What is the for-range loop in Golang?

A for loop is used to iterate over elements in a variety of data structures (e.g., a slice, an array, a map, or a string).

The for statement supports one additional form that uses the keyword range to iterate over an expression that evaluates to an array, slice, map, string, or channel.

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Syntax

The basic syntax for a for-range loop is:

for index, value := range mydatastructure {
		fmt.Println(value)
	}
  • index is the index of the value we are accessing.
  • value is the actual value we have on each iteration.
  • mydatastructure holds the data structure whose values will be accessed in the loop.

Note that the above example is highly generalized. The case-by-case examples given below will help you understand the syntax further.​

Code

1. Iterating over a string

The for-range loop can be used to access individual characters in a string.

package main
import "fmt"
func main() {
  for i, ch := range "World" {
    fmt.Printf("%#U starts at byte position %d\n", ch, i)
  }
}

2. Iterating over a map

The for-range loop can be used to access individual key-value pairs in a map.

package main
import "fmt"
func main() {
  m := map[string]int{
    "one":   1,
    "two":   2,
    "three": 3,
  }
  for key, value := range m {
      fmt.Println(key, value)
  }
}

3. Iterating over channels

For channels, the iteration values are the successive values sent on the channel until its close.

package main
import "fmt"
func main() {
  mychannel := make(chan int)
  go func() {
      mychannel <- 1
      mychannel <- 2
      mychannel <- 3
      close(mychannel)
  }()
  for n := range mychannel {
      fmt.Println(n)
  }
}

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