带有和不带有空白标识符的Golang中的计时

Question

I am playing around with some elementary timing in Go and have a question. I want to time Golang in taking the square root of each element of an array but I get two somewhat different answers whether or not I keep the output. Here is the my first version:

package main

import ("fmt"
        "time" 
        "math"
        "math/rand"
)

// A random array of integers 
func randomArray(max int ,len int) []int {
    a := make([]int, len)
    for i := 0; i <= len-1; i++ {
        a[i] = rand.Intn(max)
    }
    return a
}

// Taking square root of each element in array
func sqrt_array(arr [] int) [] float64 {
    var len_arr = len(arr)
    a := make([]float64, len_arr)
    for i, v := range arr {
    a[i] = math.Sqrt(float64(v))
    }
    return a
}


func main() {

    arr := randomArray(100, 10e6)
    sqrt := make([]float64, len(arr))
    start := time.Now()
    sqrt = sqrt_array(arr)
    end := time.Now()

    fmt.Println("time taken: ", end.Sub(start), sqrt[0])
}

gives on average around 36ms:

time taken:  36.542019ms 9

Now when I replace the output "sqrt" with the blank identifier, I get something much slower. Specifically, I replace main() with

func main() {

    arr := randomArray(100, 10e6)
    // sqrt := make([]float64, len(arr))
    start := time.Now()
    _ = sqrt_array(arr)
    end := time.Now()

    fmt.Println("time taken: ", end.Sub(start))
}

and get on average something like 92ms

time taken:  92.121481ms

Would someone be able to explain what is happening? I feel that if I understood I might learn something about Go.

Incidentally I find that the same computation in Python was around 20ms if broadcasted and several hundred ms if in a loop.

Answer 1

If you want the same results, do the same thing! Obviously, allocations are not going to be free! Don't comment out an allocation!

For example,

Output:

time taken:  77.726729ms 9
time taken:  78.213242ms 0

main.go:

package main

import (
    "fmt"
    "math"
    "math/rand"
    "testing"
    "time"
)

// A random array of integers
func randomArray(max int, len int) []int {
    a := make([]int, len)
    for i := 0; i <= len-1; i++ {
        a[i] = rand.Intn(max)
    }
    return a
}

// Taking square root of each element in array
func sqrt_array(arr []int) []float64 {
    var len_arr = len(arr)
    a := make([]float64, len_arr)
    for i, v := range arr {
        a[i] = math.Sqrt(float64(v))
    }
    return a
}

func main1() {

    arr := randomArray(100, 10e6)
    sqrt := make([]float64, len(arr))
    start := time.Now()
    sqrt = sqrt_array(arr)
    end := time.Now()

    fmt.Println("time taken: ", end.Sub(start), sqrt[0])
}

func main2() {

    arr := randomArray(100, 10e6)
    // sqrt := make([]float64, len(arr))
    sqrt := make([]float64, len(arr))
    start := time.Now()
    _ = sqrt_array(arr)
    end := time.Now()

    fmt.Println("time taken: ", end.Sub(start), sqrt[0])
}

func main() {
    main1()
    main2()
}