	Often optimized to jump table	Evaluated sequentially

✓Use switch for:

• Menu options (1, 2, 3, 4...)
• Day of week (1-7)
• Month selection
• Character commands ('a', 'b', 'c')
• Enum values

✓Use if-else for:

• Range checks (marks >= 90)
• Boolean conditions
• Comparing floats
• Complex expressions
• Variable comparisons

12Jump Statements: break, continue, goto

Jump statements transfer control to another part of the program. They interrupt the normal sequential flow of execution.

Statement	Purpose	Used In	Effect
break	Exit immediately	loops, switch	Stops execution, exits the block
continue	Skip current iteration	loops only	Jumps to next iteration
goto	Jump to label	anywhere in function	Unconditional jump

The break Statement

break immediately exits the innermost loop or switch statement. Execution continues with the statement following the loop/switch.

When to Use break

✓Exit a switch case (required to prevent fall-through)
✓Stop searching when you find what you need
✓Exit an infinite loop when a condition is met
✓Handle error conditions that require early exit

▶ Try it: Searches for a target value and exits the loop immediately when found.

break_search.c

1#include <stdio.h>
2
3int main() {
4    int numbers[] = {10, 25, 33, 47, 56, 68, 72};
5    int size = 7;
6    int target = 47;
7    int found = 0;
8    
9    printf("Searching for %d...\n", target);
10    
11    for (int i = 0; i < size; i++) {
12        printf("Checking index %d: %d\n", i, numbers[i]);
13        
14        if (numbers[i] == target) {
15            printf(">>> Found %d at index %d!\n", target, i);
16            found = 1;
17            break;  // Exit loop immediately - no need to check rest
18        }
19    }
20    
21    if (!found) {
22        printf("Value not found.\n");
23    }
24    
25    printf("Search complete.\n");
26    return 0;
27}

Output

Searching for 47...

Checking index 0: 10

Checking index 1: 25

Checking index 2: 33

Checking index 3: 47

>>> Found 47 at index 3!

Search complete.

The continue Statement

continue skips the rest of the current iteration and jumps to the next iteration of the loop. The loop itself continues running.

When to Use continue

✓Skip invalid or unwanted data
✓Process only items matching criteria (filter)
✓Avoid deep nesting by handling edge cases early

break vs continue

break

Loop: 1 2 3 [break] done

Exits loop completely

continue

Loop: 1 2 [skip] 4 5 done

Skips to next iteration

▶ Try it: Prints only positive numbers from an array, skipping negative ones.

continue_filter.c

1#include <stdio.h>
2
3int main() {
4    int numbers[] = {5, -3, 10, -7, 8, -2, 15};
5    int size = 7;
6    int sum = 0;
7    
8    printf("Processing numbers (skipping negatives):\n");
9    
10    for (int i = 0; i < size; i++) {
11        // Skip negative numbers
12        if (numbers[i] < 0) {
13            printf("Skipping %d (negative)\n", numbers[i]);
14            continue;  // Jump to next iteration
15        }
16        
17        // This code only runs for positive numbers
18        printf("Adding %d\n", numbers[i]);
19        sum += numbers[i];
20    }
21    
22    printf("\nSum of positive numbers: %d\n", sum);
23    return 0;
24}

Output

Processing numbers (skipping negatives):

Adding 5

Skipping -3 (negative)

Adding 10

Skipping -7 (negative)

Adding 8

Skipping -2 (negative)

Adding 15

Sum of positive numbers: 38

The goto Statement

goto performs an unconditional jump to a labeled statement anywhere within the same function.

Syntax

goto label_name;    // Jump to label

// ... other code ...

label_name:         // Label definition (ends with :)
    // Code continues here

Why goto is Controversial

goto is considered bad practice by most programmers because:

• Creates "spaghetti code" that's hard to read and maintain
• Makes program flow unpredictable
• Can be replaced by structured constructs (loops, functions)
• Makes debugging difficult

✓Acceptable Uses of goto

There are a few cases where goto can make code cleaner:

1. Breaking out of deeply nested loops - When you need to exit multiple loops at once
2. Centralized cleanup code - Jumping to cleanup/error handling at end of function
3. State machines - Some state machine implementations

Acceptable use: Breaking out of nested loops to find a target in a 2D grid.

goto_nested.c

1#include <stdio.h>
2
3int main() {
4    int grid[3][3] = {
5        {1, 2, 3},
6        {4, 5, 6},
7        {7, 8, 9}
8    };
9    int target = 5;
10    int found_row = -1, found_col = -1;
11    
12    printf("Searching for %d in 3x3 grid...\n\n", target);
13    
14    // Nested loops - break would only exit inner loop
15    for (int i = 0; i < 3; i++) {
16        for (int j = 0; j < 3; j++) {
17            printf("Checking [%d][%d] = %d\n", i, j, grid[i][j]);
18            
19            if (grid[i][j] == target) {
20                found_row = i;
21                found_col = j;
22                goto found;  // Exit BOTH loops at once
23            }
24        }
25    }
26    
27    // Only reaches here if not found
28    printf("\nValue not found in grid.\n");
29    goto end;
30    
31found:
32    printf("\n>>> Found %d at position [%d][%d]!\n", target, found_row, found_col);
33    
34end:
35    printf("Search complete.\n");
36    return 0;
37}

Output

Searching for 5 in 3x3 grid...

