The for Loop

The Classic Three-Part for Loop

Go's most familiar for loop has three parts separated by semicolons: an initializer, a condition checked before each iteration, and a post statement run after each iteration. This mirrors the classic C-style for loop found in many languages.

Example: The Classic Three-Part for Loop

package main

import "fmt"

func main() {
	for i := 0; i < 5; i++ {
		fmt.Println("iteration", i)
	}
}

for as a while Loop

Dropping the init and post clauses and keeping only a condition turns for into the equivalent of a while loop found in other languages -- it keeps running as long as the condition stays true.

Example: for as a while Loop

package main

import "fmt"

func main() {
	n := 1
	for n < 100 {
		n *= 2
	}
	fmt.Println("first power of two >= 100:", n)
}

Infinite Loops

Omitting all three clauses produces an infinite loop, 'for { }', which runs forever unless something inside it breaks out or returns from the function. This form is often used for servers or workers that should run until explicitly stopped.

Note: Always pair an infinite for loop with a break, return, or os.Exit so the program has a way out.

Example: Infinite Loops

package main

import "fmt"

func main() {
	count := 0
	for {
		count++
		if count == 3 {
			break
		}
	}
	fmt.Println("stopped at:", count)
}

Nested for Loops

Because for is Go's only loop construct, nested loops (a loop inside a loop) are written by simply placing one for statement inside another, commonly used for iterating over grids or combinations of two sequences.

Example: Nested for Loops

package main

import "fmt"

func main() {
	for i := 1; i <= 3; i++ {
		for j := 1; j <= 2; j++ {
			fmt.Printf("(%d,%d) ", i, j)
		}
	}
	fmt.Println()
}