How to create an array/slice of 5 value, all of same value

Question

Problem

In go programming language, how to create an array of length 5, with all elements has same value, eg, 42.

Preference order

Answer 1

For example,

package main

import (
    "fmt"
)

func main() {
    s := make([]int, 5)
    for i := range s {
        s[i] = 42
    }
    fmt.Println(len(s), s)
}

Playground: https://play.golang.org/p/GjTXruMsJ5h

Output:

5 [42 42 42 42 42]

---

Some benchmarks:

package main

import (
    "fmt"
    "testing"
)

func BenchmarkStack(b *testing.B) {
    for N := 0; N < b.N; N++ {
        s := make([]int, 5)
        for i := range s {
            s[i] = 42
        }
    }
}

func BenchmarkHeap(b *testing.B) {
    var s []int
    for N := 0; N < b.N; N++ {
        s = make([]int, 5)
        for i := range s {
            s[i] = 42
        }
    }
}

func BenchmarkHygull(b *testing.B) {
    for N := 0; N < b.N; N++ {
        var s []int
        for i := 0; i < 5; i++ {
            s = append(s, 42)
        }
    }
}

Output:

$ go test slice42_test.go -bench=. -benchmem
BenchmarkStack-8     1000000000      2.05 ns/op     0 B/op    0 allocs/op
BenchmarkHeap-8       100000000     26.9 ns/op     48 B/op    1 allocs/op
BenchmarkHygull-8      10000000    123 ns/op      120 B/op    4 allocs/op
$ 

BenchmarkHygull demonstrates how inefficient @hygull's solution is.

---

A "one-liner":

package main

import (
    "fmt"
)

func newInts(n, v int) []int {
    s := make([]int, n)
    for i := range s {
        s[i] = v
    }
    return s
}

func main() {
    s := newInts(5, 42)
    fmt.Println(len(s), s)
}

Playground: https://play.golang.org/p/t8J-AjYQ72l

Output:

5 [42 42 42 42 42]

Answer 2

The best way to do will be to use slice for that as it can grow dynamically.

Currently, you are looking only for 5 values so using arrays also is not a problem but what if you are looking for dynamic size (i.e. if you specify size of array in run time, there may be a chance of either going beyond the range or not using all the allocated spaces).

So I think the following code is sufficient to understand that.

package main  
import "fmt" 

func main() { 
    var s []int
    value := 42 // It can be changed to other values 

    for i := 0; i < 5; i++ {
        s = append(s, value)
    }

    fmt.Println(s); // [42 42 42 42 42]
}