Loops

for loops

We’ve already seen the basic structure of a for loop:

for (initializer ; condition ; iterator) {
     // do stuff
}

Tip

The behavior of the prefix and postfix operator is essentially the same when it is on its own line. Also in the for construction:

for (int i = 0; i < 10; ++i) {
    ...

it doesn’t matter much which version you use—although you will commonly see the prefix version used since it does not make a copy, and therefore can be faster.

We can do this for just a simple integer counter:

for (int i = 0; i < 10; i += 2) {
     // we'll see i = 0, 2, 4, 6, 8
}

or with an iterator, like:

std::string my_string{"this is my string"};

for (auto it = my_string.begin(); it < my_string.end(); ++it) {
     // work on the string character by character
}

Note

Just like with if, there is a single-statement form of for that doesn’t use brackets for the loop body—this should be avoided.

We also saw the range-for loop that works with a variety of containers. For example:

std::vector<double> x{0.0, 1.0, 2.0, 3.0};

for (auto e : x) {
    // work on the current element in x
}

while and do-while loops

There are two types of while loops in C++. The first takes the form:

while (condition) {
    // do stuff
}

where the body is executed so long as condition is true. For example:

int i{0};

while (i < 10) {
   i = 2*i;
}

The loop body is only ever executed if the condition is true. The other form puts the while at the end:

do {
    // do stuff
} while (condition);

In this case, all of the statements in the loop body are executed at least once.

Note

The do {} while (condition) form is discouraged.

Finally, you can loop over a range simply by using an initialization list:

Listing 45 list_loop.cpp

#include <iostream>

int main() {

    for (auto year : {2020, 2021, 2022, 2023}) {
        std::cout << "the year is " << year << std::endl;
    }

}