Common Beginner Mistakes in C

?Why Learn About Common Mistakes?

C is a powerful but unforgiving language. Unlike Python or JavaScript, C won't catch your mistakes at runtime — it just does something unexpected.

›

Save Hours

Recognize bugs instantly

›

Prevention

Avoid mistakes before they happen

›

Level Up

Write more professional code

Learning from mistakes is part of programming. Even experienced programmers encounter these issues — the key is recognizing them quickly!

01Common C Programming Mistakes

This guide covers the most common errors beginners encounter and how to avoid them. Each mistake includes wrong vs. correct examples.

Categories of Common Mistakes

Operator Errors

= vs ==, / issues

Array Errors

Bounds, size issues

String Errors

\0, buffer overflow

Pointer Errors

NULL, dangling, &

Initialization

Garbage values

Memory Errors

Leaks, double free

Danger Level

Compile Error

Caught early ✓

Wrong Output

Visible issue

Silent Bug

Works sometimes!

The worst bugs are the ones that "work" — they pass tests but fail in production!

02Using = Instead of ==

The most common C mistake! Using single = (assignment) when you meant double == (comparison).

Wrong

if (x = 5) { // ALWAYS true!

printf("Equal");

}

This ASSIGNS 5 to x, then checks if x (5) is truthy

Correct

if (x == 5) { // Compares x to 5

printf("Equal");

}

This COMPARES x with 5

Pro Tip: Yoda Conditions

Write if (5 == x) instead. If you accidentally use =, the compiler will error because you can't assign to a constant!

03Missing or Extra Semicolons

Missing Semicolon

int x = 10 // Missing ; here!

int y = 20;

Error message points to NEXT line, confusing!

Extra Semicolon After if

if (x > 5); // Empty statement!

{

printf("Always runs!");

}

The block runs regardless of condition!

Extra Semicolon in Loop

for (int i = 0; i < 10; i++); // Loop body is empty!

{

printf("%d", i); // Runs once, i=10

}

04Integer Division Surprise

When you divide two integers, C performs integer division — the decimal part is thrown away!

integer_division.c

1#include <stdio.h>
2
3int main() {
4    // WRONG: Integer division
5    float result1 = 5 / 2;      // = 2.0, NOT 2.5!
6    float result2 = 1 / 3;      // = 0.0, NOT 0.333!
7    
8    printf("5 / 2 = %.2f (wrong!)\n", result1);
9    printf("1 / 3 = %.2f (wrong!)\n", result2);
10    
11    // CORRECT: Cast to float BEFORE dividing
12    float result3 = (float)5 / 2;   // = 2.5
13    float result4 = 5.0 / 2;        // = 2.5
14    float result5 = 1.0 / 3;        // = 0.333...
15    
16    printf("(float)5 / 2 = %.2f (correct!)\n", result3);
17    printf("5.0 / 2 = %.2f (correct!)\n", result4);
18    printf("1.0 / 3 = %.2f (correct!)\n", result5);
19    
20    // Common mistake: calculating average
21    int sum = 17, count = 5;
22    
23    float avg_wrong = sum / count;       // = 3.0 (wrong!)
24    float avg_right = (float)sum / count; // = 3.4 (correct!)
25    
26    printf("Average (wrong): %.2f\n", avg_wrong);
27    printf("Average (right): %.2f\n", avg_right);
28    
29    return 0;
30}

05Array Out of Bounds

C doesn't check array boundaries! Accessing memory outside your array causes undefined behavior — crashes, wrong values, or security vulnerabilities.

