The For Loop

Most languages give you a toolbox of loops: while, do-while, for, sometimes loop or repeat. Go gives you one keyword — for — and lets it take a few shapes. One keyword means one thing to learn, one thing to read, and no debates about which loop to use.

The classic three-part form

This is the for you may know from other languages: an init statement, a condition, and a post statement, separated by semicolons — and, like if, with no parentheses around them:

package main

import "fmt"

func main() {
	for i := 1; i <= 5; i++ {
		fmt.Println(i)
	}
}

i := 1 runs once before the loop; i <= 5 is checked before every iteration; i++ runs after each one. The variable i exists only inside the loop — a small scope is a safe scope.

Drop the semicolons and it's a while

Leave out the init and post statements and for becomes what other languages call while: keep looping as long as the condition holds.

package main

import "fmt"

func main() {
	n := 1
	for n < 100 {
		n *= 2
	}
	fmt.Println(n) // 128
}

Same keyword, same braces — just less punctuation.

Drop everything and it loops forever

for with no condition at all loops until something stops it. Pair it with break, which exits the loop immediately:

package main

import "fmt"

func main() {
	countdown := 3
	for {
		fmt.Println(countdown)
		countdown--
		if countdown == 0 {
			break
		}
	}
	fmt.Println("Liftoff!")
}

Infinite loops sound alarming, but they're idiomatic Go — servers, for example, are essentially a for {} that accepts one request per iteration.

continue skips to the next round

Where break leaves the loop, continue abandons the current iteration and jumps straight to the next one:

package main

import "fmt"

func main() {
	for i := 1; i <= 10; i++ {
		if i%2 != 0 {
			continue // odd — skip it
		}
		fmt.Println(i) // prints 2, 4, 6, 8, 10
	}
}

Used near the top of a loop body, continue filters out the cases you don't care about, so the interesting code below it stays unindented.

A preview: for range

There is one more shape — for i, v := range collection — which walks over slices, maps, strings, and more. It's the loop you'll eventually use most, but it belongs with the collections that feed it, so we'll cover it properly in the chapter on slices and maps. For counting and conditions, the forms above are all you need.

The exercise below asks you to put a loop to work: summing the numbers from 1 to n.