比较C ++ Stl排序中的函数

Question

I need to obtain an non-increasing order from a given random ordered elements.After working a lot, I realized that both the following codes produce different outputs. Why the codes produce different outputs?

bool cmp(int i,int j)
{
    if(i<j)
        return 0;
    return 1;
}
bool cmp(int i,int j)
{
   return i>j;
}

Answer 1

First function should be

bool cmp(int i,int j)
{
    if(i>j)
        return 1;
    return 0;
}

to get equivalent outputs.

Or alternatively use the following function:

bool cmp(int i,int j)
{
    return -1 * i < -1 * j;
}

Answer 2

First things first:

Your two functions produce different results when they have the same input. So cmp(1,1) produces different results.

Undefined behavior:

Given this cmp() function:

bool cmp(int i,int j) {
    if(i<j)
        return 0;
    return 1;
}

The following statement will produce undefined behavior :

std::vector<int> x;
std::sort(x.begin(), x.end(), cmp);

The comparator passed to std::sort is required to implement Compare . One of the requirements is that cmp(1,1) will return false. Your code returns true.

Answer 3

Your first function is equivalent to "not less than" ie "greater or equal" when what you want is "greater". Also, given that this is C++11, why not just do:

std::sort(v.begin(), v.end(), std::greater<int>());