<algorithm> sort custom condition

Question

Ok, so I tried using sorting for the element vector, so the size of the two additive elements is <= 2d. So here is my attempt:

struct item{
    long number;
    long size;
};

// d is global variable.
bool check(const item& x, const item& y)
{
    return ((x.size + y.size) <= (2 * d));
}

// Items is a vector of item.
sort(items.begin(), items.end(), check); 

What am I doing wrong or even impossible to sort using this condition?

Answer 1

is it even impossible to sort using such a condition?

No. The comparator sortmust satisfy the criteria of strict weak ordering , which your explicitly does not do (for example, it is not non-reflective).

Answer 2

O (N log N) . , NP-, . , , , , , . : , .

. , O (N log N), , .

, , , size 10 * d.

Answer 3

sort().

STL . "" , :

• (A, B) == false AND A!= B, (B, A) true.
• if check (A, B) == false AND check (B, C) == false AND A, B, C are different, then check (A, C) returns false.

A good idea to use STL sort () is, given your list of S elements and the order you want:

• If the order of elements in S is changed, the order of output should remain unchanged.
• The output is unique.
• All items in the output order have some relation, which is a strict partial order relation .

If so, then you can probably write a validation function to work for you :)