Checking [0][0] = 1

Checking [0][1] = 2

Checking [0][2] = 3

Checking [1][0] = 4

Checking [1][1] = 5

>>> Found 5 at position [1][1]!

Search complete.

Another acceptable use: Centralized error handling and resource cleanup.

goto_cleanup.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    FILE *file = NULL;
6    char *buffer = NULL;
7    int result = 0;
8    
9    // Allocate memory
10    buffer = malloc(100);
11    if (buffer == NULL) {
12        printf("Error: Memory allocation failed\n");
13        goto cleanup;  // Jump to cleanup
14    }
15    
16    // Open file
17    file = fopen("data.txt", "r");
18    if (file == NULL) {
19        printf("Error: Cannot open file\n");
20        goto cleanup;  // Jump to cleanup
21    }
22    
23    // Process file...
24    printf("Processing file...\n");
25    result = 1;  // Success
26    
27cleanup:  // Single cleanup point for all exits
28    if (file != NULL) {
29        fclose(file);
30        printf("File closed.\n");
31    }
32    if (buffer != NULL) {
33        free(buffer);
34        printf("Memory freed.\n");
35    }
36    
37    return result ? 0 : 1;
38}

Output (if file doesn't exist)

Error: Cannot open file

Memory freed.

Alternatives to goto

In most cases, you can avoid goto using:

• Flag variables - Use a boolean to break out of nested loops
• Functions - Put nested loops in a function and use return
• break + flag - Set a flag before break, check in outer loop

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Using = instead of == in conditions

This is the most common bug! = assigns a value,== compares values.

main.c

// WRONG - assigns 5 to x, condition is always TRUE (5 != 0)
if (x = 5) {
    printf("This always runs!\n");
}
// CORRECT - compares x to 5
if (x == 5) {
    printf("x is 5\n");
}

Forgetting break in switch

Without break, execution falls through to the next case!

main.c

// WRONG - falls through to next cases
switch (day) {
    case 1:
        printf("Monday\n");
        // Missing break! Falls through...
    case 2:
        printf("Tuesday\n");  // Also prints!
}
// CORRECT
switch (day) {
    case 1:
        printf("Monday\n");
        break;  // Exits switch
    case 2:
        printf("Tuesday\n");
        break;
}

Semicolon after if condition

A semicolon after if() creates an empty statement!

main.c

// WRONG - semicolon makes if do nothing
if (x > 5);  // Empty statement!
{
    printf("This ALWAYS runs!\n");  // Not part of if
}
// CORRECT
if (x > 5) {
    printf("x is greater than 5\n");
}

Dangling else problem

Without braces, else binds to the nearest if.

main.c

// CONFUSING - which if does else belong to?
if (a > 0)
    if (b > 0)
        printf("a and b positive\n");
else
    printf("This runs when b <= 0, NOT when a <= 0!\n");
// CLEAR - use braces to show intent
if (a > 0) {
    if (b > 0) {
        printf("a and b positive\n");
    }
} else {
    printf("a is not positive\n");
}

Using switch with non-constant cases

Switch case values must be compile-time constants, not variables.

main.c

int option = 2;
// WRONG - case values must be constants
switch (choice) {
    case option:  // ERROR! Variables not allowed
        break;
}
// CORRECT - use constant values
switch (choice) {
    case 2:  // Literal constant - OK
        break;
    case MAX_VALUE:  // #define constant - OK
        break;
}

Wrong order in else-if ladder

More specific conditions should come first; otherwise, they're never reached.

main.c

int score = 95;
// WRONG - first condition catches everything >= 60
if (score >= 60) {
    printf("D\n");  // 95 matches here!
} else if (score >= 70) {
    printf("C\n");  // Never reached for 95
} else if (score >= 90) {
    printf("A\n");  // Never reached
}
// CORRECT - most specific first
if (score >= 90) {
    printf("A\n");  // 95 matches correctly
} else if (score >= 80) {
    printf("B\n");
} else if (score >= 60) {
    printf("D\n");
}

Using floats in switch

Switch only works with integer types (int, char, enum). Use if-else for floats.

main.c

float grade = 3.5;
// WRONG - float not allowed in switch
switch (grade) {  // ERROR!
    case 4.0:
        break;
}
// CORRECT - use if-else for floats
if (grade >= 4.0) {
    printf("A\n");
} else if (grade >= 3.0) {
    printf("B\n");
}

Watch Out When Copying Code!

Why copied Conditionals Need Your Review!

When you ask someone to write if-else or switch statements, it often generates code that looks correct but has subtle logical flaws:

1. Incomplete Edge Cases

Copied code might write a grade calculator that handles A, B, C, D but forgets what happens when score is negative or above 100.

main.c

// copied - looks fine but incomplete
if (score >= 90) printf("A");
else if (score >= 80) printf("B");
// What if score = -5 or score = 150?

2. Missing Break in Switch

Beginners sometimes generates switch statements with intentional fall-through but doesn't comment it, or accidentally forgets break statements.

3. Condition Order Bugs

Copied code might place score >= 60 before score >= 90, meaning a score of 95 would match the first condition (60+) and never reach the A grade check.

Your Job: Always trace through copied conditionals with extreme values (0, -1, max, boundary values) to catch logic errors that "look right" but aren't.

15Frequently Asked Questions

Q:When should I use switch instead of if-else?

