Modify make_heap in C++ algorithm STL to work as a minimum heap

Question

I am sure I am making some really silly mistake but haven't been able to point it out. The make_heap function in the algorithm STL of C++ has the syntax

void make_heap(v.begin(), v.end(),Compare comp ); where v is declared as an int vector. Now the comp input defaults to < but I want to change it so that I can use it to make both min and max heaps. Looking at other examples what makes sense to me is doing something like,

void make_heap(v.begin(), v.end(),std::less<int> );

OR

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

but I keep on getting the compiler error expected primary expression before ')' token

I can't figure out what am I doing wrong? Any help?

You're passing a type for the third parameter. It needs to be an object instance of a type fulfilling the comparator requirements. Put some () on the end of that thing. And the void make no sense at all. — WhozCraig
– WhozCraig, Commented Apr 8, 2017 at 3:12
You are missing a pair of parentheses after std::less<int>() — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Apr 8, 2017 at 3:12

Jonas · Accepted Answer · 2017-04-08 04:48:11Z

Put () at the end

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

Reason

priority_queue signature -

template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue;

So, it takes a Compare class (not Compare class object) as third argument. So priority_queue<int, vector<int>, greater<int>()> Q; will yield compilation error because you need to pass a Compare class (a binary predicate). Correct syntax is -

priority_queue<int, vector<int>, greater<int>> Q;

make_heap signature is -

template <class RandomAccessIterator, class Compare> void make_heap (RandomAccessIterator first, RandomAccessIterator last, Compare comp );

which take Compare class object. So make_heap(v.begin(), v.end(), std::greater<int>()); will give compilation error. You need to a Compare class object (a binary function) here.

Hope it helps!

Collectives™ on Stack Overflow

Modify make_heap in C++ algorithm STL to work as a minimum heap

1 Answer 1

Reason

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Reason

Comments

Related