Go Keyword range

Internal

• Keywords
• for
• Channels

Overview

range keyword is used to iterate over strings, arrays, slices, maps, channels and variadic function arguments. It returns two values. On the first position is the index/key, and on the second position is the copy of the value in that element. For slices and arrays, range always starts to iterate from index 0. If you need more control over the start index, use a for loop.

Note 1: that only a first identifier is declared, that is the index and not the value, as the intuition would suggest. See examples below.

Note 2: range returns a copy of the value, in the same way it would pass an argument to a function. It does NOT return a reference to the element in the structure it iterates over.

Iterating over Indices

var a [5]int
for i := range a {
   // i is the index
}

Iterating over Values

var a [5]int
for _, value := range a {
   // 'value' is the value
}

Iterating over Indices and Values

var a [5]int
for i, value := range a {
   // i is the index
   // 'value' is the value
}

range and Channels

range can be used to read from a channel in a for loop. range blocks until a value is available on the channel. The iteration values produced are the successive values sent on the channel until the channel is closed. When the channel is closed, the for loop exists:

var c chan string

for m := range c {
   // do something with the message
}

If the channel is nil, the range expression blocks forever.