C Language CheatSheet

"Access a comprehensive cheatsheet for C programming. This downloadable PDF offers concise explanations, examples, and quick references for functions, syntax, and key concepts. Perfect for both beginners and seasoned programmers."

This cheatsheet is designed to help you quickly revise C syntax before exams. Covers basics, control flow, arrays, strings, pointers, functions, and file I/O—the topics most commonly asked in practicals and viva.

1. Quick Start & Compilation

Basic Program

#include <stdio.h>
 
int main(void) {
    printf("Hello, World!\n");
    return 0;
}

Compilation

# Compile and run
gcc program.c -o program
./program
 
# With warnings (recommended)
gcc -Wall program.c -o program

2. Data Types & Variables

Primitive Types

char c = 'A';          // 1 byte
int i = 42;            // Typically 4 bytes
float f = 3.14f;       // 4 bytes
double d = 3.14159;    // 8 bytes

Size and Ranges

printf("Size of int: %zu\n", sizeof(int));

3. Input / Output

Output with `printf`

printf("Integer: %d\n", 10);
printf("Float: %.2f\n", 3.14);
printf("Char: %c\n", 'A');
printf("String: %s\n", "Hello");
// This is a single line comment
/* This is a 
   multi-line comment */

Input with `scanf`

int num;
scanf("%d", &num);
 
char name[50];
scanf("%49s", name);   // Avoid buffer overflow

gets and puts

char str[100];
gets(str);
puts(str);
// NOTE: gets() is unsafe. Use fgets(str, sizeof(str), stdin) instead.

4. Control Flow

if-else

if (a > b) {
    printf("a is greater");
} else {
    printf("b is greater");
}

switch

switch (ch) {
    case 1: printf("One"); break;
    case 2: printf("Two"); break;
    default: printf("Other");
}

Loops

// for loop
for (int i = 0; i < 5; i++) printf("%d ", i);
 
// while loop
int i = 0;
while (i < 5) i++;
 
// do-while loop
int j = 0;
do { j++; } while (j < 5);

5. Arrays

int arr[5] = {1, 2, 3, 4, 5};
 
// Array size
size_t size = sizeof(arr) / sizeof(arr[0]);
 
// Traversal
for (size_t i = 0; i < size; i++)
    printf("%d ", arr[i]);

6. Strings

#include <string.h>
 
char str1[20] = "Hello";
char str2[] = "World";
 
printf("Length: %zu\n", strlen(str1));
strcpy(str1, "Hi");          // Copy
strcat(str1, str2);          // Concatenate
if (strcmp(str1, str2) == 0) // Compare
    printf("Equal");

String Functions

Function	Description	Usage Example
`strlen`	Get string length	`size_t len = strlen(str);`
`strcpy`	Copy string	`strcpy(dest, src);`
`strncpy`	Copy n chars	`strncpy(dest, src, n);`
`strcat`	Concatenate strings	`strcat(dest, src);`
`strncat`	Concatenate n chars	`strncat(dest, src, n);`
`strcmp`	Compare strings	`strcmp(str1, str2);`
`strncmp`	Compare n chars	`strncmp(str1, str2, n);`
`strchr`	Find char in string	`strchr(str, 'a');`
`strrchr`	Find last char in string	`strrchr(str, 'a');`
`strstr`	Find substring	`strstr(str, "sub");`
`strtok`	Tokenize string	`strtok(str, " ,");`

7. Pointers

int x = 10;
int *ptr = &x;
 
printf("Value: %d\n", *ptr); // Dereference
*ptr = 20;                  // Modify value

8. Functions

// Declaration
int add(int a, int b);
 
// Definition
int add(int a, int b) {
    return a + b;
}
 
// Call
int sum = add(5, 10);

Call by Reference (Swap Example)

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

9. Structures

struct Student {
    char name[50];
    int age;
};
 
struct Student s1 = {"John", 20};
printf("%s %d", s1.name, s1.age);

typedef

typedef is used to create an alias for a data type.

typedef struct {
    char name[50];
    int age;
} Student;
 
Student s1 = {"John", 20};
printf("%s %d", s1.name, s1.age);

10. File I/O

#include <stdio.h>
 
FILE *fp = fopen("data.txt", "w");
fprintf(fp, "Hello File\n");
fclose(fp);
 
fp = fopen("data.txt", "r");
char line[100];
while (fgets(line, sizeof(line), fp))
    printf("%s", line);
fclose(fp);

11. Preprocessor Directives

#include <stdio.h>
#define PI 3.14
#define MAX(a,b) ((a) > (b) ? (a) : (b))
 
#ifdef DEBUG
    printf("Debug info\n");
#endif

12. Command-Line Arguments

int main(int argc, char *argv[]) {
    printf("Program: %s\n", argv[0]);
    for (int i = 1; i < argc; i++)
        printf("Arg %d: %s\n", i, argv[i]);
}

13. Memory Management

#include <stdlib.h>
 
int *arr = (int *)malloc(5 * sizeof(int)); // Dynamic allocation using malloc
// int *arr = (int *)calloc(5, sizeof(int)); // Zero-initialized using calloc
 
if (arr == NULL) {
    // Handle allocation failure
}
// int *arr = realloc(arr, 10 * sizeof(int)); // Resize array using realloc
free(arr); // Deallocate memory

14. Quick Reference Tables

Format Specifiers

Type	Specifier
int	%d
unsigned int	%u
float/double	%f
char	%c
string	%s
hex	%x
octal	%o
pointer	%p

Escape Sequences

Sequence	Meaning
`\n`	New line
`\t`	Tab
`\\`	Backslash
`\"`	Double quote
`\'`	Single quote

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