array_bounds.c

1#include <stdio.h>
2
3int main() {
4    int arr[5] = {10, 20, 30, 40, 50};
5    
6    // Valid indices: 0, 1, 2, 3, 4
7    printf("arr[0] = %d\n", arr[0]);  // ✓ OK
8    printf("arr[4] = %d\n", arr[4]);  // ✓ OK
9    
10    // DANGER: Out of bounds access
11    // printf("arr[5] = %d\n", arr[5]);   // ✗ Undefined behavior!
12    // printf("arr[-1] = %d\n", arr[-1]); // ✗ Undefined behavior!
13    // arr[10] = 100;                      // ✗ May corrupt memory!
14    
15    // Common mistake: using <= instead of <
16    for (int i = 0; i <= 5; i++) {  // BUG: should be i < 5
17        // arr[5] is out of bounds!
18        printf("%d ", arr[i]);  // Last access is undefined
19    }
20    
21    // CORRECT loop
22    for (int i = 0; i < 5; i++) {  // ✓ Correct: 0, 1, 2, 3, 4
23        printf("%d ", arr[i]);
24    }
25    printf("\n");
26    
27    return 0;
28}

Remember

Array of size N has indices 0 to N-1. Use < not <= in loops!

06Uninitialized Variables

Local variables in C contain garbage values until you initialize them!

uninitialized.c

1#include <stdio.h>
2
3int main() {
4    // WRONG: Uninitialized variable
5    int x;
6    printf("x = %d\n", x);  // Random garbage value!
7    
8    // WRONG: Uninitialized array
9    int arr[5];
10    for (int i = 0; i < 5; i++) {
11        printf("%d ", arr[i]);  // Garbage values!
12    }
13    printf("\n");
14    
15    // CORRECT: Initialize variables
16    int a = 0;
17    int b = 10;
18    
19    // CORRECT: Initialize arrays
20    int arr2[5] = {0};  // All elements = 0
21    int arr3[5] = {1, 2, 3, 4, 5};
22    
23    // CORRECT: Initialize in loop
24    int arr4[5];
25    for (int i = 0; i < 5; i++) {
26        arr4[i] = i * 2;  // Initialize before use
27    }
28    
29    // Sum example - MUST initialize accumulator
30    int numbers[5] = {1, 2, 3, 4, 5};
31    int sum = 0;  // MUST be 0, not uninitialized!
32    
33    for (int i = 0; i < 5; i++) {
34        sum += numbers[i];
35    }
36    printf("Sum = %d\n", sum);  // 15
37    
38    return 0;
39}

07String Mistakes

Using == to Compare Strings

char str1[] = "hello";

char str2[] = "hello";

if (str1 == str2) // Compares ADDRESSES, not content!

✓ Fix: Use strcmp(str1, str2) == 0

Using = to Copy Strings

char dest[20];

dest = "hello"; // ERROR: Can't assign to array!

✓ Fix: Use strcpy(dest, "hello")

Forgetting Null Terminator

char str[5] = {'h', 'e', 'l', 'l', 'o'}; // No '\\0'!

printf("%s", str); // Prints garbage after "hello"

✓ Fix: Use char str[6] = "hello" or add '\0'

Buffer Overflow with gets()

char name[10];

gets(name); // DANGEROUS: No length check!

✓ Fix: Use fgets(name, 10, stdin)

08Pointer Pitfalls

pointer_mistakes.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    // WRONG: Uninitialized pointer
6    int *ptr;
7    // *ptr = 10;  // CRASH: ptr points to random memory!
8    
9    // CORRECT: Initialize pointer
10    int x = 10;
11    int *ptr1 = &x;  // Points to valid memory
12    
13    // WRONG: Using NULL pointer
14    int *ptr2 = NULL;
15    // *ptr2 = 5;  // CRASH: Dereferencing NULL!
16    
17    // CORRECT: Check for NULL
18    if (ptr2 != NULL) {
19        *ptr2 = 5;
20    }
21    
22    // WRONG: Dangling pointer (pointing to freed memory)
23    int *ptr3 = (int*)malloc(sizeof(int));
24    *ptr3 = 42;
25    free(ptr3);
26    // *ptr3 = 10;  // DANGER: Memory already freed!
27    
28    // CORRECT: Set to NULL after free
29    ptr3 = NULL;
30    
31    // WRONG: Memory leak (forgetting to free)
32    int *arr = (int*)malloc(100 * sizeof(int));
33    // ... use arr ...
34    // Forgot free(arr)! Memory leaked!
35    
36    // CORRECT: Always free allocated memory
37    free(arr);
38    arr = NULL;
39    
40    // WRONG: Returning address of local variable
41    // int* bad_function() {
42    //     int local = 10;
43    //     return &local;  // local is destroyed after function!
44    // }
45    
46    return 0;
47}