A: Use switch when comparing ONE variable against multiple exact values (like menu choices, day numbers, character codes). Use if-else for ranges, multiple conditions, floating-point comparisons, or string comparisons. Switch is often cleaner for >3 exact-match cases.

Q:Why do I need braces if there's only one statement after if?

A: You don't technically need them for single statements, but you should always use them anyway. Why? When you add a second line later, forgetting to add braces causes a bug where only the first line runs conditionally. This "Apple SSL bug" actually crashed iOS security in 2014! Always use braces — it costs nothing and prevents disasters.

Q:What's the difference between = and == again?

A: = is assignment (put value into variable). == is comparison (check if equal). Writing if (x = 5) assigns 5 to x, then evaluates to true (since 5 ≠ 0). This is the #1 C bug. Tip: Write if (5 == x) — if you accidentally use =, the compiler catches it since you can't assign to 5.

Q:What happens if I forget break in a switch case?

A: Execution "falls through" to the next case! If case 1 matches but has no break, it runs case 1's code, then case 2's code, then case 3's, etc. until hitting a break or the switch ends. This is sometimes useful (grouping cases), but usually a bug. Modern compilers warn about this with -Wimplicit-fallthrough.

Q:Can I use strings in a switch statement?

A: No! Switch only works with integer types:int, char,enum, short,long. For strings, you must use if-else with strcmp(). Note: char works because characters are integers (ASCII codes), so case 'A': meanscase 65:.

Q:What does the ternary operator ?: do?

A: It's a compact if-else in one line:condition ? value_if_true : value_if_false. Example:int max = (a > b) ? a : b; assigns the larger of a or b to max. Use it for simple value selection, not complex logic. Nested ternaries become unreadable — prefer if-else for anything complex.

Q:Why does if(x) work when x is a number, not a boolean?

A: In C, there's no built-in boolean type (until C99's stdbool.h). Instead, C uses integers: 0 = false,any non-zero = true. So if(5) is true,if(-1) is true, if(0) is false. This is why if(x = 5) evaluates as true — the assignment returns 5, which is non-zero!

16Summary

Key Takeaways

•TRUE/FALSE: 0 = false, any non-zero = true
•Comparison: ==, !=, >, <, >=, <= (return 1 or 0)
•Logical: && (AND), || (OR), ! (NOT)
•if: Execute code only when condition is true
•if-else: Two paths - one for true, one for false
•else-if: Check multiple conditions in order
•Ternary: condition ? valueIfTrue : valueIfFalse
•switch: Match exact values (int/char), always use break!
•break: Exit loop/switch immediately
•continue: Skip to next loop iteration
•= vs ==: Assignment vs comparison - #1 bug source!

Quick Reference

Decision Statements

if (condition) { ... }

if (cond) { ... } else { ... }

switch (var) { case 1: ... break; }

Operators

== != > < >= <= (comparison)

&& || ! (logical)

?: (ternary)

01What is Control Flow?

Simple Definition

Control flow lets your program make decisions. Instead of running every line from top to bottom, your program can choose which code to run based on conditions.

Real World Example

️

Is it raining?

→

YES

☂️ Take umbrella

Wear sunglasses

This is exactly what if-else does in programming!

Type	Purpose	Keywords
Decision Making	Choose which code to run	if, else, switch
Looping	Repeat code multiple times	for, while, do-while
Jump	Skip or exit code blocks	break, continue, goto

This Tutorial Covers Decision Making

This tutorial focuses on if-else and switch statements. For loops (for, while, do-while), see the Loops Tutorial.

02Understanding True and False in C

Before using if statements, you must understand how C interprets true and false. Unlike some languages, C doesn't have a built-in bool type (before C99).

The Golden Rule in C

= FALSE

The condition is NOT met

Code block is SKIPPED

≠ 0

= TRUE

Any non-zero value

Code block EXECUTES

What Values Are True or False?

Expression / Value	Numeric Value	Result	if() Behavior
0	0	FALSE	Block skipped
1	1	TRUE	Block executes
-1, -5, -100	non-zero	TRUE	Block executes
42, 100, 999	non-zero	TRUE	Block executes
0.0 (float)	0	FALSE	Block skipped
3.14 (float)	non-zero	TRUE	Block executes
'\0' (null char)	0	FALSE	Block skipped
'A' (any char)	65	TRUE	Block executes
NULL (pointer)	0	FALSE	Block skipped
&variable (address)	non-zero	TRUE	Block executes

Comparison Expressions Return 0 or 1

When you use comparison operators (>, <,==, etc.), the result is always 1 (true) or 0 (false).

Expression	Evaluates To	Meaning
5 > 3	1	TRUE (5 is greater than 3)
5 < 3	0	FALSE (5 is not less than 3)
5 == 5	1	TRUE (5 equals 5)
5 != 5	0	FALSE (5 is not different from 5)
10 >= 10	1	TRUE (10 is >= 10)
(5 > 3) && (2 < 1)	0	FALSE (true AND false = false)
(5 > 3) \|\| (2 < 1)	1	TRUE (true OR false = true)

▶ Try it: Demonstrates how C evaluates different values as true or false.

