How to declare an stl::priority_queue with initial data and custom comparator?

Question

I need a priority_queue for my structs. I want to construct one using both initial data from an array and a custom comparator:

class mycomparison
{
public:
    mycomparison(const bool& maxOnTop=false, const int& compField = 0);
    ...
};

mergeRecord initialRecords[N]; // my array
mycomparison(false, field); // my custom comparator

std::priority_queue<mergeRecord, std::vector<mergeRecord>, mycomparison>
    recsHeap(???);

On the cplusplus reference, there is an example of how to initialize a priority_queue object with a comparator OR an array with initial values. How would I do both? I know I could insert them one by one, but this would cost O(nlogn) whereas passing them all at once is O(n).

Answer 1

The range constructor for std::priority_queue allows you to pass a comparator function in as an optional argument:

template <class InputIterator>
priority_queue (InputIterator first, InputIterator last,
                const Compare& comp = Compare(),
                const Container& ctnr = Container());

So just call it like this:

std::priority_queue<mergeRecord,std::vector<mergeRecord>,mycomparison>
    recsHeap(std::begin(initialRecords), std::end(initialRecords), mycomparison(false,field));

Answer 2

In below example std::greater<int> is the compare function and myints is an integer array. We can create a priority_queue my_priority_queue as follows:

int myints[]= {10,60,50,20};
std::priority_queue<int, std::vector<int>, std::greater<int> >
                            my_priority_queue(myints,myints+4);

In your example:

priority_queue<mergeRecord,std::vector<mergeRecord>,mycomparison>
 recsHeap (mergeRecord, mergeRecord+ kWayMerge,mycomparison(false,field));