Pointers and functions

Give the function the address

A function gets copies of its arguments, so swap(a, b) can't swap a and b. Pass their addresses instead: swap(&a, &b), with void swap(int *x, int *y). Inside, *x is main's a.

That's exactly why scanf needs &: it has to write into your variable.

The same trick lets a function return several results: pass pointers to where each answer should go.

A swap that works

#include <stdio.h>

void swap(int *x, int *y) {
    int t = *x;
    *x = *y;
    *y = t;
}

int main() {
    int a = 3, b = 8;
    swap(&a, &b);
    printf("%d %d\n", a, b);
    return 0;
}

1. Line 10: a is 3, b is 8.
2. Line 11: Pass the addresses of a and b.
3. Line 4: x points at a, y at b. Save *x (3) in t.
4. Line 5: Write b's value into a: a is now 8.
5. Line 6: Write the saved 3 into b.
6. Line 12: Back in main, a and b really changed.

Exercise: Fix swap()

swap() should swap main's two numbers, but nothing changes. Fix the function and the call.

Exercise: Two answers from one function

Complete void minMax(int a[], int n, int *lo, int *hi) so it stores the smallest and largest of the array in *lo and *hi. main prints them.