How to find indexes of 5 the biggest elements in vector?

Question

How to find indexes of 5 the biggest elements in vector ? For example std::vector<int> how to find indexes of 5 biggest values but not to change original vector ?

Answer 1

std::partial_sort( v.begin(), v.begin()+5, v.end() ) sorts a vector in a way, that the 5 smallest values are sorted and at the beginning of v. The rest is left unsorted.

Since you want the indices and keep the original:
Fill a new vector with numbers from 0..n-1 and supply a comparison function that does v[a] > v[b] instead of a > b:

struct Comp{
    Comp( const vector<int>& v ) : _v(v) {}
    bool operator ()(int a, int b) { return _v[a] > _v[b]; }
    const vector<int>& _v;
}

vector<int> vx;
vx.resize(v.size());
for( int i= 0; i<v.size(); ++i ) vx[i]= i;
partial_sort( vx.begin(), vx.begin()+5, vx.end(), Comp(v) );

vx[0..4] contains the indices.