C Functions Declaration, Recursion, Pointers: Tutorial, Examples, FAQs & Interview Tips

What is a Function?

A function is a reusable block of code that performs a specific task. Functions help break a large program into smaller, manageable pieces. Every C program has at least one function - main().

Function Declaration, Definition and Call

Declaration (Prototype) - tells the compiler about the function's name, return type, and parameters. Placed before main().
Definition - the actual body of the function with its implementation.
Call - invoking the function to execute it.

// Declaration (prototype)
int add(int a, int b);

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

// Call
int result = add(5, 3);  // result = 8

Variable Scope

Local variables - declared inside a function, exist only within that function
Global variables - declared outside all functions, accessible everywhere
Static local variables - retain their value between function calls

Function Declaration, Definition and Call

#include <stdio.h>

// Function prototypes (declarations)
int add(int a, int b);
float average(int a, int b, int c);
void greet(char name[]);
void countCalls();

int main() {
    printf("Sum: %d\n", add(10, 5));
    printf("Average: %.2f\n", average(10, 20, 30));
    greet("Alice");

    // Static variable demo
    countCalls();
    countCalls();
    countCalls();

    return 0;
}

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

float average(int a, int b, int c) {
    return (a + b + c) / 3.0f;
}

void greet(char name[]) {
    printf("Hello, %s!\n", name);
    // no return statement needed for void
}

void countCalls() {
    static int count = 0;  // retains value between calls
    count++;
    printf("Function called %d time(s)\n", count);
}

/*
Output:
Sum: 15
Average: 20.00
Hello, Alice!
Function called 1 time(s)
Function called 2 time(s)
Function called 3 time(s)
*/

Recursion - Factorial and Fibonacci

#include <stdio.h>

// Recursive factorial: n! = n * (n-1)!
long long factorial(int n) {
    if (n == 0 || n == 1) return 1;  // base case
    return n * factorial(n - 1);     // recursive call
}

// Recursive Fibonacci: fib(n) = fib(n-1) + fib(n-2)
int fibonacci(int n) {
    if (n <= 1) return n;            // base cases: fib(0)=0, fib(1)=1
    return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
    // Factorial
    for (int i = 0; i <= 10; i++) {
        printf("%d! = %lld\n", i, factorial(i));
    }

    // Fibonacci series
    printf("\nFibonacci (first 10): ");
    for (int i = 0; i < 10; i++) {
        printf("%d ", fibonacci(i));
    }
    printf("\n");

    return 0;
}

/*
0! = 1
1! = 1
...
10! = 3628800

Fibonacci (first 10): 0 1 1 2 3 5 8 13 21 34
*/

Call by Value vs Call by Reference

#include <stdio.h>

// Call by value - a copy is passed; original is NOT modified
void doubleByValue(int x) {
    x = x * 2;
    printf("Inside doubleByValue: %d\n", x);
}

// Call by reference - pointer is passed; original IS modified
void doubleByRef(int *x) {
    *x = *x * 2;
    printf("Inside doubleByRef: %d\n", *x);
}

// Swap using pointers
void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int num = 10;

    doubleByValue(num);
    printf("After doubleByValue: %d\n\n", num);  // still 10

    doubleByRef(&num);
    printf("After doubleByRef: %d\n\n", num);    // now 20

    int a = 5, b = 8;
    printf("Before swap: a=%d, b=%d\n", a, b);
    swap(&a, &b);
    printf("After swap:  a=%d, b=%d\n", a, b);

    return 0;
}

/*
Inside doubleByValue: 20
After doubleByValue: 10

Inside doubleByRef: 20
After doubleByRef: 20

Before swap: a=5, b=8
After swap:  a=8, b=5
*/

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C Functions Declaration, Recursion, Pointers: Tutorial, Examples, FAQs & Interview Tips

What is a Function?

Function Declaration, Definition and Call

Variable Scope

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