09Off-by-One Errors

Being off by one in loops or array access is extremely common!

off_by_one.c

1#include <stdio.h>
2#include <string.h>
3
4int main() {
5    // Array indices start at 0, not 1
6    int arr[5] = {10, 20, 30, 40, 50};
7    
8    // WRONG: Starting at 1
9    for (int i = 1; i <= 5; i++) {
10        printf("%d ", arr[i]);  // Misses first, accesses out of bounds
11    }
12    
13    // CORRECT: Start at 0, use <
14    for (int i = 0; i < 5; i++) {
15        printf("%d ", arr[i]);  // 10 20 30 40 50
16    }
17    printf("\n");
18    
19    // String length confusion
20    char str[] = "Hello";
21    int len = strlen(str);  // 5 (doesn't count '\0')
22    
23    // WRONG: Trying to access null terminator position
24    // str[len] is '\0', str[len+1] is out of bounds!
25    
26    // CORRECT: Last character is at index len-1
27    printf("Last char: %c\n", str[len - 1]);  // 'o'
28    
29    // Copying N elements
30    int src[5] = {1, 2, 3, 4, 5};
31    int dest[5];
32    
33    // CORRECT: Copy 5 elements (indices 0-4)
34    for (int i = 0; i < 5; i++) {  // Not <= 5
35        dest[i] = src[i];
36    }
37    
38    return 0;
39}

Remember These Rules

• Arrays: index 0 to (size-1)
• Loop N times: for(i=0; i<N; i++)
• String length: doesn't include '\0'

10Wrong Format Specifiers

Using the wrong format specifier causes undefined behavior or wrong output.

Type	Printf	Scanf
int	%d	%d
float	%f	%f
double	%lf or %f	%lf (must use!)
char	%c	%c
char[]	%s	%s
long	%ld	%ld
pointer	%p	—

format_specifiers.c

1#include <stdio.h>
2
3int main() {
4    int i = 42;
5    float f = 3.14f;
6    double d = 3.14159;
7    char c = 'A';
8    char str[] = "Hello";
9    
10    // CORRECT format specifiers
11    printf("int: %d\n", i);
12    printf("float: %f\n", f);
13    printf("double: %lf\n", d);
14    printf("char: %c\n", c);
15    printf("string: %s\n", str);
16    
17    // WRONG - will produce garbage or crash
18    // printf("%d\n", f);    // int format for float
19    // printf("%s\n", i);    // string format for int
20    
21    // SCANF - MUST use & for non-arrays
22    int num;
23    printf("Enter number: ");
24    scanf("%d", &num);  // &num, not num!
25    
26    // But NOT for strings (already an address)
27    char name[50];
28    printf("Enter name: ");
29    scanf("%s", name);  // name, not &name
30    
31    // For double in scanf, MUST use %lf
32    double price;
33    scanf("%lf", &price);  // %lf, not %f!
34    
35    return 0;
36}

!Code Pitfalls: Common Mistakes & What to Watch For

copied C code often contains the exact mistakes listed on this page, hidden behind correct-looking syntax.

search online to "write a function that returns a string," and it might return a pointer to a local array — a dangling pointer that seems to work in testing but causes crashes or security vulnerabilities in production. copied code frequently forgets to null-terminate strings, uses = instead of == in conditions, and ignores return values.

The Trap: Online sources are excellent at producing code that compiles and "works" for simple test cases. But they lack understanding of undefined behavior, memory safety, and edge cases. The mistakes on this page are the exact pitfalls that copied C code regularly falls into.

The Reality: Always compile copied code with maximum warnings (-Wall -Wextra -Werror). Use static analyzers and memory checkers like Valgrind. Test with edge cases: empty input, maximum values, NULL pointers. Never trust that copied code is correct just because it compiles.

12Frequently Asked Questions

Q:Why does my program crash with no error message?