true_false.c

1#include <stdio.h>
2
3int main() {
4    // Demonstrating true/false values
5    printf("=== True/False in C ===\n\n");
6    
7    // Zero is FALSE
8    if (0) {
9        printf("0: This won't print\n");
10    } else {
11        printf("0: FALSE (block skipped)\n");
12    }
13    
14    // Non-zero is TRUE
15    if (1) printf("1: TRUE\n");
16    if (-5) printf("-5: TRUE (negative numbers are non-zero)\n");
17    if (100) printf("100: TRUE\n");
18    
19    // Comparison results
20    printf("\n=== Comparisons Return 0 or 1 ===\n");
21    printf("5 > 3  = %d (TRUE)\n", 5 > 3);
22    printf("5 < 3  = %d (FALSE)\n", 5 < 3);
23    printf("5 == 5 = %d (TRUE)\n", 5 == 5);
24    printf("5 != 5 = %d (FALSE)\n", 5 != 5);
25    
26    // You can store comparison results
27    int isAdult = (25 >= 18);  // isAdult = 1
28    printf("\n25 >= 18 stored in variable: %d\n", isAdult);
29    
30    return 0;
31}

Output

=== True/False in C ===

0: FALSE (block skipped)

1: TRUE

-5: TRUE (negative numbers are non-zero)

100: TRUE

=== Comparisons Return 0 or 1 ===

5 > 3 = 1 (TRUE)

5 < 3 = 0 (FALSE)

5 == 5 = 1 (TRUE)

5 != 5 = 0 (FALSE)

25 >= 18 stored in variable: 1

Common Mistake: = vs ==

= is assignment, == is comparison. Writing if (x = 5) assigns 5 to x and is always TRUE (5 ≠ 0)!

if (x = 5)   // WRONG! Assigns 5 to x, always TRUE
if (x == 5)  // CORRECT! Compares x to 5

03Comparison Operators

Before writing conditions, you need to know how to compare values. These operators compare two values and return 1 (true) or 0 (false).

Operator	Meaning	Example	Result
==	Equal to	5 == 5	1 (true)
!=	Not equal to	5 != 3	1 (true)
>	Greater than	5 > 3	1 (true)
<	Less than	5 < 3	0 (false)
>=	Greater than or equal	5 >= 5	1 (true)
<=	Less than or equal	3 <= 5	1 (true)

Quick Example

main.c

#include <stdio.h>
int main() {
    int a = 10, b = 5;
    
    printf("a = %d, b = %d\n", a, b);
    printf("a == b: %d\n", a == b);  // 0 (false)
    printf("a != b: %d\n", a != b);  // 1 (true)
    printf("a > b:  %d\n", a > b);   // 1 (true)
    printf("a < b:  %d\n", a < b);   // 0 (false)
    
    return 0;
}

a = 10, b = 5

a == b: 0

a != b: 1

a > b: 1

a < b: 0

Common Mistake: = vs ==

= (Assignment)

x = 5; // Sets x to 5

== (Comparison)

x == 5; // Is x equal to 5?

04Logical Operators

Logical operators let you combine multiple conditions.

Operator	Name	Meaning	Example
&&	AND	Both must be true	a > 0 && b > 0
\|\|	OR	At least one must be true	a > 0 \|\| b > 0
!	NOT	Reverses true/false	!(a > 0)

&& (AND)

Both must be TRUE

true && true= true ✓

true && false= false ✗

false && true= false ✗

false && false= false ✗

|| (OR)

At least one TRUE

true || true= true ✓

true || false= true ✓

false || true= true ✓

false || false= false ✗

! (NOT)

Reverses the value

!true= false

!false= true

In C:

!0 = 1, !1 = 0

Real World Examples

&& (AND) - Both conditions required

Can drive = age >= 18 && hasLicense

Must be 18+ AND have license

|| (OR) - Either condition works

Free entry = age < 5 || age > 65

Either under 5 OR over 65

Example: Combining Conditions

main.c

#include <stdio.h>
int main() {
    int age = 25;
    int hasLicense = 1;  // 1 = true
    
    // AND: Both conditions must be true
    if (age >= 18 && hasLicense) {
        printf("You can drive!\n");
    }
    
    // OR: At least one condition must be true
    int isWeekend = 1;
    int isHoliday = 0;
    if (isWeekend || isHoliday) {
        printf("No work today!\n");
    }
    
    // NOT: Reverses the condition
    int isRaining = 0;
    if (!isRaining) {
        printf("No umbrella needed.\n");
    }
    
    return 0;
}

You can drive!

No work today!

No umbrella needed.

05If Statement

The if statement executes code only when the condition is true.

How if Works

Check condition

↓

if (condition)

↓ TRUE

Run code block

↓ FALSE

Skip code block

Syntax

if (condition) {

// Code runs if condition is TRUE

}

1.Simple Example: Check Temperature

main.c

#include <stdio.h>
int main() {
    int temp = 35;
    
    if (temp > 30) {
        printf("It's hot today!\n");
    }
    
    printf("Done.\n");
    return 0;
}

It's hot today!

Done.

Since 35 > 30 is true, the message prints.

2.Another Example: Check if Positive

main.c

#include <stdio.h>
int main() {
    int num = 10;
    
    if (num > 0) {
        printf("%d is positive\n", num);
    }
    
    return 0;
}

10 is positive

06If-Else Statement

Add else to handle the case when the condition is FALSE. Now you have two paths - one for true, one for false.

How if-else Works

if (condition)

↓ TRUE

Run if block

↓ FALSE

Run else block

↓

Continue...

