Loops in C

01Why Do We Need Loops?

Simple Definition

A loop repeats a block of code multiple times. Instead of writing the same code 100 times, you write it once inside a loop!

How a Loop Works

Check condition

↓ TRUE

Run code

↩️ Repeat

↓ FALSE

Exit loop

Without Loop (5 lines!)

printf("Hello\n");
printf("Hello\n");
printf("Hello\n");
printf("Hello\n");
printf("Hello\n");

✓With Loop (3 lines!)

for (int i = 0; i < 5; i++) {
    printf("Hello\n");
}

Three Types of Loops in C

for

When you know how many times

Count from 1 to 10

while

When you don't know how many times

Until user enters 0

do-while

Must run at least once

Show menu, then ask

02Loop Conditions

Every loop depends on a condition that determines when the loop should stop.

Quick Reminder: True and False in C

In C: 0 = FALSE (loop exits) and any non-zero = TRUE (loop continues).

For detailed explanation, see Control Flow Tutorial.

Common Loop Conditions

Expression	Value	Result	Loop Behavior
0	0	FALSE	Loop doesn't run / exits
1	1	TRUE	Loop runs / continues
-5	-5 (non-zero)	TRUE	Loop runs / continues
100	100 (non-zero)	TRUE	Loop runs / continues
5 > 3	1	TRUE	Loop runs / continues
5 < 3	0	FALSE	Loop doesn't run / exits
'\0' (null char)	0	FALSE	Loop doesn't run / exits
NULL pointer	0	FALSE	Loop doesn't run / exits

▶ Try it: Demonstrates how C interprets different values as true or false in loop conditions.

true_false_demo.c

1#include <stdio.h>
2
3int main() {
4    // Demonstrating true/false values
5    printf("Testing loop conditions:\n\n");
6    
7    // while(0) - FALSE, never runs
8    printf("while(0): ");
9    while (0) {
10        printf("This never prints");
11    }
12    printf("Skipped (0 is FALSE)\n");
13    
14    // while(1) runs once with break
15    printf("while(1): ");
16    while (1) {
17        printf("Runs! (1 is TRUE)\n");
18        break;
19    }
20    
21    // Negative numbers are TRUE
22    int x = -5;
23    printf("while(-5): ");
24    while (x) {
25        printf("Runs! (-5 is TRUE, non-zero)\n");
26        break;
27    }
28    
29    // Comparison results
30    printf("\nComparison expressions:\n");
31    printf("5 > 3 evaluates to: %d (TRUE)\n", 5 > 3);
32    printf("5 < 3 evaluates to: %d (FALSE)\n", 5 < 3);
33    printf("5 == 5 evaluates to: %d (TRUE)\n", 5 == 5);
34    
35    return 0;
36}

Output

Testing loop conditions:

while(0): Skipped (0 is FALSE)

while(1): Runs! (1 is TRUE)

while(-5): Runs! (-5 is TRUE, non-zero)

Comparison expressions:

5 > 3 evaluates to: 1 (TRUE)

5 < 3 evaluates to: 0 (FALSE)

5 == 5 evaluates to: 1 (TRUE)

Common Idiom: while(1) for Infinite Loops

Since any non-zero value is true, while(1) creates an infinite loop. Use break to exit when needed.

03While Loop

The while loop checks the condition first. If the condition is false from the start, the loop never runs.

While Loop Syntax

while (condition) {

}

Check condition → If true, run body → Repeat

1.Simple Counting

main.c

#include <stdio.h>
int main() {
    int i = 1;
    
    while (i <= 5) {
        printf("%d ", i);
        i++;  // Don't forget this!
    }
    
    return 0;
}

Output: 1 2 3 4 5

2.Sum Until Zero

main.c

#include <stdio.h>
int main() {
    int num, sum = 0;
    
    printf("Enter numbers (0 to stop):\n");
    scanf("%d", &num);
    
    while (num != 0) {
        sum += num;
        scanf("%d", &num);
    }
    
    printf("Sum: %d\n", sum);
    return 0;
}

Input: 5 10 3 0

Sum: 18

When to Use While

• You don't know how many times to loop
• Loop depends on user input or external data
• Reading until end of file

Common Bug: Infinite Loop!

Always update your loop variable! Forgetting creates an endless loop.

Missing i++

while (i < 5) {

printf("%d", i);

}

✓Correct

while (i < 5) {

printf("%d", i);

i++;

}

04For Loop (The Counter Loop)

The for loop is the most commonly used loop. It puts initialization, condition, and update all in one line, making it perfect for counting.

For Loop Syntax

for (init; condition; update) { ... }

① Init

Run once at start

int i = 0

② Condition

Check before each run

i < 5

③ Update

Run after each loop

i++

1.Count from 1 to 5

main.c

#include <stdio.h>
int main() {
    for (int i = 1; i <= 5; i++) {
        printf("%d ", i);
    }
    return 0;
}

Output: 1 2 3 4 5

2.Count Down from 5 to 1

main.c

#include <stdio.h>
int main() {
    for (int i = 5; i >= 1; i--) {
        printf("%d ", i);
    }
    printf("Blast off!\n");
    return 0;
}

Output: 5 4 3 2 1 Blast off!

3.Skip by 2 (Even Numbers)

main.c

#include <stdio.h>
int main() {
    for (int i = 2; i <= 10; i += 2) {
        printf("%d ", i);
    }
    return 0;
}

Output: 2 4 6 8 10

C99 Feature: Declare Variable in Loop

Since C99, you can declare the loop variable inside the for statement. The variable only exists inside the loop (better scope control).

Old style (C89)

int i;

for (i = 0; ...)

✓Modern (C99+)

for (int i = 0; ...)

