The for loop
- C++ course
- Beginner
- Lesson 42/116
- 4 min
Learn to write counted loops with for and use a counter.
The for loop
When you need a fixed number of iterations you use the for loop. It has three parts: initialization, condition and increment. for (int i = 0; i < 5; i++) { std::cout << i; } starts i at zero, repeats while i is below 5 and increases i by one each step.
The initialization runs only on the first iteration; the condition is checked at the start of every iteration; the increment is applied at the end. The loop body goes inside braces and can hold several statements.
The loop variable
The variable such as i in a for loop is called the loop variable. It is usually declared as int and lives only inside the loop. After the loop ends i is no longer accessible from outside.
The counter typically starts at 0 because arrays and vectors are also 0-indexed. The increment is usually i++ (increase by one) or i += 2 (increase by two). A decreasing loop is also possible: for (int i = 5; i > 0; i--).
Practice Print 0 to 4
Use a for (int i = 0; i < 5; i++) loop to print the numbers from 0 to 4 with std::cout << i; on a single line. The output must be 01234.
Starter code
#include <iostream>
int main() {
// Print 0 to 4
return 0;
}Sample solution
#include <iostream>
int main() {
for (int i = 0; i < 5; i++) {
std::cout << i;
}
std::cout << std::endl;
return 0;
}Expected output
01234
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How many times does for (int i = 0; i < 3; i++) run?
for (int i = 0; i < 3; i++) { ... }- 2 times
- 4 times
- 3 times Correct answer
- Forever
Explanation: i = 0, 1, 2 means 3 iterations.
2 Write the for initialization
for (____; i < 5; i++) { std::cout << i; }for (int i = 0; i < 5; i++) { std::cout << i; }Explanation: int i = 0 declares the type and the initial value.
3 Match each for part to its role
| Term | Meaning |
|---|---|
int i = 0 |
Initialization |
i < 5 |
Condition |
i++ |
Increment |
{ ... } |
Body |