C Language Cheat Sheet

1. Basic Syntax

Structure of a C Program:

#include 

int main() {
    // Your code here
    return 0;
}

2. Data Types

Type	Description	Example
`int`	Integer type	`int a = 10;`
`float`	Floating point number	`float f = 3.14;`
`double`	Double precision floating point	`double d = 9.81;`
`char`	Character type	`char c = 'A';`
`void`	No return type	`void`

3. Input/Output

Print to Console:

printf("Hello, World!\n");

Scan Input:

int age;
scanf("%d", &age);

4. Operators

Operator	Description	Example
`+`	Addition	`a + b`
`-`	Subtraction	`a - b`
`*`	Multiplication	`a * b`
`/`	Division	`a / b`
`%`	Modulus	`a % b`
`++`	Increment	`a++`
`--`	Decrement	`a--`

5. Control Structures

Conditional Statements

if-else Statement:

if (condition) {
    // code
} else {
    // code
}

switch Statement:

switch (expression) {
    case 1:
        // code
        break;
    case 2:
        // code
        break;
    default:
        // code
}

Loops

for Loop:

for (int i = 0; i < 10; i++) {
    // code
}

while Loop:

while (condition) {
    // code
}

do-while Loop:

do {
    // code
} while (condition);

6. Functions

Function Declaration & Definition:

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

Function Call:

int result = add(10, 20);

Passing by Reference:

void updateValue(int *x) {
    *x = 100;
}

7. Arrays

One-dimensional Array:

int arr[5] = {1, 2, 3, 4, 5};

Multi-dimensional Array:

int matrix[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

8. Pointers

Declaration:

int *ptr;

Pointer Initialization:

int a = 10;
int *ptr = &a;

Dereferencing a Pointer:

int value = *ptr; // value is 10

9. Strings

String Declaration:

char str[] = "Hello";
String Functions (from ):

strlen(str): Get length of a string.
strcpy(dest, src): Copy a string.
strcat(str1, str2): Concatenate strings.
strcmp(str1, str2): Compare two strings.

10. Structs

Define a Structure:

struct Person {
    char name[50];
    int age;
};

Accessing Struct Members:

struct Person person;
strcpy(person.name, "John");
person.age = 25;

11. Dynamic Memory Allocation

Memory Allocation (using ):

malloc(): Allocate memory.
free(): Deallocate memory.

int *ptr = (int*)malloc(sizeof(int) * 5); // Allocate memory for 5 integers
free(ptr); // Free allocated memory

12. File I/O

Opening a File:

FILE *fptr;
fptr = fopen("file.txt", "r");

Writing to a File:

FILE *fptr = fopen("file.txt", "w");
fprintf(fptr, "Writing to a file");
fclose(fptr);

Reading from a File:

char buffer[100];
FILE *fptr = fopen("file.txt", "r");
fgets(buffer, 100, fptr);
fclose(fptr);

13. Preprocessor Directives

Include Header Files:

#include

Macros:

#define PI 3.14

Conditional Compilation:

#ifdef DEBUG
    // code to compile if DEBUG is defined
#endif

14. Commonly Used Library Functions

Function	Description	Library
`printf()`	Prints to the console	`<stdio.h>`
`scanf()`	Reads input from the user	`<stdio.h>`
`malloc()`	Allocates dynamic memory	`<stdlib.h>`
`free()`	Frees allocated memory	`<stdlib.h>`
`strlen()`	Gets the length of a string	`<string.h>`
`strcpy()`	Copies one string to another	`<string.h>`
`strcmp()`	Compares two strings	`<string.h>`
`fopen()`	Opens a file	`<stdio.h>`
`fclose()`	Closes a file	`<stdio.h>`
`fgets()`	Reads a line from a file	`<stdio.h>`

15. Common Escape Sequences

Escape Sequence	Description
`\n`	Newline
`\t`	Tab
`\\`	Backslash
`\"`	Double Quote
`\'`	Single Quote

16. Comments

Single-line comment:

// This is a comment

Multi-line comment:

/* This is a 
   multi-line comment */

17. Example Program

A simple program to find the sum of two numbers:

#include 

int main() {
    int num1, num2, sum;

    // Input two numbers
    printf("Enter two numbers: ");
    scanf("%d %d", &num1, &num2);

    // Calculate the sum
    sum = num1 + num2;

    // Output the result
    printf("The sum is: %d\n", sum);
    
    return 0;
}

This cheat sheet provides a quick reference to basic concepts and syntax in the C programming language.

cheatsheet