C Programming Cheat Sheet

Basic Syntax

Concept	Syntax	Example
Include Header	#include <header>	#include <stdio.h>
Main Function	int main() { ... }	int main() { return 0; }
Print Statement	printf("text");	printf("Hello World");
Input Statement	scanf("%d", &var);	scanf("%d", &num);
Comments	// single line /* multi */	// This is comment

Data Types

Type	Size	Range	Example
int	4 bytes	-2,147,483,648 to 2,147,483,647	int age = 25;
char	1 byte	-128 to 127	char grade = 'A';
float	4 bytes	6 decimal precision	float price = 19.99;
double	8 bytes	15 decimal precision	double pi = 3.14159;
void	0 bytes	No value	void func();

Operators

Category	Operator	Example	Description
Arithmetic	+ - * / %	a + b	Addition, Subtraction, Multiplication, Division, Modulus
Relational	== != < > <= >=	a == b	Comparison operators
Logical	&& \|\| !	a && b	AND, OR, NOT
Assignment	= += -= *= /= %=	a += b	Assignment operators
Bitwise	& \| ^ ~ << >>	a & b	Bitwise operations

Control Structures

Structure	Syntax	Example
If Statement	if(condition) { ... }	if(x > 0) { printf("Positive"); }
If-Else	if(c) { ... } else { ... }	if(x > 0) { printf("Pos"); } else { printf("Neg"); }
Switch	switch(var) { case x: ... break; }	switch(day) { case 1: printf("Mon"); break; }
For Loop	for(init; cond; inc) { ... }	for(i=0; i<10; i++) { printf("%d", i); }
While Loop	while(condition) { ... }	while(x > 0) { x--; }
Do-While	do { ... } while(condition);	do { scanf("%d", &x); } while(x != 0);

Functions

Concept	Syntax	Example
Function Declaration	return_type name(type param);	int add(int a, int b);
Function Definition	return_type name(type param) { ... }	int add(int a, int b) { return a+b; }
Function Call	name(arg1, arg2);	result = add(5, 3);
Recursion	function calls itself	int fact(int n) { if(n<=1) return 1; else return n*fact(n-1); }

Arrays

Concept	Syntax	Example
Array Declaration	type name[size];	int arr[10];
Array Initialization	type name[] = {a, b, c};	int nums[] = {1, 2, 3};
Access Element	name[index]	arr[0] = 5;
2D Array	type name[row][col];	int matrix[3][3];

Pointers

Concept	Syntax	Example
Pointer Declaration	type *name;	int *ptr;
Address Operator	&variable	ptr = &var;
Dereference Operator	*pointer	value = *ptr;
Pointer Arithmetic	ptr++, ptr--, ptr+n	ptr++; // move to next address

Strings

Function	Declaration	Purpose	Example
strlen	int strlen(char *str);	Get string length	len = strlen("hello");
strcpy	char strcpy(char dest, char *src);	Copy string	strcpy(dest, src);
strcat	char strcat(char dest, char *src);	Concatenate strings	strcat(str1, str2);
strcmp	int strcmp(char str1, char str2);	Compare strings	result = strcmp(s1, s2);
gets	char gets(char str);	Read string	gets(buffer);
puts	int puts(char *str);	Print string	puts("Hello");

Memory Management

Function	Declaration	Purpose	Example
malloc	void *malloc(size_t size);	Allocate memory	ptr = malloc(10 * sizeof(int));
calloc	void *calloc(size_t n, size_t size);	Allocate and initialize to 0	ptr = calloc(10, sizeof(int));
realloc	void realloc(void ptr, size_t size);	Resize allocated memory	ptr = realloc(ptr, 20 * sizeof(int));
free	void free(void *ptr);	Free allocated memory	free(ptr);

Preprocessor Directives

Directive	Usage	Example	Description
#include	Include header files	#include <stdio.h>	Includes header files
#define	Define macros	#define PI 3.14159	Defines constants/macros
#ifdef	Conditional compilation	#ifdef DEBUG ... #endif	Compile conditionally
#ifndef	Check if not defined	#ifndef HEADER_H ... #endif	Guard against multiple inclusion
#if #else #endif	Conditional compilation	#if x>5 ... #else ... #endif	Conditional compilation

Structures and Unions

Concept	Syntax	Example
Structure	struct name { ... };	struct Student { int id; char name[20]; };
Structure Variable	struct name var;	struct Student s1;
Access Member	var.member	s1.id = 10;
Union	union name { ... };	union Data { int i; float f; };
Typedef	typedef old new;	typedef struct Student Std;

File Operations

Function	Declaration	Purpose	Example
fopen	FILE fopen(char name, char *mode);	Open file	fp = fopen("file.txt", "r");
fclose	int fclose(FILE *fp);	Close file	fclose(fp);
fread	size_t fread(void ptr, size_t size, size_t n, FILE fp);	Read from file	fread(buffer, 1, 10, fp);
fwrite	size_t fwrite(void ptr, size_t size, size_t n, FILE fp);	Write to file	fwrite(buffer, 1, 10, fp);
fprintf	int fprintf(FILE fp, char format, ...);	Formatted write	fprintf(fp, "%d", num);
fscanf	int fscanf(FILE fp, char format, ...);	Formatted read	fscanf(fp, "%d", &num);