Changing values

= means "store", not "equals"

score = score + 10; reads the old score, adds 10, and stores the result back in score. The old value is gone.

Shortcuts: score += 10; does the same thing, count++; adds 1, and count--; takes 1 away.

Swapping needs a spare box

#include <stdio.h>

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

1. Line 4: a holds 3.
2. Line 5: b holds 8.
3. Line 6: Copy a into temp before we overwrite it.
4. Line 7: a gets b's value. The 3 would be lost now without temp.
5. Line 8: b gets the saved 3.
6. Line 9: Swapped.

Follow the changes

#include <stdio.h>

int main() {
    int x = 5;
    x += 3;
    x = x * 2;
    x--;
    printf("%d\n", x);
    return 0;
}

What does this print?

• 15
• 16
• 9

Exercise: Swap without losing anything

The program should print 20 10, but it prints 20 20. Fix the swap using a third variable.