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Go make function

last modified May 28, 2022

Go make function tutorial describes the make function and shows how to use it.

The make is a built-in function which allocates and initializes an object of type slice, map, or chan.

func make(t Type, size ...IntegerType) Type

In case of a slice, the size specifies its length. A second integer may be specified to set the capacity of the slice; it must not be smaller than the length. The make([]int, 0, 5) allocates an underlying array of size 5 and returns a slice of length 0 and capacity 5 that is backed by this underlying array.

In case of a map, an empty map is allocated with enough space to hold the specified number of elements. We can omit the size parameter, in which case a small starting size is allocated.

In case of a channel, the channel's buffer is initialized with the specified buffer capacity. If zero, or the size is omitted, the channel is unbuffered.

Go make slice

In the first example, we create an empty slice of integers with make.

main.go
package main

import (
    "fmt"
)

func main() {

    vals := make([]int, 5)
    fmt.Println(vals)

    vals[0] = 12
    vals[1] = 18
    vals[2] = 13
    vals[3] = 19
    vals[4] = 38

    fmt.Println(vals)

    vals2 := []int{12, 18, 13, 19, 38}
    fmt.Println(vals2)

    vals3 := []int{}
    fmt.Println(vals3)

    // vals3[0] = 12
    // vals3[1] = 18

    vals3 = append(vals3, 1)
    vals3 = append(vals3, 2)
    vals3 = append(vals3, 5)
    vals3 = append(vals3, 6)

    fmt.Println(vals3)
}

In the example, we create slice collections using make funtion and literals.

vals := make([]int, 5)
fmt.Println(vals)

We create a new slice with five elements initialized to 0.

vals[0] = 12
vals[1] = 18
vals[2] = 13
vals[3] = 19
vals[4] = 38

fmt.Println(vals)

Later, we fill the slice with some data.

vals2 := []int{12, 18, 13, 19, 38}
fmt.Println(vals2)

We can initialize the slice when we define it with literal notation.

vals3 := []int{}
fmt.Println(vals3)

Here we create an empty slice.

// vals3[0] = 12
// vals3[1] = 18

It is not possible to assign values to an empty slice.

vals3 = append(vals3, 1)
vals3 = append(vals3, 2)
vals3 = append(vals3, 5)
vals3 = append(vals3, 6)

To add new values, the append function can be uzed.

$ go run main.go
[0 0 0 0 0]
[12 18 13 19 38]
[12 18 13 19 38]
[]
[1 2 5 6]

Go make slice of slices

In the following example, we create a slice of slices with make.

main.go
package main

import "fmt"

func main() {

    w := make([][]string, 3)

    w1 := make([]string, 4)
    w1[0] = "war"
    w1[1] = "water"
    w1[2] = "wrath"
    w1[3] = "wrong"

    w2 := make([]string, 3)
    w2[0] = "car"
    w2[1] = "cup"
    w2[2] = "cloud"

    w3 := make([]string, 2)
    w3[0] = "boy"
    w3[1] = "brown"

    w[0] = w1
    w[1] = w2
    w[2] = w3

    fmt.Println(w)
}

The example creates a slice of a string slices of various length.

w := make([][]string, 3)

We create a slice of slices.

w1 := make([]string, 4)
w1[0] = "war"
w1[1] = "water"
w1[2] = "wrath"
w1[3] = "wrong"

Each of the subslices is separately created with make and initialized.

$ go run main.go
[[war water wrath wrong] [car cup cloud] [boy brown]]

Go make map

In the next example, we create a new map with make.

main.go
package main

import "fmt"

func main() {

    benelux := make(map[string]string)

    benelux["be"] = "Belgium"
    benelux["nl"] = "Netherlands"
    benelux["lu"] = "Luxembourgh"

    fmt.Println(benelux)
    fmt.Printf("%q\n", benelux)
}

The example creates a new map and adds three pairs to it.

$ go run main.go
map[be:Belgium lu:Luxembourgh nl:Netherlands]
map["be":"Belgium" "lu":"Luxembourgh" "nl":"Netherlands"]

Go make channel

In the following example, we use the make function to create a channel. A channel is an object through which goroutines communicate.

main.go
package main

import (
    "fmt"
)

func fib(n int, c chan int) {

    x, y := 0, 1

    for i := 0; i < n; i++ {
        c <- x
        x, y = y, x+y
    }
    close(c)
}

func main() {

    c := make(chan int, 10)

    go fib(cap(c), c)

    for i := range c {
        fmt.Println(i)
    }
}

A series of fibonacci values is generated inside the fib goroutine. The values are one by one send to the caller goroutine via a channel.

$ go run main.go
0
1
1
2
3
5
8
13
21
34

In this tutorial, we have showed how to use the make built-in function.

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