A: Silent crashes usually mean segmentation fault — accessing invalid memory (NULL pointer, freed memory, array out of bounds). Use a debugger like GDB or add NULL checks to find it.

Q:How do I find memory leaks?

A: Use Valgrind (Linux/Mac) or AddressSanitizer: gcc -fsanitize=address. These tools track every malloc/free and report leaks when your program exits.

Q:Why does my variable have garbage value?

A: Local variables in C are uninitialized by default — they contain whatever was in that memory before. Always initialize variables when declaring: int x = 0;

Q:Why is scanf skipping my input?

A: Leftover newline from previous scanf! After reading a number, the '\n' stays in buffer. Fix: add a space before %c: scanf(" %c", &ch) or usegetchar() to consume the newline.

12Quick Checklist

✓Always Do

• Initialize all variables
• Use == for comparison
• Check array bounds
• Use & in scanf for non-arrays
• Free allocated memory
• Check pointers for NULL
• Use strcmp() for strings

Never Do

• Use = in if conditions
• Access arr[size] (out of bounds)
• Use gets() function
• Dereference NULL pointers
• Return address of local variable
• Compare strings with ==
• Forget semicolons

Compile with Warnings!

Always compile with gcc -Wall -Wextra to catch many of these mistakes at compile time.

?Why Learn About Common Mistakes?

C is a powerful but unforgiving language. Unlike Python or JavaScript, C won't catch your mistakes at runtime — it just does something unexpected.

›

Save Hours

Recognize bugs instantly

›

Prevention

Avoid mistakes before they happen

›

Level Up

Write more professional code

Learning from mistakes is part of programming. Even experienced programmers encounter these issues — the key is recognizing them quickly!

01Common C Programming Mistakes

This guide covers the most common errors beginners encounter and how to avoid them. Each mistake includes wrong vs. correct examples.

Categories of Common Mistakes

Operator Errors

= vs ==, / issues

Array Errors

Bounds, size issues

String Errors

\0, buffer overflow

Pointer Errors

NULL, dangling, &

Initialization

Garbage values

Memory Errors

Leaks, double free

Danger Level

Compile Error

Caught early ✓

Wrong Output

Visible issue

Silent Bug

Works sometimes!

The worst bugs are the ones that "work" — they pass tests but fail in production!

02Using = Instead of ==

The most common C mistake! Using single = (assignment) when you meant double == (comparison).

Wrong

if (x = 5) { // ALWAYS true!

printf("Equal");

}

This ASSIGNS 5 to x, then checks if x (5) is truthy

Correct

if (x == 5) { // Compares x to 5

printf("Equal");

}

This COMPARES x with 5

Pro Tip: Yoda Conditions

Write if (5 == x) instead. If you accidentally use =, the compiler will error because you can't assign to a constant!

03Missing or Extra Semicolons

Missing Semicolon

int x = 10 // Missing ; here!

int y = 20;

Error message points to NEXT line, confusing!

Extra Semicolon After if

if (x > 5); // Empty statement!

{

printf("Always runs!");

}

The block runs regardless of condition!

Extra Semicolon in Loop

for (int i = 0; i < 10; i++); // Loop body is empty!

{

printf("%d", i); // Runs once, i=10

}

04Integer Division Surprise

When you divide two integers, C performs integer division — the decimal part is thrown away!

integer_division.c

1#include <stdio.h>
2
3int main() {
4    // WRONG: Integer division
5    float result1 = 5 / 2;      // = 2.0, NOT 2.5!
6    float result2 = 1 / 3;      // = 0.0, NOT 0.333!
7    
8    printf("5 / 2 = %.2f (wrong!)\n", result1);
9    printf("1 / 3 = %.2f (wrong!)\n", result2);
10    
11    // CORRECT: Cast to float BEFORE dividing
12    float result3 = (float)5 / 2;   // = 2.5
13    float result4 = 5.0 / 2;        // = 2.5
14    float result5 = 1.0 / 3;        // = 0.333...
15    
16    printf("(float)5 / 2 = %.2f (correct!)\n", result3);
17    printf("5.0 / 2 = %.2f (correct!)\n", result4);
18    printf("1.0 / 3 = %.2f (correct!)\n", result5);
19    
20    // Common mistake: calculating average
21    int sum = 17, count = 5;
22    
23    float avg_wrong = sum / count;       // = 3.0 (wrong!)
24    float avg_right = (float)sum / count; // = 3.4 (correct!)
25    
26    printf("Average (wrong): %.2f\n", avg_wrong);
27    printf("Average (right): %.2f\n", avg_right);
28    
29    return 0;
30}

