Range Iteration in

About Range Iteration

You now have the knowledge on how to iterate a slice, or a map, in Go.

As a recap, you learnt how every iteration returns two values: the index/key and a copy of the element at that index/key.

Iterate over a slice

Easy as pie, loops over a slice, ordered as expected.

xi := []int{10, 20, 30}
for i, x := range xi {
  fmt.Println(i, x)
}
// outputs:
// 0, 10
// 1, 20
// 2, 30

Iterate over a map

Iterating over a map raises a new problem. The order is now random.

hash := map[int]int{9: 10, 99: 20, 999: 30}
for k, v := range hash {
  fmt.Println(k, v)
}
// outputs, for example:
// 99 20
// 999 30
// 9 10

Iteration omitting key or value

In Go an unused variable will raise an error at build time. Sometimes you only need the value, as per the first example:

xi := []int{10, 20, 30}
for i, x := range xi {
  fmt.Println(x)
}
// Go build failed: i declared but not used

You can replace the i with _ which tells the compiler we don't use that value:

xi := []int{10, 20, 30}
for _, x := range xi {
  fmt.Println(x)
}
// outputs:
// 10
// 20
// 30

If you want to only print the index, you can replace the x with _, or simply omit the declaration:

xi := []int{10, 20, 30}
// for i, _ := range xi {
for i := range xi {
  fmt.Println(i)
}
// outputs:
// 0
// 1
// 2

Last but not least, if you are required to perform some action but you are not interested in values nor keys of the slice or map, you can omit both index and value:

xi := []int{10, 20, 30}
count := 0
for range xi {
  count++
}
// count value:
// 3