错误的合并功能（归并排序） [英] Error in the Merge Function(Merge Sort)

问题描述

我想通过CLRS的算法书中提到实现合并排序算法。我想出了以下code。

I am trying to implement the merge sort algorithm as mentioned in the algorithms book by CLRS. I have come up with the following code.

#include<stdio.h>
#include<stdlib.h>
void merge_sort(int *arr,int start_index,int end_index);
void merge(int *arr,int start_index,int middle_index,int end_index);

int main(){

int arr[]={5,2,1,6,0,3,3,4}; //8 elements last index 7
int i;
printf("Before sorting.\n");
for(i=0;i<8;i++)
printf("%d,",arr[i]);
merge_sort(arr,0,7);
printf("\nAfter sorting.\n");
for(i=0;i<8;i++)
printf("%d,",arr[i]);

return 0;}

void merge_sort(int *arr,int start_index,int end_index){
    int middle_index;
    if(start_index<end_index)
    {
        middle_index=(start_index+end_index)/2;
        merge_sort(arr,start_index,middle_index);
        merge_sort(arr,(middle_index+1),end_index);
        merge(arr,start_index,middle_index,end_index);
    }

}

void merge(int *arr, int start_index,int middle_index, int end_index){

    int n1,n2,i,l,m;
    n1=middle_index-start_index+1;
    n2=end_index-middle_index;
    int sub_arr1[n1+1],sub_arr2[n2+1];
    for(i=0;i<=(n1-1);i++)
        sub_arr1[i]=arr[i];
     for(i=0;i<=(n2-1);i++)
        sub_arr2[i]=arr[middle_index+1+i];

    sub_arr1[n1]=100;
    sub_arr2[n2]=100;
        l=0,m=0;
    for(i=0;i<=end_index;i++){


        if(sub_arr1[l]<sub_arr2[m])
        {arr[i]=sub_arr1[l];
        l=l+1;}
        else
         {arr[i]=sub_arr2[m];
        m=m+1;}

    }}
There seems to be some error in the merge function due to which I am getting an erroneous output which is as follows.

Before sorting.
5,2,1,6,0,3,3,4,
After sorting.
2,4,100,-1076668400,2,4,100,7,
RUN FINISHED; exit value 0; real time: 0ms; user: 0ms; system: 0ms

因为，我用这种方法排序数量也少，我用100作为一个警戒值。能这样错误的可能来源？任何帮助AP preciated。

Since the numbers that I am sorting with this method are small, I have used 100 as a sentinel value. Can that be a possible source of error? Any help appreciated.

推荐答案

您有您的合并功能的错误：你所忽略的START_INDEX。这些行：

You have an error in your merge function: you are ignoring start_index. These lines:

for(i=0;i<=(n1-1);i++)
    sub_arr1[i]=arr[i];

应改为：

for(i=0;i<=(n1-1);i++)
    sub_arr1[i]=arr[start_index+i];

和最后一个for循环，应该，而是开始与我= START_INDEX：

And the last for loop, should, instead, start with i = start_index:

for (i=start_index;i<=end_index;i++) { /* ... */ }

我缩进提高一点点。下面是合并的最后工作版本（）：

I improved indentation a little bit. Here's the final working version of merge():

void merge(int *arr, int start_index,int middle_index, int end_index) {
    int n1,n2,i,l,m;
    n1=middle_index-start_index+1;
    n2=end_index-middle_index;
    int sub_arr1[n1+1],sub_arr2[n2+1];
    for (i=0;i<=(n1-1);i++)
        sub_arr1[i]=arr[start_index+i];
     for (i=0;i<=(n2-1);i++)
        sub_arr2[i]=arr[middle_index+1+i];
    sub_arr1[n1]=100;
    sub_arr2[n2]=100;
    l=0,m=0;
    for (i=start_index;i<=end_index;i++) {
        if(sub_arr1[l]<sub_arr2[m]) {
            arr[i]=sub_arr1[l];
            l=l+1;
        }
        else {
            arr[i]=sub_arr2[m];
            m=m+1;
        }
    }
}

测试和工作。

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