The for loop

Learn to write counted loops with for and use a counter.

The for loop

For tasks that repeat a known number of times you use the for loop. It has three parts: init, condition and update. for (int i = 0; i < 5; i++) { ... } runs the block 5 times; i starts at 0 and grows by one each step.

The counter usually starts at 0; this fits well with arrays. The condition is checked before each step; when it is false the loop ends. i++ runs after each step.

Counter and accumulation

Inside a for loop you can accumulate a variable to compute sums or products. The pattern int sum = 0; for (...) sum += i; gives the sum of numbers from 1 to N.

It is important to reset the accumulator BEFORE the loop; otherwise the old value stays. sum += i is a shortcut for sum = sum + i and adds i to the current value each step.

Exercise Print 1 through 5

Use a for loop to print the numbers 1 through 5 (inclusive) with Console.WriteLine. The counter must start at 1 and the condition must be i <= 5.

Show example solution

Write for (int i = 1; i <= 5; i++) { Console.WriteLine(i); }
Example solution:
using System;

class Program
{
    static void Main()
    {
        for (int i = 1; i <= 5; i++)
        {
            Console.WriteLine(i);
        }
    }
}

Test yourself

How many lines does for (int i = 0; i < 3; i++) Console.WriteLine(i); print?
```
for (int i = 0; i < 3; i++) Console.WriteLine(i);
```
- 2 lines: 0, 1
- 4 lines: 0, 1, 2, 3
- 3 lines: 0, 1, 2
- 1 line: 3
Show answer

3 lines: 0, 1, 2 — Correct. The condition holds for i = 0, 1, 2; it stops when i = 3.

Complete the for header

for (int i = 0; ____; i++)
{
    Console.WriteLine(i);
}

Show answer

for (int i = 0; i < 5; i++)
{
    Console.WriteLine(i);
}

Match each for part to its meaning

int i = 0

Init
i < 5

Condition
i++

Update
{ ... }

Loop body

Next lesson: The while loop

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