1.Check Even or Odd

main.c

#include <stdio.h>
int main() {
    int num = 7;
    
    if (num % 2 == 0) {
        printf("%d is EVEN\n", num);
    } else {
        printf("%d is ODD\n", num);
    }
    
    return 0;
}

7 is ODD

% is the modulo operator (gives remainder). 7 % 2 = 1, not 0, so it's odd.

2.Check Pass or Fail

main.c

#include <stdio.h>
int main() {
    int marks = 55;
    
    if (marks >= 40) {
        printf("Result: PASS\n");
    } else {
        printf("Result: FAIL\n");
    }
    
    return 0;
}

Result: PASS

3.Check Voting Eligibility

main.c

#include <stdio.h>
int main() {
    int age = 16;
    
    if (age >= 18) {
        printf("You can vote!\n");
    } else {
        printf("Too young to vote.\n");
        printf("Wait %d more years.\n", 18 - age);
    }
    
    return 0;
}

Too young to vote.

Wait 2 more years.

07Else-If Ladder

When you have multiple conditions to check, use an else-if ladder. Conditions are evaluated top-to-bottom; only the first true condition executes.

Syntax

if (condition1) {
    // Runs if condition1 is TRUE
}
else if (condition2) {
    // Runs if condition1 is FALSE and condition2 is TRUE
}
else if (condition3) {
    // Runs if condition1, condition2 are FALSE and condition3 is TRUE
}
else {
    // Runs if ALL conditions are FALSE
}

▶ Try it: Converts marks to letter grades using multiple conditions.

grade_calculator.c

1#include <stdio.h>
2
3int main() {
4    int marks = 73;
5    char grade;
6    
7    printf("Marks: %d\n", marks);
8    
9    if (marks >= 90) {
10        grade = 'A';
11        printf("Excellent work!\n");
12    }
13    else if (marks >= 80) {
14        grade = 'B';
15        printf("Great job!\n");
16    }
17    else if (marks >= 70) {
18        grade = 'C';
19        printf("Good effort!\n");
20    }
21    else if (marks >= 60) {
22        grade = 'D';
23        printf("You passed.\n");
24    }
25    else {
26        grade = 'F';
27        printf("Need improvement.\n");
28    }
29    
30    printf("Grade: %c\n", grade);
31    
32    return 0;
33}

Output

Marks: 73

Good effort!

Grade: C

Order Matters!

With 73 marks: We check marks >= 90 (false), then marks >= 80 (false), then marks >= 70 (TRUE!). The rest are skipped.

08Nested If Statements

You can place an if inside another if to check multiple related conditions. This is called nesting.

▶ Try it: Checks if a person is eligible to vote (must be a citizen AND at least 18).

nested_if.c

1#include <stdio.h>
2
3int main() {
4    int age = 20;
5    int isCitizen = 1;  // 1 = true, 0 = false
6    
7    printf("Age: %d, Citizen: %s\n", age, isCitizen ? "Yes" : "No");
8    
9    if (isCitizen) {
10        // Only check age if they're a citizen
11        if (age >= 18) {
12            printf("✓ You can vote!\n");
13        }
14        else {
15            printf("✗ Too young to vote (must be 18+).\n");
16        }
17    }
18    else {
19        printf("✗ Only citizens can vote.\n");
20    }
21    
22    return 0;
23}

Output

Age: 20, Citizen: Yes

✓ You can vote!

Simplify with Logical AND

Nested ifs can often be simplified using &&:

// Instead of nesting:
if (isCitizen && age >= 18) {
    printf("You can vote!\n");
}

09Ternary Operator (? :)

The ternary operator is a shorthand for simple if-else statements. It's called "ternary" because it takes three operands.

Syntax

condition ? value_if_true : value_if_false

If condition is true → returns first value, else → returns second value

Using if-else (4 lines)

main.c

int max;
if (a > b) {
    max = a;
} else {
    max = b;
}

Using ternary (1 line)

main.c

int max = (a > b) ? a : b;

Same result, much shorter!

Example: Find Maximum

main.c

#include <stdio.h>
int main() {
    int a = 10, b = 20;
    
    int max = (a > b) ? a : b;
    
    printf("Max of %d and %d is %d\n", a, b, max);
    return 0;
}

Max of 10 and 20 is 20

Example: Even/Odd String

main.c

#include <stdio.h>
int main() {
    int num = 7;
    
    // Ternary in printf
    printf("%d is %s\n", num, (num % 2 == 0) ? "even" : "odd");
    
    return 0;
}

7 is odd

Example: Absolute Value

main.c

#include <stdio.h>
int main() {
    int num = -5;
    
    int abs_val = (num >= 0) ? num : -num;
    
    printf("Absolute value of %d is %d\n", num, abs_val);
    return 0;
}

Absolute value of -5 is 5

When to Use Ternary

• Use for simple one-liner assignments
• Use inside printf for dynamic strings
• Avoid for complex logic (hard to read)
• Avoid nesting ternary operators

10Switch Statement

The switch statement selects one of many code blocks based on the value of an expression. It is cleaner than multiple if-else when comparing one variable to many constant values.

