Go for loop
last modified April 11, 2024
In this article we show how to create loops in Golang with for statement. There are three forms of for loops in Go.
Go for statement
The for statement specifies repeated execution of a block.
There are three forms of the for statement: the classic C-style for statement,
the single condition for statement, and the for statement with the range
clause.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
Classic C style for loop
The following example is similar (not entirely equivalent) to the classic C-style for statement.
package main
import "fmt"
func main() {
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
fmt.Println(sum)
}
The program calculates the sum of values 1..9.
for i := 0; i < 10; i++ {
sum += i
}
The for statement consists of three parts: the initialization, the condition, and the increment. The initialization part is executed only once. The body of the for statement is executed when the condition is true. If the condition returns false, the for loop is terminated. After the statements in the block are executed, the for loop switches to the third part, where the counter is incremented. The cycle continues until the condition is not true anymore. Note that is it possible to create endless loops.
$ go run main.go 45
The sum of values 1..9 is 45.
Single condition for
We can define single condition for statements in Go.
package main
import "fmt"
func main() {
sum := 0
i := 9
for i > 0 {
sum += i
i--
}
fmt.Println(sum)
}
The single condition for statements are functionally equivalent to the C
while loop. We sum the values 9..1. In this example we define
the i counter variable.
$ go run main.go 45
Using range clause
The next example uses the range clause with the for
statement.
package main
import "fmt"
func main() {
nums := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
sum := 0
for _, num := range nums {
sum += num
}
fmt.Println(sum)
}
We calculate the sum of integer values.
nums := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
We define an array of values.
for _, num := range nums {
sum += num
}
We iterate over the array with the range clause. The
range returns the index and the value in each iteration. Since we
do not use the index, we specify the discard _ operator. (The
Golang documentation calls it the blank identifier.)
In the next example, we use the index value.
package main
import "fmt"
func main() {
words := []string{"sky", "cup", "cloud", "news", "water"}
for idx, word := range words {
fmt.Printf("%s has index %d\n", word, idx)
}
}
We iterate over a slice of words. We print the word and its index.
$ go run main.go sky has index 0 cup has index 1 cloud has index 2 news has index 3 water has index 4
Ranging over integers
In Go version 1.22, a new syntax allowing to range over integers was added.
package main
import "fmt"
func main() {
for i := range 5 {
fmt.Println(i)
}
for range 6 {
fmt.Println("falcon")
}
}
The example prints values 0..5 and prints falcon word six times.
$ go run main.go 0 1 2 3 4 falcon falcon falcon falcon falcon falcon
Infinite loop
In the next example, we create an infinite loop.
package main
import (
"fmt"
"math/rand"
)
func main() {
for {
r := rand.Intn(30)
fmt.Printf("%d ", r)
if r == 22 {
break
}
}
}
The example prints randomly values from <0, 30) in an infinite loop.
We terminate the loop with the break keyword when we encounter
value 22.
Source
The Go Programming Language Specification
In this article we have covered for loops in Golang.
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