Writing functions

A function is a named recipe

int square(int x) { return x * x; } takes an int called x and gives back (returns) an int. square(7) runs it with x = 7 and becomes 49.

Functions go above main, so C has seen them before they're used. A function that returns nothing has the type void.

A call jumps in and comes back

#include <stdio.h>

int square(int x) {
    return x * x;
}

int main() {
    int a = square(3);
    int b = square(a + 1);
    printf("%d %d\n", a, b);
    return 0;
}

1. Line 8: main calls square with 3.
2. Line 4: Inside square, x is 3. It returns 9.
3. Line 8: Back in main: a gets the 9.
4. Line 4: Called again, this time x is a + 1 = 10. Returns 100.
5. Line 9: b gets 100.
6. Line 10: Printed.

void functions

#include <stdio.h>

void line(int n) {
    for (int i = 0; i < n; i++) printf("-");
    printf("\n");
}

int main() {
    line(3);
    line(1);
    return 0;
}

What does this print?

• --- -
• - ---
• ----

Exercise: Write max()

Write int max(int a, int b) that returns the larger of the two. main already reads three numbers and prints the biggest using it.

Exercise: Write power()

Write long long power(int base, int exp) using a loop (no math.h). Read base and exp (exp ≥ 0) and print the result.