Syntax

switch (expression) {
    case value1:
        // code for value1
        break;        // Exit switch
    case value2:
        // code for value2
        break;
    default:
        // code if no case matches
}

Component	Description
switch(expr)	Expression must be int or char (integer types)
case value:	Must be a constant (not variable), unique values only
break;	Exits switch. Without it, execution "falls through" to next case
default:	Optional. Runs if no case matches (like else)

▶ Try it: Displays the day name based on a number (1-7).

switch_day.c

1#include <stdio.h>
2
3int main() {
4    int day = 3;
5    
6    printf("Day number: %d\n", day);
7    printf("Day name: ");
8    
9    switch (day) {
10        case 1:
11            printf("Monday\n");
12            break;
13        case 2:
14            printf("Tuesday\n");
15            break;
16        case 3:
17            printf("Wednesday\n");
18            break;
19        case 4:
20            printf("Thursday\n");
21            break;
22        case 5:
23            printf("Friday\n");
24            break;
25        case 6:
26            printf("Saturday\n");
27            break;
28        case 7:
29            printf("Sunday\n");
30            break;
31        default:
32            printf("Invalid day (use 1-7)\n");
33    }
34    
35    return 0;
36}

Output

Day number: 3

Day name: Wednesday

Don't Forget break!

Without break, execution falls through to the next case. This is sometimes intentional, but usually a bug.

Intentional Fall-Through

Sometimes fall-through is useful to group multiple cases with the same code:

▶ Try it: Uses fall-through to group weekdays and weekends.

switch_fallthrough.c

1#include <stdio.h>
2
3int main() {
4    int day = 6;
5    
6    printf("Day %d is a ", day);
7    
8    switch (day) {
9        case 1:
10        case 2:
11        case 3:
12        case 4:
13        case 5:
14            printf("Weekday\n");
15            break;
16        case 6:
17        case 7:
18            printf("Weekend!\n");
19            break;
20        default:
21            printf("Invalid day\n");
22    }
23    
24    return 0;
25}

Output

Day 6 is a Weekend!

11Switch vs If-Else: When to Use Which

Criteria	switch	if-else
Comparison Type	Exact value match only	Any condition (>, <, &&, etc.)
Data Types	int, char (integer types)	Any type (int, float, pointers)
Case Values	Must be constants	Can use variables
Range Checking	Not supported	Supported (x > 10 && x < 20)
Readability	Better for many exact values	Better for complex conditions
Performance	Often optimized to jump table	Evaluated sequentially

✓Use switch for:

• Menu options (1, 2, 3, 4...)
• Day of week (1-7)
• Month selection
• Character commands ('a', 'b', 'c')
• Enum values

✓Use if-else for:

• Range checks (marks >= 90)
• Boolean conditions
• Comparing floats
• Complex expressions
• Variable comparisons

12Jump Statements: break, continue, goto

Jump statements transfer control to another part of the program. They interrupt the normal sequential flow of execution.

Statement	Purpose	Used In	Effect
break	Exit immediately	loops, switch	Stops execution, exits the block
continue	Skip current iteration	loops only	Jumps to next iteration
goto	Jump to label	anywhere in function	Unconditional jump

The break Statement

break immediately exits the innermost loop or switch statement. Execution continues with the statement following the loop/switch.

When to Use break

✓Exit a switch case (required to prevent fall-through)
✓Stop searching when you find what you need
✓Exit an infinite loop when a condition is met
✓Handle error conditions that require early exit

▶ Try it: Searches for a target value and exits the loop immediately when found.

break_search.c

1#include <stdio.h>
2
3int main() {
4    int numbers[] = {10, 25, 33, 47, 56, 68, 72};
5    int size = 7;
6    int target = 47;
7    int found = 0;
8    
9    printf("Searching for %d...\n", target);
10    
11    for (int i = 0; i < size; i++) {
12        printf("Checking index %d: %d\n", i, numbers[i]);
13        
14        if (numbers[i] == target) {
15            printf(">>> Found %d at index %d!\n", target, i);
16            found = 1;
17            break;  // Exit loop immediately - no need to check rest
18        }
19    }
20    
21    if (!found) {
22        printf("Value not found.\n");
23    }
24    
25    printf("Search complete.\n");
26    return 0;
27}

Output

Searching for 47...

Checking index 0: 10

Checking index 1: 25

Checking index 2: 33

Checking index 3: 47

>>> Found 47 at index 3!

Search complete.

The continue Statement

continue skips the rest of the current iteration and jumps to the next iteration of the loop. The loop itself continues running.

When to Use continue

✓Skip invalid or unwanted data
✓Process only items matching criteria (filter)
✓Avoid deep nesting by handling edge cases early

break vs continue

break

Loop: 1 2 3 [break] done

Exits loop completely

continue

Loop: 1 2 [skip] 4 5 done

Skips to next iteration

▶ Try it: Prints only positive numbers from an array, skipping negative ones.

continue_filter.c

1#include <stdio.h>
2
3int main() {
4    int numbers[] = {5, -3, 10, -7, 8, -2, 15};
5    int size = 7;
6    int sum = 0;
7    
8    printf("Processing numbers (skipping negatives):\n");
9    
10    for (int i = 0; i < size; i++) {
11        // Skip negative numbers
12        if (numbers[i] < 0) {
13            printf("Skipping %d (negative)\n", numbers[i]);
14            continue;  // Jump to next iteration
15        }
16        
17        // This code only runs for positive numbers
18        printf("Adding %d\n", numbers[i]);
19        sum += numbers[i];
20    }
21    
22    printf("\nSum of positive numbers: %d\n", sum);
23    return 0;
24}

Output

Processing numbers (skipping negatives):

Adding 5

Skipping -3 (negative)

Adding 10

Skipping -7 (negative)

Adding 8

Skipping -2 (negative)

Adding 15

Sum of positive numbers: 38

The goto Statement

goto performs an unconditional jump to a labeled statement anywhere within the same function.