Step-by-Step Execution

For: for (int i = 0; i < 3; i++)

Step 1: Init i = 0

Step 2: Check 0 < 3? ✓ TRUE → Run body

Step 3: Update i++ → i = 1

Step 4: Check 1 < 3? ✓ TRUE → Run body

Step 5: Update i++ → i = 2

Step 6: Check 2 < 3? ✓ TRUE → Run body

Step 7: Update i++ → i = 3

Step 8: Check 3 < 3? ✗ FALSE → Exit loop

Execution Order (Step by Step)

1. Initialization: int i = 0

2. Check condition: i < 3 → 0 < 3 = TRUE

3. Execute body: printf(i) → prints 0

4. Update: i++ → i becomes 1

5. Check condition: i < 3 → 1 < 3 = TRUE

... (continues until i = 3, then 3 < 3 = FALSE, loop ends)

When to Use For Loop

✓Use For When	Don't Use When
You know exact number of iterations	Iterations depend on runtime input
Counting from A to B	Waiting for external condition
Iterating through array elements	Complex state-based loops
Nested counting (2D arrays, patterns)	Loop must run at least once

▶ Try it: Demonstrates common for loop patterns: counting up, counting down, and skipping values.

for_patterns.c

1#include <stdio.h>
2
3int main() {
4    // Pattern 1: Count up (0 to 4)
5    printf("Count up: ");
6    for (int i = 0; i < 5; i++) {
7        printf("%d ", i);
8    }
9    printf("\n");
10    
11    // Pattern 2: Count down (5 to 1)
12    printf("Count down: ");
13    for (int i = 5; i >= 1; i--) {
14        printf("%d ", i);
15    }
16    printf("\n");
17    
18    // Pattern 3: Skip values (even numbers)
19    printf("Even numbers: ");
20    for (int i = 0; i <= 10; i += 2) {
21        printf("%d ", i);
22    }
23    printf("\n");
24    
25    // Pattern 4: Multiple variables
26    printf("Two counters: ");
27    for (int i = 0, j = 10; i < j; i++, j--) {
28        printf("(%d,%d) ", i, j);
29    }
30    printf("\n");
31    
32    return 0;
33}

Output

Count up: 0 1 2 3 4

Count down: 5 4 3 2 1

Even numbers: 0 2 4 6 8 10

Two counters: (0,10) (1,9) (2,8) (3,7) (4,6)

05Nested Loops

A nested loop is a loop inside another loop. The inner loop completes all its iterations for each iteration of the outer loop.

Total Iterations = Outer × Inner

If outer loop runs 3 times

and inner loop runs 4 times

Total = 3 × 4 = 12 inner iterations

▶ Try it: Prints a multiplication table using nested loops.

multiplication_table.c

1#include <stdio.h>
2
3int main() {
4    printf("Multiplication Table (1-5):\n\n");
5    
6    // Print header row
7    printf("    ");
8    for (int j = 1; j <= 5; j++) {
9        printf("%4d", j);
10    }
11    printf("\n    --------------------\n");
12    
13    // Outer loop: rows (1 to 5)
14    for (int i = 1; i <= 5; i++) {
15        printf("%d | ", i);
16        
17        // Inner loop: columns (1 to 5)
18        for (int j = 1; j <= 5; j++) {
19            printf("%4d", i * j);
20        }
21        printf("\n");
22    }
23    
24    return 0;
25}

Output

Multiplication Table (1-5):

1 2 3 4 5

--------------------

1 | 1 2 3 4 5

2 | 2 4 6 8 10

3 | 3 6 9 12 15

4 | 4 8 12 16 20

5 | 5 10 15 20 25

▶ Try it: Creates a triangle pattern using nested loops.

triangle_pattern.c

1#include <stdio.h>
2
3int main() {
4    int rows = 5;
5    
6    printf("Right Triangle Pattern:\n");
7    
8    for (int i = 1; i <= rows; i++) {
9        // Inner loop runs 'i' times each row
10        for (int j = 1; j <= i; j++) {
11            printf("* ");
12        }
13        printf("\n");
14    }
15    
16    return 0;
17}

Output

Right Triangle Pattern:

* *

* * *

* * * *

* * * * *

06Do-While Loop (Exit-Controlled)

The do-while loop checks the condition after each iteration. This guarantees the loop body runs at least once, even if the condition is initially false.

Syntax & Flow

do {
    // code runs at least once
} while (condition);  // ← Semicolon!

Flow:

Execute → Check → Repeat

↑ If condition TRUE

When to Use Do-While Loop

✓Use Do-While When	Don't Use When
Loop must execute at least once	You might not need any iterations
Menu systems (show menu first)	Simple counting tasks
Input validation (get input, then check)	Iterating through arrays
Game loops (play, then ask continue?)	Reading files that might be empty

▶ Try it: Validates user input using do-while - keeps asking until valid input is received.

input_validation.c

1#include <stdio.h>
2
3int main() {
4    int age;
5    
6    // Input validation with do-while
7    do {
8        printf("Enter your age (1-120): ");
9        scanf("%d", &age);
10        
11        if (age < 1 || age > 120) {
12            printf("Invalid age! Please try again.\n");
13        }
14    } while (age < 1 || age > 120);
15    
16    printf("\nValid age entered: %d\n", age);
17    
18    if (age >= 18) {
19        printf("You are an adult.\n");
20    } else {
21        printf("You are a minor.\n");
22    }
23    
24    return 0;
25}

Output (Sample Run)

Enter your age (1-120): -5

Invalid age! Please try again.

Enter your age (1-120): 150

Invalid age! Please try again.

Enter your age (1-120): 25

Valid age entered: 25

You are an adult.

While vs Do-While Comparison

while (condition FALSE)

int x = 10;
while (x < 5) {
    printf("Hi");
}
// Prints: NOTHING

do-while (condition FALSE)

int x = 10;
do {
    printf("Hi");
} while (x < 5);
// Prints: Hi (once)

07Break, Continue, and Goto

These jump statements give you control over loop execution flow.

