Golang：传入切片作为参考问题

Posted 2023-02-23

【中文标题】Golang：传入切片作为参考问题

【英文标题】：Golang: Passing in Slice as Reference issue

【发布时间】：2014-10-09 19:16:04

【问题描述】：

我正在尝试编写一个计算数组中反转的程序，但由于引用问题，我的数组没有正确排序，因此即使我认为切片是通过 Golang 中的引用传递的，我的计数也会混乱。

这是我的代码：

package main

import (
    "fmt"
)

func InversionCount(a []int) int 
    if len(a) <= 1 
        return 0
    
    mid := len(a) / 2
    left := a[:mid]
    right := a[mid:]
    leftCount := InversionCount(left) //not being sorted properly due to reference issues 
    rightCount := InversionCount(right) //not being sorted properly due to reference issues

    res := make([]int, 0, len(right)+len(left)) //temp slice to hold the sorted left side and right side

    iCount := mergeCount(left, right, &res)

    a = res        //assigns the original slice with the temp slice values
    fmt.Println(a) //a in the end is not sorted properly for most cases 
    return iCount + leftCount + rightCount


    func mergeCount(left, right []int, res *[]int) int 
        count := 0

        for len(left) > 0 || len(right) > 0 
            if len(left) == 0 
                *res = append(*res, right...)
                break
            
            if len(right) == 0 
                *res = append(*res, left...)
                break
            
        if left[0] <= right[0] 
            *res = append(*res, left[0])
            left = left[1:]
         else  //Inversion has been found
            count += len(left)
            *res = append(*res, right[0])
            right = right[1:]
        
    

    return count


func main() 
    test := []int4,2,3,1,5
    fmt.Print(InversionCount(test))

解决此问题的最佳方法是什么？我试图通过强制mergeCount函数接受数组的引用来做类似于我对res数组所做的事情，但它看起来很混乱，它会给我带来错误。

【参考方案1】：

您要么必须将指针传递给切片，例如：

func InversionCount(a *[]int) int 
    if len(*a) <= 1 
        return 0
    
    mid := len(*a) / 2
    left := (*a)[:mid]
    right := (*a)[mid:]
    leftCount := InversionCount(&left)   //not being sorted properly due to reference issues
    rightCount := InversionCount(&right) //not being sorted properly due to reference issues

    res := make([]int, 0, len(right)+len(left)) //temp slice to hold the sorted left side and right side

    iCount := mergeCount(left, right, &res)

    *a = res
    fmt.Println(a) //a in the end is not sorted properly for most cases
    return iCount + leftCount + rightCount

playground

或使用copy 并将a = res 更改为copy(a, res)。

playground

【讨论】：

@DavidTrinh 记得投票并标记对您有帮助的答案。

【参考方案2】：

与其对切片进行变异，不如让函数返回在合并步骤中获得的切片。

这是这种形式的代码，包括一些类似单元测试的代码，它将有效版本与天真的 O(N^2) 计数进行比较。

package main

import "fmt"

// Inversions returns the input sorted, and the number of inversions found.
func Inversions(a []int) ([]int, int) 
    if len(a) <= 1 
        return a, 0
    
    left, lc := Inversions(a[:len(a)/2])
    right, rc := Inversions(a[len(a)/2:])
    merge, mc := mergeCount(left, right)
    return merge, lc + rc + mc


func mergeCount(left, right []int) ([]int, int) 
    res := make([]int, 0, len(left)+len(right))
    n := 0
    for len(left) > 0 && len(right) > 0 
        if left[0] >= right[0] 
            res = append(res, left[0])
            left = left[1:]
         else 
            res = append(res, right[0])
            right = right[1:]
            n += len(left)
        
    
    return append(append(res, left...), right...), n


func dumbInversions(a []int) int 
    n := 0
    for i := range a 
        for j := i + 1; j < len(a); j++ 
            if a[i] < a[j] 
                n++
            
        
    
    return n


func main() 
    cases := [][]int
        ,
        1,
        1, 2, 3, 4, 5,
        2, 1, 3, 4, 5,
        5, 4, 3, 2, 1,
        2, 2, 1, 1, 3, 3, 4, 4, 1, 1,
    
    for _, c := range cases 
        want := dumbInversions(c)
        _, got := Inversions(c)
        if want != got 
            fmt.Printf("Inversions(%v)=%d, want %d\n", c, got, want)