Condition

For

Go has only one looping construct, the for loop.

package main

import "fmt"

func main() {
    sum := 0
    for i := 0; i < 10; i++ {
        sum += i
    }
    fmt.Println(sum)
}

{% hint style="warning" %} Unlike other languages like C, Java, or JavaScript there are no parentheses surrounding the three components of the for statement and the braces { } are always required. {% endhint %}

The init and post statements are optional.

package main

import "fmt"

func main() {
    sum := 1
    for ; sum < 1000; {
        sum += sum
    }
    fmt.Println(sum)
}

// Excepted results:
1024

For is Go's "while"

At that point you can drop the semicolons: C's while is spelled for in Go.

package main

import "fmt"

func main() {
    sum := 1
    for sum < 1000 {
        sum += sum
    }
    fmt.Println(sum)
}

Forever

If you omit the loop condition it loops forever, so an infinite loop is compactly expressed.

package main

func main() {
    for {
    }
}

If

Go's if statements are like its for loops; the expression need not be surrounded by parentheses ( ) but the braces { } are required.

package main

import (
    "fmt"
    "math"
)

func sqrt(x float64) string {
    if x < 0 {
        return sqrt(-x) + "i"
    }
    return fmt.Sprint(math.Sqrt(x))
}

func main() {
    fmt.Println(sqrt(2), sqrt(-4))
}

If with a short statement

Like for, the if statement can start with a short statement to execute before the condition.

Variables declared by the statement are only in scope until the end of the if.

package main

import (
    "fmt"
    "math"
)

func pow(x, n, lim float64) float64 {
    if v := math.Pow(x, n); v < lim {
        return v
    }
    return lim
}

func main() {
    fmt.Println(
        pow(3, 2, 10),
        pow(3, 3, 20),
    )
}

// Expected results:
9 20

If and else

Variables declared inside an if short statement are also available inside any of the else blocks.

(Both calls to pow return their results before the call to fmt.Println in main begins.)

package main

import (
    "fmt"
    "math"
)

func pow(x, n, lim float64) float64 {
    if v := math.Pow(x, n); v < lim {
        return v
    } else {
        fmt.Printf("%g >= %g\n", v, lim)
    }
    // can't use v here, though
    return lim
}

func main() {
    fmt.Println(
        pow(3, 2, 10),
        pow(3, 3, 20),
    )
}

// Expected results:
27 >= 20
9 20

Switch

A switch statement is a shorter way to write a sequence of if - else statements. It runs the first case whose value is equal to the condition expression.

Go's switch is like the one in C, C++, Java, JavaScript, and PHP, except that Go only runs the selected case, not all the cases that follow. In effect, the break statement that is needed at the end of each case in those languages is provided automatically in Go. Another important difference is that Go's switch cases need not be constants, and the values involved need not be integers.

package main

import (
    "fmt"
    "runtime"
)

func main() {
    fmt.Print("Go runs on ")
    switch os := runtime.GOOS; os {
    case "darwin":
        fmt.Println("OS X.")
    case "linux":
        fmt.Println("Linux.")
    default:
        // freebsd, openbsd,
        // plan9, windows...
        fmt.Printf("%s.\n", os)
    }
}

There is no automatic fall through, but cases can be presented in comma-separated lists.

func shouldEscape(c byte) bool {
    switch c {
    case ' ', '?', '&', '=', '#', '+', '%':
        return true
    }
    return false
}

Switch evaluation order

Switch cases evaluate cases from top to bottom, stopping when a case succeeds.

For example,

switch i {
    case 0:
    case f():
}

does not call f if i==0.

Switch with no condition

Switch without a condition is the same as switch true.

{% hint style="info" %} This construct can be a clean way to write long if-then-else chains. {% endhint %}

package main

import (
    "fmt"
    "time"
)

func main() {
    t := time.Now()
    switch {
    case t.Hour() < 12:
        fmt.Println("Good morning!")
    case t.Hour() < 17:
        fmt.Println("Good afternoon.")
    default:
        fmt.Println("Good evening.")
    }
}

For#

For is Go's "while"#

Forever#

If#

If with a short statement#

If and else#

Switch#

Switch evaluation order#

Switch with no condition#

For