Statement	Effect	Common Use
break	Exit loop immediately	Found what you need, stop searching
continue	Skip to next iteration	Skip unwanted items, keep looping
goto	Jump to labeled statement	Avoid! (breaks out of nested loops)

▶ Try it: Demonstrates break (finding a number) and continue (skipping values).

break_continue.c

1#include <stdio.h>
2
3int main() {
4    // BREAK: Exit loop when target found
5    printf("Finding first multiple of 7 (1-20):\n");
6    for (int i = 1; i <= 20; i++) {
7        if (i % 7 == 0) {
8            printf("Found: %d\n", i);
9            break;  // Stop searching
10        }
11        printf("%d ", i);
12    }
13    
14    // CONTINUE: Skip certain values
15    printf("\nOdd numbers from 1-10:\n");
16    for (int i = 1; i <= 10; i++) {
17        if (i % 2 == 0) {
18            continue;  // Skip even numbers
19        }
20        printf("%d ", i);
21    }
22    printf("\n");
23    
24    return 0;
25}

Output

Finding first multiple of 7 (1-20):

1 2 3 4 5 6 Found: 7

Odd numbers from 1-10:

1 3 5 7 9

About goto

goto jumps to a labeled statement. It's considered bad practice because it creates "spaghetti code" that's hard to follow. See Control Flow Tutorial for more details.

Acceptable use of goto: breaking out of multiple nested loops at once.

goto_nested.c

1#include <stdio.h>
2
3int main() {
4    // Finding coordinates in a 2D search
5    printf("Searching for (2,2) in 4x4 grid:\n");
6    
7    for (int i = 0; i < 4; i++) {
8        for (int j = 0; j < 4; j++) {
9            printf("Checking (%d,%d)\n", i, j);
10            if (i == 2 && j == 2) {
11                printf(">>> FOUND! <<<\n");
12                goto found;  // Exit BOTH loops
13            }
14        }
15    }
16    printf("Not found.\n");
17    goto end;
18    
19    found:
20    printf("Search complete - target located.\n");
21    
22    end:
23    return 0;
24}

Output

Searching for (2,2) in 4x4 grid:

Checking (0,0)

... (continues through grid)

Checking (2,1)

Checking (2,2)

>>> FOUND! <<<

Search complete - target located.

08Infinite Loops

Sometimes you intentionally create a loop that runs forever. Use break to exit when a condition is met.

Common Infinite Loop Patterns

while(1)

Most common

for(;;)

Empty for loop

while(true)

With stdbool.h

▶ Try it: Implements a simple calculator menu using an infinite loop.

calculator.c

1#include <stdio.h>
2
3int main() {
4    int choice, a, b;
5    
6    while (1) {  // Infinite loop
7        printf("\n=== Calculator ===\n");
8        printf("1. Add\n");
9        printf("2. Subtract\n");
10        printf("3. Multiply\n");
11        printf("4. Exit\n");
12        printf("Choice: ");
13        scanf("%d", &choice);
14        
15        if (choice == 4) {
16            printf("Goodbye!\n");
17            break;  // Exit infinite loop
18        }
19        
20        if (choice < 1 || choice > 4) {
21            printf("Invalid choice!\n");
22            continue;  // Skip to next iteration
23        }
24        
25        printf("Enter two numbers: ");
26        scanf("%d %d", &a, &b);
27        
28        switch (choice) {
29            case 1: printf("Result: %d\n", a + b); break;
30            case 2: printf("Result: %d\n", a - b); break;
31            case 3: printf("Result: %d\n", a * b); break;
32        }
33    }
34    
35    return 0;
36}

Output (Sample Run)

=== Calculator ===

1. Add

2. Subtract

3. Multiply

4. Exit

Choice: 1

Enter two numbers: 10 5

Result: 15

... (menu repeats)

09Looping Through Arrays

One of the most common uses of loops is iterating through arrays. Here are the best practices.

1.Basic Array Iteration

main.c

#include <stdio.h>
int main() {
    int numbers[] = {10, 20, 30, 40, 50};
    int size = 5;
    
    for (int i = 0; i < size; i++) {
        printf("numbers[%d] = %d\n", i, numbers[i]);
    }
    
    return 0;
}

numbers[0] = 10

numbers[1] = 20

numbers[2] = 30

...

2.Calculate Array Size with sizeof

main.c

#include <stdio.h>
int main() {
    int arr[] = {1, 2, 3, 4, 5, 6, 7};
    
    // Calculate size automatically
    int size = sizeof(arr) / sizeof(arr[0]);
    
    printf("Array has %d elements:\n", size);
    
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
    
    return 0;
}

Array has 7 elements:

1 2 3 4 5 6 7

sizeof(arr)/sizeof(arr[0]) = total bytes / bytes per element = number of elements

3.Looping Through a String

main.c

#include <stdio.h>
int main() {
    char str[] = "Hello";
    
    // Method 1: Using index
    for (int i = 0; str[i] != '\0'; i++) {
        printf("%c ", str[i]);
    }
    printf("\n");
    
    // Method 2: Using pointer
    for (char *p = str; *p != '\0'; p++) {
        printf("%c ", *p);
    }
    
    return 0;
}

H e l l o

Best Practice: Use size_t for Array Indices

For array sizes and indices, use size_t (from <stddef.h> or <stdlib.h>). It's an unsigned type that can hold any array size.

#include <stddef.h>

size_t size = sizeof(arr) / sizeof(arr[0]);

for (size_t i = 0; i < size; i++) { ... }

10Choosing the Right Loop

Use this decision guide to pick the best loop for your situation:

Loop Selection Flowchart

Q1:Do you know exactly how many times to loop?

Yes →for(best for counting)

No →Continue to Q2

Q2:Must the loop run at least once?

