To find intersection of two arrays in C++

Question

Here my code:

  void intersection(int input1[], int input2[], int size1, int size2) {
    unordered_map<int,int> mymap;
    for(int i=0;i<size1;i++){
        if(!mymap.count(input1[i]))
            mymap[input1[i]] = 1;
        else
            mymap[input1[i]]++;
    }

    for(int i=0; i<size2; i++){
        if(mymap.count(input2[i]) > 0){
            cout<<input2[i]<<endl;
            mymap[input2[i]]--;  //Line1
        }   
    }
}

I tried decreasing the "key" in "Line1" but not getting correct output. Sample Input:

size1: 6

array1: 2 6 8 5 4 3

size2: 7

array2: 2 2 3 4 7 4 3

Your Output: 2 2 3 4 4 3

Expected Output: 2 3 4

Answer 1

Underlying logic is correct, but syntax is wrong:

mymap.count(x) returns the number of time key is present. You want mymap[x] (or use a std::multi_set ).

Line1 should then be:

mymap[input2[i]]--;

In addition, your first loop (which has similar error BTW) can be simplified to:

std::unordered_map<int,int> mymap;
for(int i=0;i<size1;i++){
    mymap[input1[i]]++;
}

So final code:

void intersection(int input1[], int input2[], int size1, int size2) {
    std::unordered_map<int,int> mymap;
    for(int i=0;i<size1;i++){
        mymap[input1[i]]++;
    }
    for(int i=0; i<size2; i++){
        if (mymap[input2[i]] > 0){
            cout << input2[i] << endl;
            mymap[input2[i]]--;
        }   
    }
}