Golang: loop

Go has only one loop construct, the for-loop.

for i := 0; i < 4; i++ { body }

Loop a fixed number times with a incremental variable.

Scope of i is the whole for statement.

Note: no parenthesis after the for keyword. Adding parenthesis creates invalid syntax. Curly brackets {} are always required.

package main

import "fmt"

func main() {
	for i := 0; i < 4; i++ {
		fmt.Printf("%v ", i)
	}
}

// 0 1 2 3

for test {body}

Test a condition before repeat. (like “while loop”)

package main

import "fmt"

func main() {
	var xx = 1

	// for-loop with just test condition
	// this acts like “while loop” in other langs
	for xx <= 3 {
		fmt.Printf("%v ", xx)
		xx++
	}
}

// 1 2 3

for {body}

Infinite loop. Break the loop by keyword break, or use keyword continue to skip rest of the inner loop.

package main

import "fmt"

func main() {
	var xx = 1

	// infinite loop
	for {
		fmt.Printf("%v ", xx)
		xx++

		// use break to exit
		if xx > 4 {
			break
		}
	}
}

// 1 2 3 4

For Range Loop

for i, x := range array { fmt.Println(i, x) }

Go thru Array or Slice .

If one of the variable you don't need, name it _, else compiler will complain. The _ is called Blank Identifier.

for key, value := range map { fmt.Println(key, val) }

Go thru Map.

keyword break

The keyword break lets you break a loop. Useful when you want to exit the loop when some condition is met.

package main

import "fmt"

func main() {

	for i := 0; i < 9; i++ {
		fmt.Printf("%v ", i)
		if i == 4 {
			break
		}
	}
}

// 0 1 2 3 4

keyword continue

The keyword continue skips the rest statements of the inner loop and continue at the beginning of the inner loop.

package main

import "fmt"

func main() {
	for i := 1; i <= 3; i++ {
		for j := 1; j <= 99; j++ {
			if j <= 2 {
				fmt.Println(i, j)
			} else {
				continue
				fmt.Printf("%v\n", "this never reached")
			}
		}
	}
}

// 1 1
// 1 2
// 2 1
// 2 2
// 3 1
// 3 2

Reference

Golang Spec #For_statements

Golang Spec #Break_statements

Golang Spec #Continue_statements