Yes →do-while(menus, input validation)

No →while(may run 0 times)

Scenario	Best Loop	Why
Print numbers 1 to 100	for	Known count, simple increment
Iterate through array	for	Array size is known
Read until end of file	while	Unknown iterations, file may be empty
Sum numbers until user enters 0	while	Unknown how many numbers
Menu system	do-while	Must show menu at least once
Validate user input	do-while	Get input first, then validate
Game loop	while(1) + break	Runs until player quits
2D pattern / matrix	nested for	Rows and columns both known

!Code Pitfalls: Common Mistakes & What to Watch For

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

Infinite loop - forgetting to update loop variable

The most common loop bug! If the condition never becomes false, the loop runs forever.

main.c

// WRONG - i never changes, loops forever!
int i = 0;
while (i < 5) {
    printf("%d\n", i);
    // Missing i++; 
}
// CORRECT
int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++;  // Update the loop variable!
}

Off-by-one error

Looping one too many or one too few times. Check your boundaries!

main.c

int arr[5] = {1, 2, 3, 4, 5};
// WRONG - accesses arr[5] which doesn't exist!
for (int i = 0; i <= 5; i++) {  // Should be i < 5
    printf("%d ", arr[i]);
}
// CORRECT - array indices 0 to 4
for (int i = 0; i < 5; i++) {
    printf("%d ", arr[i]);
}

Semicolon after for/while

A semicolon creates an empty loop body!

main.c

// WRONG - semicolon makes loop body empty
for (int i = 0; i < 5; i++);  // Loop does nothing
{
    printf("%d\n", i);  // Only runs once, i = 5
}
// CORRECT
for (int i = 0; i < 5; i++) {  // No semicolon!
    printf("%d\n", i);
}

Modifying loop variable inside loop

Changing the loop counter inside the loop body causes unpredictable behavior.

main.c

// WRONG - modifying i inside loop
for (int i = 0; i < 10; i++) {
    printf("%d ", i);
    if (i == 5) {
        i = 8;  // Skips 6, 7 - confusing!
    }
}
// BETTER - use continue or break for control
for (int i = 0; i < 10; i++) {
    if (i >= 6 && i <= 7) {
        continue;  // Skip 6 and 7 clearly
    }
    printf("%d ", i);
}

Using = instead of == in condition

Assignment in condition causes unexpected behavior!

main.c

int x = 0;
// WRONG - assigns 1 to x, always true!
while (x = 1) {  // Should be x == 1
    printf("Infinite loop!\n");
}
// CORRECT
while (x == 1) {
    printf("x is 1\n");
}

Wrong loop type for the task

Using while when you need do-while, or for when while is clearer.

main.c

// WRONG - duplicated code to show menu first
printf("Menu...\n");
scanf("%d", &choice);
while (choice != 0) {
    process(choice);
    printf("Menu...\n");  // Duplicated!
    scanf("%d", &choice);
}
// CORRECT - do-while shows menu at least once
do {
    printf("Menu...\n");
    scanf("%d", &choice);
    if (choice != 0) process(choice);
} while (choice != 0);

Missing semicolon after do-while

Unlike for and while, do-while requires a semicolon at the end!

main.c

// WRONG - missing semicolon
do {
    printf("Hello\n");
} while (condition)  // ERROR! Missing ;
// CORRECT
do {
    printf("Hello\n");
} while (condition);  // Semicolon required!

Watch Out When Copying Loop Code!

Loops are where copied code makes its most dangerous mistakes because off-by-one errors can go unnoticed until runtime.

Off-By-One Errors

Copied code often uses <= instead of <, causing array out-of-bounds access.

Infinite Loop Conditions

Watch for conditions that never become false, especially with unsigned integers where i >= 0 is always true.

Missing Loop Updates

In while loops, beginners often place the increment in the wrong place or forget it entirely.

Your Defense: Always trace loop boundaries manually. Ask: "What's the first value? What's the last value? Does the loop stop correctly?"

13Frequently Asked Questions

Q:When should I use for vs while vs do-while?

A: Use for when you know exactly how many times to loop (iterating arrays, counting to N). Use while when you don't know the count — you loop until a condition changes (reading until EOF, waiting for user input). Use do-while when the loop must run at least once (menu systems, input validation where you need to get input before checking it).

Q:Why do loops use i, j, k as variable names?

A: This convention comes from mathematics, where i, j, k are traditionally used for indices and iteration variables. In early Fortran, variables starting with I-N were automatically integers, making i, j, k natural choices. The convention stuck because it's universally understood. For better readability in complex loops, use meaningful names likerow, col, oridx.

Q:What's the difference between break and continue?

A: break exits the entire loop immediately — no more iterations run. continueskips the rest of the current iteration and jumps to the next one. Think of break as "I'm done completely" and continue as "skip this one, try the next." Both only affect the innermost loop they're in.

Q:Why is do-while rarely used?

A: Most loops naturally check conditions before running. Do-while's "run first, check later" pattern is only needed in specific cases: menu systems (show menu at least once), input validation (need to get input before validating), and some algorithms. When you do need it, do-while is perfect. But for 90%+ of loops, for/while are more natural fits.

Q:Can I have multiple variables in a for loop?

A: Yes! Use commas:for (int i = 0, j = 10; i < j; i++, j--). This initializes both i and j, checks one condition, and updates both. It's useful for two-pointer algorithms or parallel counters. However, don't overuse it — if logic gets complex, a while loop with explicit updates is clearer.

Q:Why does my loop run one time too many (or too few)?

A: This is the famous "off-by-one error" — the most common programming bug! Check: Are you using < vs<=? Starting from 0 or 1? For arrays, always usei < size (not <=). Trace manually: if size=5, i goes 0,1,2,3,4 (5 elements) with <.

Q:How do I exit from nested loops?

A: break only exits the innermost loop. For nested loops, options include: (1) Set a flag variable and check it in the outer loop, (2) Extract the nested loop into a function and use return, (3) Use goto (controversial but sometimes cleanest). Most experienced programmers prefer option 2 — functions make code cleaner anyway.

14Summary

Key Takeaways

•for: When you know the count → for (int i = 0; i < n; i++)
•while: When count is unknown → while (condition)
•do-while: Must run at least once → do { ... } while (cond);
•break: Exit loop immediately
•continue: Skip to next iteration
•Arrays: Use sizeof(arr)/sizeof(arr[0]) for size
•C99+: Declare loop var inside for: for (int i = 0; ...)

Quick Reference

for loop

for (int i=0; i<n; i++) {

}

while loop

while (condition) {

}

do-while loop

do {

} while (cond);

01Why Do We Need Loops?

Simple Definition

A loop repeats a block of code multiple times. Instead of writing the same code 100 times, you write it once inside a loop!