Syntax

goto label_name;    // Jump to label

// ... other code ...

label_name:         // Label definition (ends with :)
    // Code continues here

Why goto is Controversial

goto is considered bad practice by most programmers because:

• Creates "spaghetti code" that's hard to read and maintain
• Makes program flow unpredictable
• Can be replaced by structured constructs (loops, functions)
• Makes debugging difficult

✓Acceptable Uses of goto

There are a few cases where goto can make code cleaner:

1. Breaking out of deeply nested loops - When you need to exit multiple loops at once
2. Centralized cleanup code - Jumping to cleanup/error handling at end of function
3. State machines - Some state machine implementations

Acceptable use: Breaking out of nested loops to find a target in a 2D grid.

goto_nested.c

1#include <stdio.h>
2
3int main() {
4    int grid[3][3] = {
5        {1, 2, 3},
6        {4, 5, 6},
7        {7, 8, 9}
8    };
9    int target = 5;
10    int found_row = -1, found_col = -1;
11    
12    printf("Searching for %d in 3x3 grid...\n\n", target);
13    
14    // Nested loops - break would only exit inner loop
15    for (int i = 0; i < 3; i++) {
16        for (int j = 0; j < 3; j++) {
17            printf("Checking [%d][%d] = %d\n", i, j, grid[i][j]);
18            
19            if (grid[i][j] == target) {
20                found_row = i;
21                found_col = j;
22                goto found;  // Exit BOTH loops at once
23            }
24        }
25    }
26    
27    // Only reaches here if not found
28    printf("\nValue not found in grid.\n");
29    goto end;
30    
31found:
32    printf("\n>>> Found %d at position [%d][%d]!\n", target, found_row, found_col);
33    
34end:
35    printf("Search complete.\n");
36    return 0;
37}

Output

Searching for 5 in 3x3 grid...

Checking [0][0] = 1

Checking [0][1] = 2

Checking [0][2] = 3

Checking [1][0] = 4

Checking [1][1] = 5

>>> Found 5 at position [1][1]!

Search complete.

Another acceptable use: Centralized error handling and resource cleanup.

goto_cleanup.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    FILE *file = NULL;
6    char *buffer = NULL;
7    int result = 0;
8    
9    // Allocate memory
10    buffer = malloc(100);
11    if (buffer == NULL) {
12        printf("Error: Memory allocation failed\n");
13        goto cleanup;  // Jump to cleanup
14    }
15    
16    // Open file
17    file = fopen("data.txt", "r");
18    if (file == NULL) {
19        printf("Error: Cannot open file\n");
20        goto cleanup;  // Jump to cleanup
21    }
22    
23    // Process file...
24    printf("Processing file...\n");
25    result = 1;  // Success
26    
27cleanup:  // Single cleanup point for all exits
28    if (file != NULL) {
29        fclose(file);
30        printf("File closed.\n");
31    }
32    if (buffer != NULL) {
33        free(buffer);
34        printf("Memory freed.\n");
35    }
36    
37    return result ? 0 : 1;
38}

Output (if file doesn't exist)

Error: Cannot open file

Memory freed.

Alternatives to goto

In most cases, you can avoid goto using:

• Flag variables - Use a boolean to break out of nested loops
• Functions - Put nested loops in a function and use return
• break + flag - Set a flag before break, check in outer loop

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Using = instead of == in conditions

This is the most common bug! = assigns a value,== compares values.

main.c

// WRONG - assigns 5 to x, condition is always TRUE (5 != 0)
if (x = 5) {
    printf("This always runs!\n");
}
// CORRECT - compares x to 5
if (x == 5) {
    printf("x is 5\n");
}

Forgetting break in switch

Without break, execution falls through to the next case!

main.c

// WRONG - falls through to next cases
switch (day) {
    case 1:
        printf("Monday\n");
        // Missing break! Falls through...
    case 2:
        printf("Tuesday\n");  // Also prints!
}
// CORRECT
switch (day) {
    case 1:
        printf("Monday\n");
        break;  // Exits switch
    case 2:
        printf("Tuesday\n");
        break;
}

Semicolon after if condition

A semicolon after if() creates an empty statement!

main.c

// WRONG - semicolon makes if do nothing
if (x > 5);  // Empty statement!
{
    printf("This ALWAYS runs!\n");  // Not part of if
}
// CORRECT
if (x > 5) {
    printf("x is greater than 5\n");
}

Dangling else problem

Without braces, else binds to the nearest if.

main.c

// CONFUSING - which if does else belong to?
if (a > 0)
    if (b > 0)
        printf("a and b positive\n");
else
    printf("This runs when b <= 0, NOT when a <= 0!\n");
// CLEAR - use braces to show intent
if (a > 0) {
    if (b > 0) {
        printf("a and b positive\n");
    }
} else {
    printf("a is not positive\n");
}

Using switch with non-constant cases

Switch case values must be compile-time constants, not variables.