05Array Out of Bounds

C doesn't check array boundaries! Accessing memory outside your array causes undefined behavior — crashes, wrong values, or security vulnerabilities.

array_bounds.c

1#include <stdio.h>
2
3int main() {
4    int arr[5] = {10, 20, 30, 40, 50};
5    
6    // Valid indices: 0, 1, 2, 3, 4
7    printf("arr[0] = %d\n", arr[0]);  // ✓ OK
8    printf("arr[4] = %d\n", arr[4]);  // ✓ OK
9    
10    // DANGER: Out of bounds access
11    // printf("arr[5] = %d\n", arr[5]);   // ✗ Undefined behavior!
12    // printf("arr[-1] = %d\n", arr[-1]); // ✗ Undefined behavior!
13    // arr[10] = 100;                      // ✗ May corrupt memory!
14    
15    // Common mistake: using <= instead of <
16    for (int i = 0; i <= 5; i++) {  // BUG: should be i < 5
17        // arr[5] is out of bounds!
18        printf("%d ", arr[i]);  // Last access is undefined
19    }
20    
21    // CORRECT loop
22    for (int i = 0; i < 5; i++) {  // ✓ Correct: 0, 1, 2, 3, 4
23        printf("%d ", arr[i]);
24    }
25    printf("\n");
26    
27    return 0;
28}

Remember

Array of size N has indices 0 to N-1. Use < not <= in loops!

06Uninitialized Variables

Local variables in C contain garbage values until you initialize them!

uninitialized.c

1#include <stdio.h>
2
3int main() {
4    // WRONG: Uninitialized variable
5    int x;
6    printf("x = %d\n", x);  // Random garbage value!
7    
8    // WRONG: Uninitialized array
9    int arr[5];
10    for (int i = 0; i < 5; i++) {
11        printf("%d ", arr[i]);  // Garbage values!
12    }
13    printf("\n");
14    
15    // CORRECT: Initialize variables
16    int a = 0;
17    int b = 10;
18    
19    // CORRECT: Initialize arrays
20    int arr2[5] = {0};  // All elements = 0
21    int arr3[5] = {1, 2, 3, 4, 5};
22    
23    // CORRECT: Initialize in loop
24    int arr4[5];
25    for (int i = 0; i < 5; i++) {
26        arr4[i] = i * 2;  // Initialize before use
27    }
28    
29    // Sum example - MUST initialize accumulator
30    int numbers[5] = {1, 2, 3, 4, 5};
31    int sum = 0;  // MUST be 0, not uninitialized!
32    
33    for (int i = 0; i < 5; i++) {
34        sum += numbers[i];
35    }
36    printf("Sum = %d\n", sum);  // 15
37    
38    return 0;
39}

07String Mistakes

Using == to Compare Strings

char str1[] = "hello";

char str2[] = "hello";

if (str1 == str2) // Compares ADDRESSES, not content!

✓ Fix: Use strcmp(str1, str2) == 0

Using = to Copy Strings

char dest[20];

dest = "hello"; // ERROR: Can't assign to array!

✓ Fix: Use strcpy(dest, "hello")

Forgetting Null Terminator

char str[5] = {'h', 'e', 'l', 'l', 'o'}; // No '\\0'!

printf("%s", str); // Prints garbage after "hello"

✓ Fix: Use char str[6] = "hello" or add '\0'

Buffer Overflow with gets()

char name[10];

gets(name); // DANGEROUS: No length check!