How a Loop Works

Check condition

↓ TRUE

Run code

↩️ Repeat

↓ FALSE

Exit loop

Without Loop (5 lines!)

printf("Hello\n");
printf("Hello\n");
printf("Hello\n");
printf("Hello\n");
printf("Hello\n");

✓With Loop (3 lines!)

for (int i = 0; i < 5; i++) {
    printf("Hello\n");
}

Three Types of Loops in C

for

When you know how many times

Count from 1 to 10

while

When you don't know how many times

Until user enters 0

do-while

Must run at least once

Show menu, then ask

02Loop Conditions

Every loop depends on a condition that determines when the loop should stop.

Quick Reminder: True and False in C

In C: 0 = FALSE (loop exits) and any non-zero = TRUE (loop continues).

For detailed explanation, see Control Flow Tutorial.

Common Loop Conditions

Expression	Value	Result	Loop Behavior
0	0	FALSE	Loop doesn't run / exits
1	1	TRUE	Loop runs / continues
-5	-5 (non-zero)	TRUE	Loop runs / continues
100	100 (non-zero)	TRUE	Loop runs / continues
5 > 3	1	TRUE	Loop runs / continues
5 < 3	0	FALSE	Loop doesn't run / exits
'\0' (null char)	0	FALSE	Loop doesn't run / exits
NULL pointer	0	FALSE	Loop doesn't run / exits

▶ Try it: Demonstrates how C interprets different values as true or false in loop conditions.

true_false_demo.c

1#include <stdio.h>
2
3int main() {
4    // Demonstrating true/false values
5    printf("Testing loop conditions:\n\n");
6    
7    // while(0) - FALSE, never runs
8    printf("while(0): ");
9    while (0) {
10        printf("This never prints");
11    }
12    printf("Skipped (0 is FALSE)\n");
13    
14    // while(1) runs once with break
15    printf("while(1): ");
16    while (1) {
17        printf("Runs! (1 is TRUE)\n");
18        break;
19    }
20    
21    // Negative numbers are TRUE
22    int x = -5;
23    printf("while(-5): ");
24    while (x) {
25        printf("Runs! (-5 is TRUE, non-zero)\n");
26        break;
27    }
28    
29    // Comparison results
30    printf("\nComparison expressions:\n");
31    printf("5 > 3 evaluates to: %d (TRUE)\n", 5 > 3);
32    printf("5 < 3 evaluates to: %d (FALSE)\n", 5 < 3);
33    printf("5 == 5 evaluates to: %d (TRUE)\n", 5 == 5);
34    
35    return 0;
36}

Output

Testing loop conditions:

while(0): Skipped (0 is FALSE)

while(1): Runs! (1 is TRUE)

while(-5): Runs! (-5 is TRUE, non-zero)

Comparison expressions:

5 > 3 evaluates to: 1 (TRUE)

5 < 3 evaluates to: 0 (FALSE)

5 == 5 evaluates to: 1 (TRUE)

Common Idiom: while(1) for Infinite Loops

Since any non-zero value is true, while(1) creates an infinite loop. Use break to exit when needed.

03While Loop

The while loop checks the condition first. If the condition is false from the start, the loop never runs.

While Loop Syntax

while (condition) {

}

Check condition → If true, run body → Repeat

1.Simple Counting

main.c

#include <stdio.h>
int main() {
    int i = 1;
    
    while (i <= 5) {
        printf("%d ", i);
        i++;  // Don't forget this!
    }
    
    return 0;
}

Output: 1 2 3 4 5

2.Sum Until Zero

main.c

#include <stdio.h>
int main() {
    int num, sum = 0;
    
    printf("Enter numbers (0 to stop):\n");
    scanf("%d", &num);
    
    while (num != 0) {
        sum += num;
        scanf("%d", &num);
    }
    
    printf("Sum: %d\n", sum);
    return 0;
}

Input: 5 10 3 0

Sum: 18

When to Use While

• You don't know how many times to loop
• Loop depends on user input or external data
• Reading until end of file

Common Bug: Infinite Loop!

Always update your loop variable! Forgetting creates an endless loop.

Missing i++

while (i < 5) {

printf("%d", i);

}

✓Correct

while (i < 5) {

printf("%d", i);

i++;

}

04For Loop (The Counter Loop)

The for loop is the most commonly used loop. It puts initialization, condition, and update all in one line, making it perfect for counting.

For Loop Syntax

for (init; condition; update) { ... }

① Init

Run once at start

int i = 0

② Condition

Check before each run

i < 5

③ Update

Run after each loop

i++

1.Count from 1 to 5

main.c

#include <stdio.h>
int main() {
    for (int i = 1; i <= 5; i++) {
        printf("%d ", i);
    }
    return 0;
}

Output: 1 2 3 4 5

2.Count Down from 5 to 1

main.c

#include <stdio.h>
int main() {
    for (int i = 5; i >= 1; i--) {
        printf("%d ", i);
    }
    printf("Blast off!\n");
    return 0;
}

Output: 5 4 3 2 1 Blast off!

3.Skip by 2 (Even Numbers)

main.c

#include <stdio.h>
int main() {
    for (int i = 2; i <= 10; i += 2) {
        printf("%d ", i);
    }
    return 0;
}

Output: 2 4 6 8 10

C99 Feature: Declare Variable in Loop

Since C99, you can declare the loop variable inside the for statement. The variable only exists inside the loop (better scope control).

Old style (C89)

int i;

for (i = 0; ...)

✓Modern (C99+)

for (int i = 0; ...)