main.c

int option = 2;
// WRONG - case values must be constants
switch (choice) {
    case option:  // ERROR! Variables not allowed
        break;
}
// CORRECT - use constant values
switch (choice) {
    case 2:  // Literal constant - OK
        break;
    case MAX_VALUE:  // #define constant - OK
        break;
}

Wrong order in else-if ladder

More specific conditions should come first; otherwise, they're never reached.

main.c

int score = 95;
// WRONG - first condition catches everything >= 60
if (score >= 60) {
    printf("D\n");  // 95 matches here!
} else if (score >= 70) {
    printf("C\n");  // Never reached for 95
} else if (score >= 90) {
    printf("A\n");  // Never reached
}
// CORRECT - most specific first
if (score >= 90) {
    printf("A\n");  // 95 matches correctly
} else if (score >= 80) {
    printf("B\n");
} else if (score >= 60) {
    printf("D\n");
}

Using floats in switch

Switch only works with integer types (int, char, enum). Use if-else for floats.

main.c

float grade = 3.5;
// WRONG - float not allowed in switch
switch (grade) {  // ERROR!
    case 4.0:
        break;
}
// CORRECT - use if-else for floats
if (grade >= 4.0) {
    printf("A\n");
} else if (grade >= 3.0) {
    printf("B\n");
}

Watch Out When Copying Code!

Why copied Conditionals Need Your Review!

When you ask someone to write if-else or switch statements, it often generates code that looks correct but has subtle logical flaws:

1. Incomplete Edge Cases

Copied code might write a grade calculator that handles A, B, C, D but forgets what happens when score is negative or above 100.

main.c

// copied - looks fine but incomplete
if (score >= 90) printf("A");
else if (score >= 80) printf("B");
// What if score = -5 or score = 150?

2. Missing Break in Switch

Beginners sometimes generates switch statements with intentional fall-through but doesn't comment it, or accidentally forgets break statements.

3. Condition Order Bugs

Copied code might place score >= 60 before score >= 90, meaning a score of 95 would match the first condition (60+) and never reach the A grade check.

Your Job: Always trace through copied conditionals with extreme values (0, -1, max, boundary values) to catch logic errors that "look right" but aren't.

15Frequently Asked Questions

Q:When should I use switch instead of if-else?

Q:Why do I need braces if there's only one statement after if?

Q:What's the difference between = and == again?

Q:What happens if I forget break in a switch case?

Q:Can I use strings in a switch statement?

Q:What does the ternary operator ?: do?

Q:Why does if(x) work when x is a number, not a boolean?

16Summary

Key Takeaways

•TRUE/FALSE: 0 = false, any non-zero = true
•Comparison: ==, !=, >, <, >=, <= (return 1 or 0)
•Logical: && (AND), || (OR), ! (NOT)
•if: Execute code only when condition is true
•if-else: Two paths - one for true, one for false
•else-if: Check multiple conditions in order
•Ternary: condition ? valueIfTrue : valueIfFalse
•switch: Match exact values (int/char), always use break!
•break: Exit loop/switch immediately
•continue: Skip to next loop iteration
•= vs ==: Assignment vs comparison - #1 bug source!

Quick Reference

Decision Statements

if (condition) { ... }

if (cond) { ... } else { ... }

switch (var) { case 1: ... break; }

Operators

== != > < >= <= (comparison)

&& || ! (logical)

?: (ternary)

What You Will Learn

01What is Control Flow?

Simple Definition

Real World Example

This Tutorial Covers Decision Making

02Understanding True and False in C

The Golden Rule in C

What Values Are True or False?

Comparison Expressions Return 0 or 1

Common Mistake: = vs ==

03Comparison Operators

Quick Example

Common Mistake: = vs ==

04Logical Operators

&& (AND)

|| (OR)

! (NOT)

Real World Examples

Example: Combining Conditions

05If Statement

How if Works

Syntax

1.Simple Example: Check Temperature

2.Another Example: Check if Positive

06If-Else Statement

How if-else Works

1.Check Even or Odd

2.Check Pass or Fail

3.Check Voting Eligibility

07Else-If Ladder

Syntax

Order Matters!

08Nested If Statements

Simplify with Logical AND

09Ternary Operator (? :)

Syntax

Using if-else (4 lines)

Using ternary (1 line)

Example: Find Maximum

Example: Even/Odd String

Example: Absolute Value

When to Use Ternary

10Switch Statement

Syntax

Don't Forget break!

Intentional Fall-Through

11Switch vs If-Else: When to Use Which

✓Use switch for:

✓Use if-else for:

12Jump Statements: break, continue, goto

The break Statement

When to Use break

The continue Statement

When to Use continue

break vs continue

The goto Statement

Syntax

Why goto is Controversial

✓Acceptable Uses of goto

Alternatives to goto

!Code Pitfalls: Common Mistakes & What to Watch For

Using = instead of == in conditions

Forgetting break in switch

Semicolon after if condition

Dangling else problem

Using switch with non-constant cases

Wrong order in else-if ladder

Using floats in switch

Watch Out When Copying Code!

Why copied Conditionals Need Your Review!

1. Incomplete Edge Cases

2. Missing Break in Switch

3. Condition Order Bugs

15Frequently Asked Questions

Q:When should I use switch instead of if-else?

Q:Why do I need braces if there's only one statement after if?

Q:What's the difference between = and == again?

Q:What happens if I forget break in a switch case?

Q:Can I use strings in a switch statement?

Q:What does the ternary operator ?: do?