✓ Fix: Use fgets(name, 10, stdin)

08Pointer Pitfalls

pointer_mistakes.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    // WRONG: Uninitialized pointer
6    int *ptr;
7    // *ptr = 10;  // CRASH: ptr points to random memory!
8    
9    // CORRECT: Initialize pointer
10    int x = 10;
11    int *ptr1 = &x;  // Points to valid memory
12    
13    // WRONG: Using NULL pointer
14    int *ptr2 = NULL;
15    // *ptr2 = 5;  // CRASH: Dereferencing NULL!
16    
17    // CORRECT: Check for NULL
18    if (ptr2 != NULL) {
19        *ptr2 = 5;
20    }
21    
22    // WRONG: Dangling pointer (pointing to freed memory)
23    int *ptr3 = (int*)malloc(sizeof(int));
24    *ptr3 = 42;
25    free(ptr3);
26    // *ptr3 = 10;  // DANGER: Memory already freed!
27    
28    // CORRECT: Set to NULL after free
29    ptr3 = NULL;
30    
31    // WRONG: Memory leak (forgetting to free)
32    int *arr = (int*)malloc(100 * sizeof(int));
33    // ... use arr ...
34    // Forgot free(arr)! Memory leaked!
35    
36    // CORRECT: Always free allocated memory
37    free(arr);
38    arr = NULL;
39    
40    // WRONG: Returning address of local variable
41    // int* bad_function() {
42    //     int local = 10;
43    //     return &local;  // local is destroyed after function!
44    // }
45    
46    return 0;
47}

09Off-by-One Errors

Being off by one in loops or array access is extremely common!

off_by_one.c

1#include <stdio.h>
2#include <string.h>
3
4int main() {
5    // Array indices start at 0, not 1
6    int arr[5] = {10, 20, 30, 40, 50};
7    
8    // WRONG: Starting at 1
9    for (int i = 1; i <= 5; i++) {
10        printf("%d ", arr[i]);  // Misses first, accesses out of bounds
11    }
12    
13    // CORRECT: Start at 0, use <
14    for (int i = 0; i < 5; i++) {
15        printf("%d ", arr[i]);  // 10 20 30 40 50
16    }
17    printf("\n");
18    
19    // String length confusion
20    char str[] = "Hello";
21    int len = strlen(str);  // 5 (doesn't count '\0')
22    
23    // WRONG: Trying to access null terminator position
24    // str[len] is '\0', str[len+1] is out of bounds!
25    
26    // CORRECT: Last character is at index len-1
27    printf("Last char: %c\n", str[len - 1]);  // 'o'
28    
29    // Copying N elements
30    int src[5] = {1, 2, 3, 4, 5};
31    int dest[5];
32    
33    // CORRECT: Copy 5 elements (indices 0-4)
34    for (int i = 0; i < 5; i++) {  // Not <= 5
35        dest[i] = src[i];
36    }
37    
38    return 0;
39}

Remember These Rules

• Arrays: index 0 to (size-1)
• Loop N times: for(i=0; i<N; i++)
• String length: doesn't include '\0'

10Wrong Format Specifiers

Using the wrong format specifier causes undefined behavior or wrong output.

Type	Printf	Scanf
int	%d	%d
float	%f	%f
double	%lf or %f	%lf (must use!)
char	%c	%c
char[]	%s	%s
long	%ld	%ld
pointer	%p	—

format_specifiers.c

1#include <stdio.h>
2
3int main() {
4    int i = 42;
5    float f = 3.14f;
6    double d = 3.14159;
7    char c = 'A';
8    char str[] = "Hello";
9    
10    // CORRECT format specifiers
11    printf("int: %d\n", i);
12    printf("float: %f\n", f);
13    printf("double: %lf\n", d);
14    printf("char: %c\n", c);
15    printf("string: %s\n", str);
16    
17    // WRONG - will produce garbage or crash
18    // printf("%d\n", f);    // int format for float
19    // printf("%s\n", i);    // string format for int
20    
21    // SCANF - MUST use & for non-arrays
22    int num;
23    printf("Enter number: ");
24    scanf("%d", &num);  // &num, not num!
25    
26    // But NOT for strings (already an address)
27    char name[50];
28    printf("Enter name: ");
29    scanf("%s", name);  // name, not &name
30    
31    // For double in scanf, MUST use %lf
32    double price;
33    scanf("%lf", &price);  // %lf, not %f!
34    
35    return 0;
36}

!Code Pitfalls: Common Mistakes & What to Watch For

copied C code often contains the exact mistakes listed on this page, hidden behind correct-looking syntax.

