Len

Consider a string in a Go program. No counting needs to be done to measure its length. The len built-in returns a stored length value.

In Go, we use built-in methods like len on strings, arrays, slices and maps. This may make code clearer—simpler to understand.

String length

We test len() on a string. We assign a variable named "length" to the result of the len function. This returns the count of characters.

package main

import "fmt"

func main() {
    value := "cat"

    // Take length of string with len.
    length := len(value)
    fmt.Println(length)

    // Loop over the string with len.
    for i := 0; i < len(value); i++ {
        fmt.Println(string(value[i]))
    }
}
3
c
a
t

Slice length

A slice can be the argument to len(). This returns the number of elements (of any value) in the slice. Here we test len on a 4-element int slice.

package main

import "fmt"

func main() {
    slice := []int{10, 20, 30, 40}

    // Get length of slice with len.
    length := len(slice)

    fmt.Println(slice)
    fmt.Println(length)

    // Append an element, and take the length again.
    slice = append(slice, 50)

    fmt.Println(slice)
    fmt.Println(len(slice))
}
[10 20 30 40]
4
[10 20 30 40 50]
5

Nil

In Go, a slice may be nil—this is a nonexistent slice. The len of a nil slice is 0. No panic results from taking the len of a nil thing.

package main

import "fmt"

func main() {
    // A slice with len of 3.
    value := []int{10, 20, 30}
    fmt.Println(len(value))

    // A nil slice has a len of 0.
    value = nil
    fmt.Println(len(value))
}
3
0

Performance

The performance of len does not depend on the length of the string. Its time is constant. This is achieved in languages by storing the string's length as a field on the object.

package main

import (
    "fmt"
    "time"
)

func main() {

    // Length is 3.
    string1 := "cat"
    // Length is 13 times 5 which is 65.
    string2 := "cat0123456789" +
        "cat0123456789" +
        "cat0123456789" +
        "cat0123456789" +
        "cat0123456789"

    t0 := time.Now()

    // Version 1: length of short string.
    for i := 0; i < 10000000; i++ {
        result := len(string1)
        if result != 3 {
            return
        }
    }

    t1 := time.Now()

    // Version 2: length of long string.
    for i := 0; i < 10000000; i++ {
        result := len(string2)
        if result != 65 {
            return
        }
    }

    t2 := time.Now()
    // Print results.
    fmt.Println(t1.Sub(t0))
    fmt.Println(t2.Sub(t1))
}
7.0049ms    len( 3 character string)
7.0189ms    len(65 character string)

Summary

Len discovers lengths of things. In an array, the length is part of the type itself. But for strings and slices, a stored value (maintained by the runtime) is returned.