Comparison Operators

The symbols < and > that you've used in conditions are comparison operators: they compare two values, and the result is either true or false. C++ has six of them:

• == equal to: 5 == 5
• != not equal to: 5 != 3
• > greater than: 5 > 3
• < less than: 3 < 5
• >= greater than or equal to: 5 >= 5
• <= less than or equal to: 3 <= 5

All the examples in the list are true. In !=, >= and <=, the equals sign always comes second, with no space in between. Watch the difference between > and >= at the boundary: 18 > 18 is false, but 18 >= 18 is true. “18 or older” is therefore age >= 18.

“Equal” is written with two equals signs, ==, because a single one already has a job: points = 10 assigns, which means it stores 10 in the variable points. points == 10 compares: it asks whether points holds 10, and changes nothing.

Writing = instead of == in a condition is one of the most common mistakes. In C++ it's especially treacherous, because the program compiles and runs:

int points = 7;

if (points = 10) {
    std::cout << "Full marks!" << std::endl;
}
std::cout << "Points: " << points << std::endl;

This prints:

Full marks!
Points: 10

points = 10 in the condition doesn't compare anything. It stores 10 in points, and that value is then used as the condition. C++ treats every number except 0 as true, so the condition is always true, whatever points held before, and the original 7 points are gone as well. The compiler only prints a warning about it, which you don't see in this editor. When a program runs a block it shouldn't, check first whether the condition has = instead of ==.

You can compare text as well as numbers:

std::string password = "kitten42";

if (password == "kitten42") {
    std::cout << "Welcome back" << std::endl;
} else {
    std::cout << "Wrong password" << std::endl;
}

This prints:

Welcome back

As in every program with std::string, don't forget #include <string> at the top. Two texts are equal only if they match exactly, character by character. Upper and lower case letters are different characters: when the variable name holds "Anna", name == "anna" is false. "kitten42 " with a space at the end is a different text, too. Don't forget the quotes around the text you compare with. Without them, password == kitten42 makes the compiler look for a variable called kitten42 and report error: use of undeclared identifier 'kitten42'.