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stl sort function

MyOnlinePersona (66)
I am going back over older code that I had written before and I have noticed something that I dont actually understand. Code below. How is the sort and compare function working in this. It seems to be returning true when the inputs are a small number and then a large number, indicating that the smaller number is larger than the two. The program prints the numbers in reverse, so what I am assuming happens is not what actually is happening, but i cant seem to figure out how exactly it is working. 

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  #include <iostream>
#include <vector>
#include <algorithm>

int main()
{
	std::vector<int>nums = { 1, 2, 3, 4, 5 };

	

	auto funct = []()
	{
		std::cout << "Lambda function\n";
	};

	funct();

	auto compare = [](int const &a, int const &b)
	{
		return a > b;	
	};

	sort(nums.begin(), nums.end(), compare);

	for (auto i = nums.begin(); i < nums.end(); i++)
	{
		std::cout << *i << '\n';
	}

}
Ganado (6809)
(Somebody else feel free to also reply in case I'm missing the question)
std::sort takes in a "compare" function (or function-like object) that it calls while doing the sorting. Normally, this compare function is expected to implement the logic of "a < b", which is used to sort things in ascending order. But if you instead implement "a > b", you'll get the opposite effect, and things will be sorted in descending order.

Here's a simple example that sorts just two items, by calling the compare function a single time (empty if statement just for clarity).
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// Example program
#include <iostream>
#include <algorithm>
#include <functional>

template <typename T, typename Func>
void sort_two_items(T& first, T& second, Func compare)
{
    if (compare(first, second))
    {
        // a < b
        // assume items are already in order   
    }
    else
    {
        // a > b
        // swap them
        std::swap(first, second);
    }
}


int main()
{
    int a = 42;
    int b = 36;
    std::cout << "a = " << a << ", b = " << b << "\n\n";
    
    std::cout << "Sorting in ascending order...\n";
    sort_two_items(a, b, std::less<>{});
    std::cout << "a = " << a << ", b = " << b << '\n';
    
    std::cout << "Sorting in descending order...\n";
    sort_two_items(a, b, std::greater<>{});
    std::cout << "a = " << a << ", b = " << b << '\n';
}

a = 42, b = 36

Sorting in ascending order...
a = 36, b = 42
Sorting in descending order...
a = 42, b = 36
Last edited on
mbozzi (3932)
The function you pass to sort should return true if its first argument is ordered-before its second argument. "Ordered before" isn't necessarily the same as "less than".
Last edited on
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