1.5.2 Iterate Using the range Operator

Another form of the for loop iterates over a range of values from a data type like a ‘string’ or a ‘slice’. To illustrate, here’s a second version of echo:

// Echo2 prints its command-line arguments.
package main

import ("fmt";"os")

func main() {
        s, sep := "",""
        for _, arg := range os.Args[1:] {
                s += sep + arg
                sep = " "
        }
        fmt.Println(s)
}

The ‘blank’ Identifier

In each iteration of the loop, range produces a pair of values:

• the index and
• the value of the element at that index.

In this example, we don’t need the index, but the syntax of a range loop requires that if we deal with the element, we must deal with the index too. One idea would be to assign the index to an obviously temporary variable like temp and ignore its value, but Go does not permit unused local variables, so this would result in a compilation error.

The solution is to use the @texinfo:@dfn{blank identifier}, whose name is ‘_’ (that is, an underscore). The blank identifier may be used whenever syntax requires a variable name but program logic does not, for instance to discard an unwanted loop index when we require only the element value.