Golang: loop

Go has only one loop construct, the for loop.

for loop can:

• Repeat a fixed number times
• Test a coniditon before repeat. (aka “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, acts like “while loop” in other langs. e.g.

for ss < 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

For Range Loop

for loop has a second syntax, with range. It lets you loop over array, slice, string, map.

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

If one of the variable you don't need, name it _, else compiler will complain.

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

The _ is called “blank identifier”.

Loop Thru Array or Slice

package main

import "fmt"

func main() {
    var arr = []string{"aa", "bb", "cc"}
    for i, x := range arr {
    fmt.Println(i, x)
    }
    // prints
    // 0 aa
    // 1 bb
    // 2 cc

}

[see Golang: Array]

[see Golang: Slice]

Loop Thru Map

package main

import "fmt"

func main() {

    var mm = map[string]int{"a": 1, "b": 2}

    for kk, vv := range mm {
        fmt.Printf("%v, %v\n", kk, vv)
    }
    // prints
    // a, 1
    // b, 2

}

[see Golang: Map]

Reference

The Go Programming Language Specification - The Go Programming Language#For_statements

The Go Programming Language Specification - The Go Programming Language#Break_statements

The Go Programming Language Specification - The Go Programming Language#Continue_statements