Step-by-Step Execution

For: for (int i = 0; i < 3; i++)

Step 1: Init i = 0

Step 2: Check 0 < 3? ✓ TRUE → Run body

Step 3: Update i++ → i = 1

Step 4: Check 1 < 3? ✓ TRUE → Run body

Step 5: Update i++ → i = 2

Step 6: Check 2 < 3? ✓ TRUE → Run body

Step 7: Update i++ → i = 3

Step 8: Check 3 < 3? ✗ FALSE → Exit loop

Execution Order (Step by Step)

1. Initialization: int i = 0

2. Check condition: i < 3 → 0 < 3 = TRUE

3. Execute body: printf(i) → prints 0

4. Update: i++ → i becomes 1

5. Check condition: i < 3 → 1 < 3 = TRUE

... (continues until i = 3, then 3 < 3 = FALSE, loop ends)

When to Use For Loop

✓Use For When	Don't Use When
You know exact number of iterations	Iterations depend on runtime input
Counting from A to B	Waiting for external condition
Iterating through array elements	Complex state-based loops
Nested counting (2D arrays, patterns)	Loop must run at least once

▶ Try it: Demonstrates common for loop patterns: counting up, counting down, and skipping values.

for_patterns.c

1#include <stdio.h>
2
3int main() {
4    // Pattern 1: Count up (0 to 4)
5    printf("Count up: ");
6    for (int i = 0; i < 5; i++) {
7        printf("%d ", i);
8    }
9    printf("\n");
10    
11    // Pattern 2: Count down (5 to 1)
12    printf("Count down: ");
13    for (int i = 5; i >= 1; i--) {
14        printf("%d ", i);
15    }
16    printf("\n");
17    
18    // Pattern 3: Skip values (even numbers)
19    printf("Even numbers: ");
20    for (int i = 0; i <= 10; i += 2) {
21        printf("%d ", i);
22    }
23    printf("\n");
24    
25    // Pattern 4: Multiple variables
26    printf("Two counters: ");
27    for (int i = 0, j = 10; i < j; i++, j--) {
28        printf("(%d,%d) ", i, j);
29    }
30    printf("\n");
31    
32    return 0;
33}

Output

Count up: 0 1 2 3 4

Count down: 5 4 3 2 1

Even numbers: 0 2 4 6 8 10

Two counters: (0,10) (1,9) (2,8) (3,7) (4,6)

05Nested Loops

A nested loop is a loop inside another loop. The inner loop completes all its iterations for each iteration of the outer loop.

Total Iterations = Outer × Inner

If outer loop runs 3 times

and inner loop runs 4 times

Total = 3 × 4 = 12 inner iterations

▶ Try it: Prints a multiplication table using nested loops.

multiplication_table.c

1#include <stdio.h>
2
3int main() {
4    printf("Multiplication Table (1-5):\n\n");
5    
6    // Print header row
7    printf("    ");
8    for (int j = 1; j <= 5; j++) {
9        printf("%4d", j);
10    }
11    printf("\n    --------------------\n");
12    
13    // Outer loop: rows (1 to 5)
14    for (int i = 1; i <= 5; i++) {
15        printf("%d | ", i);
16        
17        // Inner loop: columns (1 to 5)
18        for (int j = 1; j <= 5; j++) {
19            printf("%4d", i * j);
20        }
21        printf("\n");
22    }
23    
24    return 0;
25}

Output

Multiplication Table (1-5):

1 2 3 4 5

--------------------

1 | 1 2 3 4 5

2 | 2 4 6 8 10

3 | 3 6 9 12 15

4 | 4 8 12 16 20

5 | 5 10 15 20 25

▶ Try it: Creates a triangle pattern using nested loops.

triangle_pattern.c

1#include <stdio.h>
2
3int main() {
4    int rows = 5;
5    
6    printf("Right Triangle Pattern:\n");
7    
8    for (int i = 1; i <= rows; i++) {
9        // Inner loop runs 'i' times each row
10        for (int j = 1; j <= i; j++) {
11            printf("* ");
12        }
13        printf("\n");
14    }
15    
16    return 0;
17}

Output

Right Triangle Pattern:

* *

* * *

* * * *

* * * * *

06Do-While Loop (Exit-Controlled)

The do-while loop checks the condition after each iteration. This guarantees the loop body runs at least once, even if the condition is initially false.

Syntax & Flow

do {
    // code runs at least once
} while (condition);  // ← Semicolon!

Flow:

Execute → Check → Repeat

↑ If condition TRUE

When to Use Do-While Loop

✓Use Do-While When	Don't Use When
Loop must execute at least once	You might not need any iterations
Menu systems (show menu first)	Simple counting tasks
Input validation (get input, then check)	Iterating through arrays
Game loops (play, then ask continue?)	Reading files that might be empty

▶ Try it: Validates user input using do-while - keeps asking until valid input is received.

input_validation.c

1#include <stdio.h>
2
3int main() {
4    int age;
5    
6    // Input validation with do-while
7    do {
8        printf("Enter your age (1-120): ");
9        scanf("%d", &age);
10        
11        if (age < 1 || age > 120) {
12            printf("Invalid age! Please try again.\n");
13        }
14    } while (age < 1 || age > 120);
15    
16    printf("\nValid age entered: %d\n", age);
17    
18    if (age >= 18) {
19        printf("You are an adult.\n");
20    } else {
21        printf("You are a minor.\n");
22    }
23    
24    return 0;
25}

Output (Sample Run)

Enter your age (1-120): -5

Invalid age! Please try again.

Enter your age (1-120): 150

Invalid age! Please try again.

Enter your age (1-120): 25

Valid age entered: 25

You are an adult.

While vs Do-While Comparison

while (condition FALSE)

int x = 10;
while (x < 5) {
    printf("Hi");
}
// Prints: NOTHING

do-while (condition FALSE)

int x = 10;
do {
    printf("Hi");
} while (x < 5);
// Prints: Hi (once)

07Break, Continue, and Goto

These jump statements give you control over loop execution flow.

Statement	Effect	Common Use
break	Exit loop immediately	Found what you need, stop searching
continue	Skip to next iteration	Skip unwanted items, keep looping
goto	Jump to labeled statement	Avoid! (breaks out of nested loops)

▶ Try it: Demonstrates break (finding a number) and continue (skipping values).

break_continue.c

1#include <stdio.h>
2
3int main() {
4    // BREAK: Exit loop when target found
5    printf("Finding first multiple of 7 (1-20):\n");
6    for (int i = 1; i <= 20; i++) {
7        if (i % 7 == 0) {
8            printf("Found: %d\n", i);
9            break;  // Stop searching
10        }
11        printf("%d ", i);
12    }
13    
14    // CONTINUE: Skip certain values
15    printf("\nOdd numbers from 1-10:\n");
16    for (int i = 1; i <= 10; i++) {
17        if (i % 2 == 0) {
18            continue;  // Skip even numbers
19        }
20        printf("%d ", i);
21    }
22    printf("\n");
23    
24    return 0;
25}

Output

Finding first multiple of 7 (1-20):

1 2 3 4 5 6 Found: 7

Odd numbers from 1-10:

1 3 5 7 9

About goto

goto jumps to a labeled statement. It's considered bad practice because it creates "spaghetti code" that's hard to follow. See Control Flow Tutorial for more details.