12Frequently Asked Questions

Q:Why does my program crash with no error message?

A: Silent crashes usually mean segmentation fault — accessing invalid memory (NULL pointer, freed memory, array out of bounds). Use a debugger like GDB or add NULL checks to find it.

Q:How do I find memory leaks?

A: Use Valgrind (Linux/Mac) or AddressSanitizer: gcc -fsanitize=address. These tools track every malloc/free and report leaks when your program exits.

Q:Why does my variable have garbage value?

A: Local variables in C are uninitialized by default — they contain whatever was in that memory before. Always initialize variables when declaring: int x = 0;

Q:Why is scanf skipping my input?

A: Leftover newline from previous scanf! After reading a number, the '\n' stays in buffer. Fix: add a space before %c: scanf(" %c", &ch) or usegetchar() to consume the newline.

12Quick Checklist

✓Always Do

• Initialize all variables
• Use == for comparison
• Check array bounds
• Use & in scanf for non-arrays
• Free allocated memory
• Check pointers for NULL
• Use strcmp() for strings

Never Do

• Use = in if conditions
• Access arr[size] (out of bounds)
• Use gets() function
• Dereference NULL pointers
• Return address of local variable
• Compare strings with ==
• Forget semicolons

Compile with Warnings!

Always compile with gcc -Wall -Wextra to catch many of these mistakes at compile time.

What You Will Learn

?Why Learn About Common Mistakes?

01Common C Programming Mistakes

Categories of Common Mistakes

Danger Level

02Using = Instead of ==

Wrong

Correct

Pro Tip: Yoda Conditions

03Missing or Extra Semicolons

Missing Semicolon

Extra Semicolon After if

Extra Semicolon in Loop

04Integer Division Surprise

05Array Out of Bounds

Remember

06Uninitialized Variables

07String Mistakes

Using == to Compare Strings

Using = to Copy Strings

Forgetting Null Terminator

Buffer Overflow with gets()

08Pointer Pitfalls

09Off-by-One Errors

Remember These Rules

10Wrong Format Specifiers

!Code Pitfalls: Common Mistakes & What to Watch For

copied C code often contains the exact mistakes listed on this page, hidden behind correct-looking syntax.

12Frequently Asked Questions

Q:Why does my program crash with no error message?

Q:How do I find memory leaks?

Q:Why does my variable have garbage value?

Q:Why is scanf skipping my input?

12Quick Checklist

✓Always Do

Never Do

Compile with Warnings!

Test Your Knowledge

Related Tutorials

Basic Syntax in C

Data Types & Variables

C Operators and Expressions

Have Feedback?

What You Will Learn

?Why Learn About Common Mistakes?

01Common C Programming Mistakes

Categories of Common Mistakes

Danger Level

02Using = Instead of ==

Wrong

Correct

Pro Tip: Yoda Conditions

03Missing or Extra Semicolons

Missing Semicolon

Extra Semicolon After if

Extra Semicolon in Loop

04Integer Division Surprise

05Array Out of Bounds

Remember

06Uninitialized Variables

07String Mistakes

Using == to Compare Strings

Using = to Copy Strings

Forgetting Null Terminator

Buffer Overflow with gets()

08Pointer Pitfalls

09Off-by-One Errors

Remember These Rules

10Wrong Format Specifiers

!Code Pitfalls: Common Mistakes & What to Watch For

copied C code often contains the exact mistakes listed on this page, hidden behind correct-looking syntax.

12Frequently Asked Questions

Q:Why does my program crash with no error message?

Q:How do I find memory leaks?

Q:Why does my variable have garbage value?

Q:Why is scanf skipping my input?

12Quick Checklist

✓Always Do

Never Do

Compile with Warnings!