Online C Compiler – Run and Test C Code in Your Browser

Use our free online C compiler to write, compile, and run C code right in your browser. Perfect for C learners and developers — no setup required.

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🚀 2 total executions (2 this month)

💡 Learn C/C++ to upgrade your skills

💡 C Basics Guide for Beginners

1. Declaring Variables and Constants

C requires you to declare the type of each variable. Use #define or const to define read-only values.

int age = 30;
double pi = 3.14159;
char grade = 'A';
char name[] = "Alice";
bool isActive = 1; // true

// Constants
#define MAX_USERS 100
const char* COMPANY = "CodeUtility";

2. Conditionals (if / switch)

Use if, else if, and switch for decision making.

int x = 2;
if (x == 1) {
    printf("One\n");
} else if (x == 2) {
    printf("Two\n");
} else {
    printf("Other\n");
}

switch (x) {
    case 1:
        printf("One\n");
        break;
    case 2:
        printf("Two\n");
        break;
    default:
        printf("Other\n");
}

3. Loops

Use for, while, and do-while for repetition.

for (int i = 0; i < 3; i++) {
    printf("%d\n", i);
}

int n = 3;
while (n > 0) {
    printf("%d\n", n);
    n--;
}

4. Arrays

Arrays store multiple elements of the same type.

int numbers[3] = {10, 20, 30};
printf("%d\n", numbers[1]);

5. Structs

struct lets you group related data.

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

struct Person p = {"Alice", 30};
printf("%s is %d years old\n", p.name, p.age);

6. Console Input/Output

Use printf and scanf for console I/O.

char name[50];
printf("Enter your name: ");
scanf("%s", name);
printf("Hello, %s\n", name);

7. Functions

Functions encapsulate reusable logic. Declare return type, name, and parameters.

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

printf("%d\n", add(3, 4));

8. Pointers

Use pointers to store memory addresses and manipulate data indirectly.

int x = 10;
int* ptr = &x;

printf("Value of x: %d\n", x);
printf("Address of x: %p\n", ptr);
printf("Value from pointer: %d\n", *ptr);

*ptr = 20;
printf("Updated x: %d\n", x);

9. File I/O

Use fopen, fprintf, fscanf, and fclose for file operations.

FILE* file = fopen("file.txt", "w");
fprintf(file, "Hello File");
fclose(file);

char line[100];
file = fopen("file.txt", "r");
fgets(line, sizeof(line), file);
printf("%s", line);
fclose(file);

10. String Handling

Use functions from <string.h> like strlen, strcpy, and strcmp.

#include <string.h>

char text[] = "Hello";
char copy[10];

strcpy(copy, text);
printf("Length: %lu\n", strlen(copy));
printf("Compare: %d\n", strcmp(copy, "Hello"));

11. Dynamic Memory

Use malloc and free for heap allocation.

int* nums = (int*) malloc(3 * sizeof(int));
nums[0] = 1; nums[1] = 2; nums[2] = 3;

for (int i = 0; i < 3; i++) {
    printf("%d ", nums[i]);
}

free(nums);