Updating Variables

You often calculate the new value of a variable from its current one. A player picks up a coin, and their score goes up by 10:

int score = 30;
score = score + 10;
std::cout << score << "\n";

This prints:

40

In math, score = score + 10 wouldn't make sense. But in C++, = means “store”: first, the right side is calculated with the current value (30 + 10), and then the result, 40, is stored in score. The old value is overwritten.

Programs do this so often that C++ has shortcuts for it:

• score += 10; means the same as score = score + 10;
• score -= 10; means score = score - 10;
• score *= 2; means score = score * 2;
• score /= 2; means score = score / 2;
• score++; adds 1, and score--; subtracts 1

int money = 100;
money -= 30;  // you bought a book
money += 50;  // you sold an old game
money *= 2;   // grandma doubled your savings
std::cout << "Money: " << money << " EUR\n";

int visits = 0;
visits++;
visits++;
std::cout << "Visits: " << visits << "\n";

This prints:

Money: 240 EUR
Visits: 2

You'll see ++ mostly in loops, where it counts 1, 2, 3 and so on. It's even in the name of the language: C++ is the C language, made one better.

Typical mistakes:

• score + 10; on its own doesn't change the variable. The result is calculated and thrown away at once, because nothing stores it. The program still compiles, so you don't see an error, only a wrong result.
• score =+ 10;, with the two signs swapped, compiles too, but it means score = +10;: it stores 10 in the variable. Write +=: the plus before the equals sign, with no space between them.