Golang: loop

By Xah Lee. Date: 2016-02-03. Last updated: 2019-01-19.

Go has only one loop construct, the for loop.

for loop can:

Repeat a fixed number times
Test a condition before repeat. (like “while loop”)
Infinite loop (exit when a condition is met)
Go thru array, slice, map. (called “for range loop”)

Loop a Fixed Number Times

for i := 0; i < 4; i++ { … }

package main

import "fmt"

func main() {
    // scope of i is the whole for statement
    for i := 0; i < 4; i++ {
        fmt.Printf("%v ", i)
    }
}

// 0 1 2 3

Note: no parenthesis after the for keyword. Adding parenthesis creates invalid syntax.

Curly brackets {} are always required.

Like While Loop

for loop with just test condition, it acts like “while loop” in other langs. Example:

for x < 3 {…}

package main

import "fmt"

func main() {
    var ss = 1

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

// 1 2 3

Infinite Loop

for {…}

If no condition is used, loop acts like a infinite loop.

You break the loop using keyword break.

package main

import "fmt"

func main() {
    var uu = 1

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

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

// 1 2 3 4

break loop

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

continue loop

The keyword continue lets you exit the inner loop of a nested 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
            }
        }
    }
}

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