Basic C

1. What is C?

C is a general-purpose, compiled programming language developed in the early 1970s. It sits close to the hardware, giving you direct control over memory and system resources.

• Source files use the .c extension; headers use .h
• Code must be compiled before it can run (e.g. gcc main.c -o main)
• Every program starts at the main() function
• All variables must be declared before use, with an explicit type
• Statements end with a semicolon ;

2. Program Structure

#include <stdio.h>   /* standard I/O library */

/* main() is the entry point of every C program */
int main(void) {
    printf("Hello, World!\n");
    return 0;   /* 0 = success */
}

Compile and run:

gcc main.c -o main
./main

3. Variables & Data Types

int    age     = 30;          /* whole number */
float  temp    = 36.6f;        /* single-precision decimal */
double pi      = 3.14159265;  /* double-precision decimal */
char   grade   = 'A';          /* single character */
int    active  = 1;            /* C has no bool: 0=false, 1=true */

/* Constants — value cannot change */
const int MAX = 100;
#define PI 3.14159            /* preprocessor constant */

4. Operators

Arithmetic

int a = 10, b = 3;
a + b    /* 13 */
a - b    /* 7  */
a * b    /* 30 */
a / b    /* 3  — integer division truncates */
a % b    /* 1  — modulus (remainder) */
a++      /* post-increment: use then add 1 */
++a      /* pre-increment: add 1 then use */

Comparison & Logical

a == b   /* equal */
a != b   /* not equal */
a <  b   /* less than */
a >  b   /* greater than */

a && b   /* logical AND */
a || b   /* logical OR  */
!a        /* logical NOT */

Assignment Shortcuts

x += 5;   /* x = x + 5 */
x -= 2;   /* x = x - 2 */
x *= 3;   /* x = x * 3 */
x /= 4;   /* x = x / 4 */

5. Input & Output

#include <stdio.h>

int main(void) {
    int age;
    char name[50];

    /* printf — formatted output */
    printf("Enter your name: ");

    /* scanf — formatted input */
    scanf("%s", name);
    scanf("%d", &age);    /* & gives scanf the address to write to */

    printf("Hello, %s! You are %d years old.\n", name, age);
    return 0;
}

6. Control Structures

if / else if / else

int score = 75;

if (score >= 90) {
    printf("A\n");
} else if (score >= 75) {
    printf("B\n");
} else {
    printf("C or below\n");
}

switch

int day = 2;

switch (day) {
    case 1:  printf("Monday\n");    break;
    case 2:  printf("Tuesday\n");   break;
    case 5:  printf("Friday\n");    break;
    default: printf("Other day\n"); break;
}

Ternary Operator

int max = (a > b) ? a : b;   /* condition ? if_true : if_false */

7. Loops

for

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}
/* 0 1 2 3 4 */

while

int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++;
}

do / while

int i = 0;
do {
    printf("%d\n", i);
    i++;
} while (i < 5);   /* always runs at least once */

8. Functions

#include <stdio.h>

/* Function declaration (prototype) — tells compiler about the function */
int add(int a, int b);

int main(void) {
    printf("%d\n", add(3, 4));   /* 7 */
    return 0;
}

/* Function definition */
int add(int a, int b) {
    return a + b;
}

/* void function — no return value */
void greet(char *name) {
    printf("Hello, %s!\n", name);
}

9. Arrays & Strings

Arrays

int nums[5] = {10, 20, 30, 40, 50};
nums[0];                   /* 10 — zero-indexed */
nums[4];                   /* 50 */
nums[2] = 99;              /* update value */

/* Iterate with a for loop */
for (int i = 0; i < 5; i++) {
    printf("%d\n", nums[i]);
}

/* 2D array */
int grid[2][3] = {{1,2,3}, {4,5,6}};
grid[1][2];                /* 6 */

Strings

#include <string.h>

/* A string is a char array ending with '\0' */
char name[20] = "Alice";
char greeting[] = "Hello";   /* size inferred */

strlen(name);               /* 5 — length (not counting '\0') */
strcpy(dest, src);          /* copy src into dest */
strcat(dest, src);          /* append src to dest */
strcmp(a, b);               /* 0 if equal, <0 or >0 otherwise */
strncpy(dest, src, 10);     /* safer: copy at most n chars */

10. Pointers

A pointer stores the memory address of another variable.

int  x   = 42;
int *ptr = &x;    /* ptr holds the address of x */

printf("%p\n", ptr);    /* prints the address */
printf("%d\n", *ptr);   /* 42 — dereference: read the value at the address */

*ptr = 99;              /* change x through the pointer */
printf("%d\n", x);      /* 99 */

/* Passing a pointer to a function allows it to modify the original */
void double_it(int *n) {
    *n = *n * 2;
}
double_it(&x);          /* x is now 198 */

/* Array name is a pointer to its first element */
int arr[] = {1, 2, 3};
int *p = arr;
printf("%d\n", *(p + 1));  /* 2 — pointer arithmetic */

11. Structs

A struct groups related variables of different types under one name.

#include <stdio.h>
#include <string.h>

/* Define the struct type */
struct Person {
    char  name[50];
    int   age;
    float height;
};

int main(void) {
    /* Declare and initialise */
    struct Person p1;
    strcpy(p1.name, "Alice");
    p1.age    = 30;
    p1.height = 1.68f;

    printf("%s is %d years old.\n", p1.name, p1.age);

    /* Pointer to struct — use -> to access members */
    struct Person *ptr = &p1;
    printf("%.2f\n", ptr->height);

    return 0;
}

/* typedef makes the syntax cleaner */
typedef struct {
    char name[50];
    int  age;
} Person;

Person p2 = {"Bob", 25};   /* no 'struct' keyword needed */

12. Dynamic Memory

Use <stdlib.h> to allocate memory on the heap at runtime.

#include <stdlib.h>
#include <stdio.h>

int main(void) {
    int n = 5;

    /* malloc — allocate n ints, uninitialised */
    int *arr = (int *)malloc(n * sizeof(int));

    if (arr == NULL) {
        fprintf(stderr, "Allocation failed\n");
        return 1;
    }

    for (int i = 0; i < n; i++)
        arr[i] = i * i;

    /* realloc — resize the allocation */
    arr = (int *)realloc(arr, 10 * sizeof(int));

    /* calloc — allocate and zero-initialise */
    int *zeroed = (int *)calloc(n, sizeof(int));

    /* free — ALWAYS free when done */
    free(arr);
    free(zeroed);

    return 0;
}