C++set intersection and union using arrays

Question

I've to find cardinality of set union and set intersection of two sets from a data file. I've created two arrays(setA[] and setB[]) to store my data. a and b are the number of elements in set A and set B respectively. setIntersection should hold the result of intersection between set A and B. But I'm stuck at how should I find the union and intersection.

int printIntersection(int setA[], int setB[], int setIntersection[], int a, int b, int k) 
{
 int i = 0;
 int j = 0;

 while(i < a && j < b)
 {
     if(setA[i] < setB[j])
     {
         i++;
     }

     else if(setA[i] > setB[j])
     {
         j++;
     }

     else if (setA[i] == setB[j])
     {
         setIntersection[k] = setA[i];
         i++;
         j++;
         k++;
     }
     cout<<"Cardinality of intersection is "<<k<<endl;
 }

This code is for intersection but I'm not getting anything. And I don't know where to start on union. can anyone help me out with the code thanks! P.S I'm only allowed to use arrays and simple code algorithm. Thanks in advance!

Answer 1

Use std::set_union and std::set_intersection, e.g.

int *c = std::set_union(setA, setA + a, setB, setB + b, setC)
int *c = std::set_intersection(setA, setA + a, setB, setB + b, setC)

where setC is an output array of enough size; c points to one past the last element of the constructed range.

If you want cardinalities, c - setC is your answer.

I would recommend using something like std::vector for representation, and iterators instead of arrays/pointers/lengths in algorithm calls.

Input ranges are assumed sorted; so is the output.

If you want to do it yourself, you'll find "possible implementations" at the links above.