Acceptable use of goto: breaking out of multiple nested loops at once.

goto_nested.c

1#include <stdio.h>
2
3int main() {
4    // Finding coordinates in a 2D search
5    printf("Searching for (2,2) in 4x4 grid:\n");
6    
7    for (int i = 0; i < 4; i++) {
8        for (int j = 0; j < 4; j++) {
9            printf("Checking (%d,%d)\n", i, j);
10            if (i == 2 && j == 2) {
11                printf(">>> FOUND! <<<\n");
12                goto found;  // Exit BOTH loops
13            }
14        }
15    }
16    printf("Not found.\n");
17    goto end;
18    
19    found:
20    printf("Search complete - target located.\n");
21    
22    end:
23    return 0;
24}

Output

Searching for (2,2) in 4x4 grid:

Checking (0,0)

... (continues through grid)

Checking (2,1)

Checking (2,2)

>>> FOUND! <<<

Search complete - target located.

08Infinite Loops

Sometimes you intentionally create a loop that runs forever. Use break to exit when a condition is met.

Common Infinite Loop Patterns

while(1)

Most common

for(;;)

Empty for loop

while(true)

With stdbool.h

▶ Try it: Implements a simple calculator menu using an infinite loop.

calculator.c

1#include <stdio.h>
2
3int main() {
4    int choice, a, b;
5    
6    while (1) {  // Infinite loop
7        printf("\n=== Calculator ===\n");
8        printf("1. Add\n");
9        printf("2. Subtract\n");
10        printf("3. Multiply\n");
11        printf("4. Exit\n");
12        printf("Choice: ");
13        scanf("%d", &choice);
14        
15        if (choice == 4) {
16            printf("Goodbye!\n");
17            break;  // Exit infinite loop
18        }
19        
20        if (choice < 1 || choice > 4) {
21            printf("Invalid choice!\n");
22            continue;  // Skip to next iteration
23        }
24        
25        printf("Enter two numbers: ");
26        scanf("%d %d", &a, &b);
27        
28        switch (choice) {
29            case 1: printf("Result: %d\n", a + b); break;
30            case 2: printf("Result: %d\n", a - b); break;
31            case 3: printf("Result: %d\n", a * b); break;
32        }
33    }
34    
35    return 0;
36}

Output (Sample Run)

=== Calculator ===

1. Add

2. Subtract

3. Multiply

4. Exit

Choice: 1

Enter two numbers: 10 5

Result: 15

... (menu repeats)

09Looping Through Arrays

One of the most common uses of loops is iterating through arrays. Here are the best practices.

1.Basic Array Iteration

main.c

#include <stdio.h>
int main() {
    int numbers[] = {10, 20, 30, 40, 50};
    int size = 5;
    
    for (int i = 0; i < size; i++) {
        printf("numbers[%d] = %d\n", i, numbers[i]);
    }
    
    return 0;
}

numbers[0] = 10

numbers[1] = 20

numbers[2] = 30

...

2.Calculate Array Size with sizeof

main.c

#include <stdio.h>
int main() {
    int arr[] = {1, 2, 3, 4, 5, 6, 7};
    
    // Calculate size automatically
    int size = sizeof(arr) / sizeof(arr[0]);
    
    printf("Array has %d elements:\n", size);
    
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
    
    return 0;
}

Array has 7 elements:

1 2 3 4 5 6 7

sizeof(arr)/sizeof(arr[0]) = total bytes / bytes per element = number of elements

3.Looping Through a String

main.c

#include <stdio.h>
int main() {
    char str[] = "Hello";
    
    // Method 1: Using index
    for (int i = 0; str[i] != '\0'; i++) {
        printf("%c ", str[i]);
    }
    printf("\n");
    
    // Method 2: Using pointer
    for (char *p = str; *p != '\0'; p++) {
        printf("%c ", *p);
    }
    
    return 0;
}

H e l l o

Best Practice: Use size_t for Array Indices

For array sizes and indices, use size_t (from <stddef.h> or <stdlib.h>). It's an unsigned type that can hold any array size.

#include <stddef.h>

size_t size = sizeof(arr) / sizeof(arr[0]);

for (size_t i = 0; i < size; i++) { ... }

10Choosing the Right Loop

Use this decision guide to pick the best loop for your situation:

Loop Selection Flowchart

Q1:Do you know exactly how many times to loop?

Yes →for(best for counting)

No →Continue to Q2

Q2:Must the loop run at least once?

Yes →do-while(menus, input validation)

No →while(may run 0 times)

Scenario	Best Loop	Why
Print numbers 1 to 100	for	Known count, simple increment
Iterate through array	for	Array size is known
Read until end of file	while	Unknown iterations, file may be empty
Sum numbers until user enters 0	while	Unknown how many numbers
Menu system	do-while	Must show menu at least once
Validate user input	do-while	Get input first, then validate
Game loop	while(1) + break	Runs until player quits
2D pattern / matrix	nested for	Rows and columns both known

!Code Pitfalls: Common Mistakes & What to Watch For

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

Infinite loop - forgetting to update loop variable

The most common loop bug! If the condition never becomes false, the loop runs forever.

main.c

// WRONG - i never changes, loops forever!
int i = 0;
while (i < 5) {
    printf("%d\n", i);
    // Missing i++; 
}
// CORRECT
int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++;  // Update the loop variable!
}

Off-by-one error

Looping one too many or one too few times. Check your boundaries!

