confusion about using std::less and std::greater with std::sort

Question

In C, sort usually implements as in the following example:

#include <stdio.h>

void Sort( int* arr, int n, bool(*cmp)(int,int) )
{
    for( int i=0; i<n-1; i++ )
    {
        for( int j=i+1; j<n; j++ )
        {
            if( cmp(arr[i], arr[j]) )
                swap( arr[i], arr[j] );
        }
    }
}

int ascending( int a, int b ) { return a > b; }    // greater
int descending( int a, int b ) { return a < b; }   // less

void main()
{
    int arr[10] = { 1,3,5,7,9,2,4,6,8,10 };

    // ascending
    Sort( arr, 10, ascending );
    for( int i=0; i<10; i++ )
        printf( "%d ", arr[i] );

    printf( "\n" );


    // descending
    Sort( arr, 10, descending );
    for( int i=0; i<10; i++ )
        printf( "%d ", arr[i] );

    printf( "\n" );
}

So I wrote some source as in the following example, expecting same result:

#include <iostream>
#include <algorithm>    // for sort
#include <functional>   // for less & greater
using namespace std;

bool gt( int a, int b ) { return a > b; }   // greater
bool ls( int a, int b ) { return a < b; }   // less

void main()
{
    int x[10] = { 1,3,5,7,9,2,4,6,8,10 };

    // ascending but descending
    sort( x, x+10, gt );
    for( int i=0; i<10; i++ )
        cout << x[i] << " ";

    cout << endl;

    // descending but ascending
    sort( x, x+10, ls );
    for( int i=0; i<10; i++ )
        cout << x[i] << " ";

    cout << endl;


    greater<int> g; // a > b
    less<int> l;    // a < b

    // ascending but descending
    sort( x, x+10, g );
    for( int i=0; i<10; i++ )
        cout << x[i] << " ";

    cout << endl;

    // descending but ascending
    sort( x, x+10, l );
    for( int i=0; i<10; i++ )
        cout << x[i] << " ";

    cout << endl;
}

But my expectation was not correct.

Why does not sort in STL work like sort in C?

Solution 1

std::sort behaves like that because it's based on the idea of a strict weak ordering, which is (usually) defined in terms of the < operator.

As to your question; it currently seems to be "I wrote a C function that behaves differently to std::sort. Why is it different?". The answer is: because you wrote a different function!

Solution 2

std::sort sorts in ascending order by default. In case you are looking for descending order, here's the trick:

int x[10] = { 1,3,5,7,9,2,4,6,8,10 };
std::vector<int> vec(x, x+10);          // construct std::vector object
std::sort(vec.rbegin(),vec.rend());     // sort it in reverse manner

This way, you explicitly say that std::sort should treat your array as its end is its beginning and vice versa, which results in your array being sorted in descending order. Here's the full example.

---

And in case you want to use std::less and std::greater, then it could look like this:

int x[10] = { 1,3,5,7,9,2,4,6,8,10 };
std::sort(x, x + 10, std::less<int>());     // for ascending order
std::sort(x, x + 10, std::greater<int>());  // for descending order

Full example with second solution is here.