对vector模板进行排序的方法——sort, stable_sort, qsort

- 日理万妓 2022-01-11 02:47 267阅读 0赞

I am trying to sort the "sth" integer value within the vector of class Entry using qsort. The code for the same is as follows. But after applying qsort, also the values remain the same. When I tried to print the values in cmpfunc2(), I found that 0 0 is getting printed.

#include<iostream>
    #include<vector>
    #include<algorithm>
    using namespace std;
    class Entry{public:
    int id;
    int sth;
    
    Entry(int,int);
    };
     Entry::Entry(int a,int b){
      id=a;
      sth=b;
    }
    int cmpfunc2 (const void * a, const void * b)
    {
       cout<<(*(Entry *)a).sth<<"  "<<(*(Entry *)b).sth <<endl;
       return ( (*(Entry*)a).sth - (*(Entry*)b).sth );
    }
    int main(){
    vector<Entry> entries;
    
    entries.push_back(Entry(2,3));
    entries.push_back(Entry(21,14));
    entries.push_back(Entry(54,12));
    
    qsort(&entries, entries.size(),sizeof(Entry),cmpfunc2);
      for(int i=0;i<entries.size();i++)
      cout<<entries[i].sth<<endl;
    }

Solution:

Change this statement

qsort(&entries, entries.size(),sizeof(Entry),cmpfunc2);

qsort( entries.data(),  entries.size(), sizeof( Entry ), cmpfunc2 );
    qsort( &entries[0],  entries.size(), sizeof( Entry ), cmpfunc2 );

However it would be better to use standard algorithm `std::sort` declared in header `<algorithm>`. For example

#include <algorithm>
    
    //...
    
    bool cmpfunc2 ( const Entry &a, const Entry &b )
    {
       return ( a.sth < b.sth );
    }
    
    // ...
    
    std::sort( entries.begin(), entries.end(), cmpfunc2 );

Or you could use `std::stable_sort`

std::stable_sort( entries.begin(), entries.end(), cmpfunc2 );

Also consider the posiibility to compare objects of type `Entry` as objects of type `std::pair`. In this case objects with equal sth will be sorted according to id. For example

#include <algorithm>
    #include <utility>
    
    //... 
    
    bool cmpfunc2 ( const Entry &a, const Entry &b )
    {
       return ( std::make_pair( a.sth, a.id )  < std::make_pair( b.sth, b.id ) );
    }