C Cheat Sheet

Learn C from the basics →

Variables & Types

int x = 5; double d = 3.14; char c = 'A';: Basic types
const int MAX = 100;: Constant (cannot be reassigned)
#define MAX 100: Preprocessor constant (text substitution)
unsigned int, long, short: Type modifiers

Control Flow

if (x > 0) { } else { }: Conditional
for (int i = 0; i < 10; i++) { }: Counting loop
while (condition) { }: While loop
switch (x) { case 1: break; default: break; }: Switch statement

Pointers & Arrays

int *p = &x;: Pointer to x
*p = 10;: Dereference — write through the pointer
int arr[5] = {1, 2, 3, 4, 5};: Fixed-size array
int *p = malloc(n * sizeof(int));: Heap allocation
free(p);: Release heap memory

Structs

struct Point { int x; int y; };: Struct definition
struct Point p = {3, 4};: Struct literal
p.x: Access a field
struct Point *ptr = &p; ptr->x: Access a field through a pointer
typedef struct { int x; int y; } Point;: typedef to drop the struct keyword

Strings & I/O

char name[20] = "Ada";: Fixed-size string (null-terminated char array)
printf("%d %s %.2f\n", n, s, f);: Formatted output
scanf("%d", &n);: Formatted input (needs &)
strlen(s); strcpy(dst, src); strcmp(a, b);: Common string.h functions

Functions

int add(int a, int b) { return a + b; }: Function definition
void modify(int *p) { *p = 10; }: Pass by pointer to modify the caller's value
int (*fn)(int, int) = add;: Function pointer