C Language

1. Quick Start & Compilation

Basic Program

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

Compilation

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

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2. Data Types & Variables

Primitive Types

char c = 'A';          // 1 byteint i = 42;            // Typically 4 bytesfloat f = 3.14f;       // 4 bytesdouble d = 3.14159;    // 8 bytes

Size and Ranges

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

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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

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

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5. Arrays

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

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6. Strings

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

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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

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8. Functions

// Declarationint add(int a, int b); // Definitionint add(int a, int b) {    return a + b;} // Callint sum = add(5, 10);

Call by Reference (Swap Example)

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

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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);

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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);

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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

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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]);}

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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 reallocfree(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