Step 3 of 6

Changing a variable

Variables are called variables because they can vary: you can store a new value in one at any time with =. The old value is thrown away and replaced.

#include <stdio.h>

int main(void) {
    int hp = 100;
    printf("Start: %d\n", hp);
    hp = hp - 27;
    printf("After a hit: %d\n", hp);
    hp = 100;
    printf("Healed: %d\n", hp);
    return 0;
}

Start: 100
After a hit: 73
Healed: 100

= is not "equals"

In math, hp = hp - 27 would be impossible. In C, = means assign: "work out the value on the right, then store it in the box on the left". So hp = hp - 27 runs in two stages:

1. Read the current value of hp (100) and subtract 27, giving 73.
2. Store 73 into hp, replacing 100.

The right side is always worked out completely before anything is stored, which is why a variable can appear on both sides.

Declaring vs assigning

int hp = 100; creates the box (with its type) and fills it. Later lines just write hp = ...; without int. Writing int hp again in the same place would try to create a second box with the same name, and the compiler reports redefinition of 'hp'.

Your turn: the player has 3200 money and buys a rifle for 2700. Subtract the price from money so the program prints Money left: 500.

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