Functions & Recursion

Concept

A function has a prototype (declaration), a definition and calls. C passes arguments by value — the function gets copies.

To let a function change a caller's variable, pass its address (a pointer).

A static local variable keeps its value between calls. Recursion needs a base case to stop.

Code (C17)

#include <stdio.h>

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

int counter(void) { static int n = 0; return ++n; }

long fib(int n) { return n < 2 ? n : fib(n - 1) + fib(n - 2); }

int main(void) {
    int x = 1, y = 2;
    swap(&x, &y);
    printf("%d %d\n", x, y);         // 2 1
    counter(); printf("%d\n", counter()); // 2
    printf("%ld\n", fib(10));        // 55
    return 0;
}

Remember

• C is always call by value.
• Pointers simulate call by reference.
• static locals persist between calls.

Interview questions