main.c

int arr[5] = {1, 2, 3, 4, 5};
// WRONG - accesses arr[5] which doesn't exist!
for (int i = 0; i <= 5; i++) {  // Should be i < 5
    printf("%d ", arr[i]);
}
// CORRECT - array indices 0 to 4
for (int i = 0; i < 5; i++) {
    printf("%d ", arr[i]);
}

Semicolon after for/while

A semicolon creates an empty loop body!

main.c

// WRONG - semicolon makes loop body empty
for (int i = 0; i < 5; i++);  // Loop does nothing
{
    printf("%d\n", i);  // Only runs once, i = 5
}
// CORRECT
for (int i = 0; i < 5; i++) {  // No semicolon!
    printf("%d\n", i);
}

Modifying loop variable inside loop

Changing the loop counter inside the loop body causes unpredictable behavior.

main.c

// WRONG - modifying i inside loop
for (int i = 0; i < 10; i++) {
    printf("%d ", i);
    if (i == 5) {
        i = 8;  // Skips 6, 7 - confusing!
    }
}
// BETTER - use continue or break for control
for (int i = 0; i < 10; i++) {
    if (i >= 6 && i <= 7) {
        continue;  // Skip 6 and 7 clearly
    }
    printf("%d ", i);
}

Using = instead of == in condition

Assignment in condition causes unexpected behavior!

main.c

int x = 0;
// WRONG - assigns 1 to x, always true!
while (x = 1) {  // Should be x == 1
    printf("Infinite loop!\n");
}
// CORRECT
while (x == 1) {
    printf("x is 1\n");
}

Wrong loop type for the task

Using while when you need do-while, or for when while is clearer.

main.c

// WRONG - duplicated code to show menu first
printf("Menu...\n");
scanf("%d", &choice);
while (choice != 0) {
    process(choice);
    printf("Menu...\n");  // Duplicated!
    scanf("%d", &choice);
}
// CORRECT - do-while shows menu at least once
do {
    printf("Menu...\n");
    scanf("%d", &choice);
    if (choice != 0) process(choice);
} while (choice != 0);

Missing semicolon after do-while

Unlike for and while, do-while requires a semicolon at the end!

main.c

// WRONG - missing semicolon
do {
    printf("Hello\n");
} while (condition)  // ERROR! Missing ;
// CORRECT
do {
    printf("Hello\n");
} while (condition);  // Semicolon required!

Watch Out When Copying Loop Code!

Loops are where copied code makes its most dangerous mistakes because off-by-one errors can go unnoticed until runtime.

Off-By-One Errors

Copied code often uses <= instead of <, causing array out-of-bounds access.

Infinite Loop Conditions

Watch for conditions that never become false, especially with unsigned integers where i >= 0 is always true.

Missing Loop Updates

In while loops, beginners often place the increment in the wrong place or forget it entirely.

Your Defense: Always trace loop boundaries manually. Ask: "What's the first value? What's the last value? Does the loop stop correctly?"

13Frequently Asked Questions

Q:When should I use for vs while vs do-while?

Q:Why do loops use i, j, k as variable names?

Q:What's the difference between break and continue?

Q:Why is do-while rarely used?

Q:Can I have multiple variables in a for loop?

Q:Why does my loop run one time too many (or too few)?

Q:How do I exit from nested loops?

14Summary

Key Takeaways

•for: When you know the count → for (int i = 0; i < n; i++)
•while: When count is unknown → while (condition)
•do-while: Must run at least once → do { ... } while (cond);
•break: Exit loop immediately
•continue: Skip to next iteration
•Arrays: Use sizeof(arr)/sizeof(arr[0]) for size
•C99+: Declare loop var inside for: for (int i = 0; ...)

Quick Reference

for loop

for (int i=0; i<n; i++) {

}

while loop

while (condition) {

}

do-while loop

do {

} while (cond);

What You Will Learn

01Why Do We Need Loops?

Simple Definition

How a Loop Works

Without Loop (5 lines!)

✓With Loop (3 lines!)

Three Types of Loops in C

02Loop Conditions

Quick Reminder: True and False in C

Common Loop Conditions

Common Idiom: while(1) for Infinite Loops

03While Loop

While Loop Syntax

1.Simple Counting

2.Sum Until Zero

When to Use While

Common Bug: Infinite Loop!

04For Loop (The Counter Loop)

For Loop Syntax

1.Count from 1 to 5

2.Count Down from 5 to 1

3.Skip by 2 (Even Numbers)

C99 Feature: Declare Variable in Loop

Step-by-Step Execution

Execution Order (Step by Step)

When to Use For Loop

05Nested Loops

Total Iterations = Outer × Inner

06Do-While Loop (Exit-Controlled)

Syntax & Flow

When to Use Do-While Loop

While vs Do-While Comparison

07Break, Continue, and Goto

About goto

08Infinite Loops

Common Infinite Loop Patterns

09Looping Through Arrays

1.Basic Array Iteration

2.Calculate Array Size with sizeof

3.Looping Through a String

Best Practice: Use size_t for Array Indices

10Choosing the Right Loop

Loop Selection Flowchart

!Code Pitfalls: Common Mistakes & What to Watch For

Infinite loop - forgetting to update loop variable

Off-by-one error

Semicolon after for/while

Modifying loop variable inside loop

Using = instead of == in condition

Wrong loop type for the task

Missing semicolon after do-while

Watch Out When Copying Loop Code!

Off-By-One Errors

Infinite Loop Conditions

Missing Loop Updates

13Frequently Asked Questions

Q:When should I use for vs while vs do-while?

Q:Why do loops use i, j, k as variable names?

Q:What's the difference between break and continue?

Q:Why is do-while rarely used?

Q:Can I have multiple variables in a for loop?

Q:Why does my loop run one time too many (or too few)?

Q:How do I exit from nested loops?

14Summary

Key Takeaways

Quick Reference

Test Your Knowledge

Related Tutorials

Control Flow & Decision Making

Functions in C

Arrays in C

Have Feedback?

What You Will Learn

01Why Do We Need Loops?

Simple Definition

How a Loop Works

Without Loop (5 lines!)

✓With Loop (3 lines!)

Three Types of Loops in C

02Loop Conditions