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C Programming Language Cheatsheet

A concise reference for the C programming language, covering syntax, data types, standard library functions, and memory management.

Basic Syntax and Data Types

Program Structure

A C program consists of preprocessor directives, type definitions, function declarations, and function definitions. The main function is the entry point.

 
#include <stdio.h>

int main() {
  printf("Hello, World!\n");
  return 0;
}

Comments can be single-line (//) or multi-line (/* ... */).

Data Types

int

Integer (e.g., 10, -5, 0)

float

Single-precision floating-point number (e.g., 3.14, -2.5)

double

Double-precision floating-point number (e.g., 3.14159265359)

char

Single character (e.g., 'A', '7', '\n')

void

Represents the absence of a type. Used for functions that do not return a value or for generic pointers.

Variables

Variables must be declared before use.

Syntax: data_type variable_name;

Example:

int age;
float salary;
char initial;

Initialization can be done during declaration: int age = 30;

Use const keyword to declare constant variables: const float PI = 3.14159;

Operators and Control Flow

Operators

Arithmetic

+, -, *, /, % (modulus)

Assignment

=, +=, -=, *=, /=, %=

Comparison

==, !=, >, <, >=, <=

Logical

&& (AND), || (OR), ! (NOT)

Bitwise

& (AND), | (OR), ^ (XOR), ~ (NOT), << (left shift), >> (right shift)

Increment/Decrement

++, -- (prefix and postfix)

Control Flow Statements

if-else Statement:

if (condition) {
  // code to execute if condition is true
} else {
  // code to execute if condition is false
}

switch Statement:

switch (expression) {
  case constant1:
    // code to execute if expression == constant1
    break;
  case constant2:
    // code to execute if expression == constant2
    break;
  default:
    // code to execute if no case matches
}

for Loop:

for (initialization; condition; increment) {
  // code to execute repeatedly
}

while Loop:

while (condition) {
  // code to execute repeatedly while condition is true
}

do-while Loop:

do {
  // code to execute at least once
} while (condition);

break and continue:
break exits the loop or switch statement.
continue skips the current iteration of the loop.

Functions and Pointers

Functions

Functions are blocks of code that perform a specific task.

Syntax:

return_type function_name(parameter_list) {
  // function body
  return value;
}

Example:

int add(int a, int b) {
  return a + b;
}

Function prototypes declare the function before its definition.

Example:

int add(int a, int b);

Pointers

Declaration

int *ptr; (pointer to an integer)

Address-of Operator

&variable (returns the address of the variable)

Dereference Operator

*ptr (accesses the value pointed to by ptr)

Example

 
int x = 10;
int *ptr = &x;
printf("%d", *ptr); // Output: 10

Arrays and Pointers

Arrays and pointers are closely related. The name of an array is a pointer to the first element of the array.

Example:

int arr[5] = {1, 2, 3, 4, 5};
int *ptr = arr; // ptr points to arr[0]
printf("%d", *ptr); // Output: 1
printf("%d", arr[0]); // Output: 1

Pointer Arithmetic:

printf("%d", *(ptr + 1)); // Output: 2 (accesses arr[1])

Standard Library and Memory Management

Standard Library Functions (stdio.h)

printf()

Formatted output to the console.

scanf()

Formatted input from the console.

fopen()

Opens a file.

fclose()

Closes a file.

fprintf()

Formatted output to a file.

fscanf()

Formatted input from a file.

Memory Management (stdlib.h)

malloc(): Allocates a block of memory on the heap.

Syntax:

void *malloc(size_t size);

calloc(): Allocates a block of memory and initializes it to zero.

Syntax:

void *calloc(size_t num, size_t size);

realloc(): Resizes a previously allocated block of memory.

Syntax:

void *realloc(void *ptr, size_t size);

free(): Deallocates a block of memory.

Syntax:

void free(void *ptr);

Example:

int *arr = (int *)malloc(5 * sizeof(int));
if (arr == NULL) {
  // Handle memory allocation failure
}
// Use arr
free(arr);
arr = NULL; // Good practice to prevent dangling pointers
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