two arrays, find the missing numbers

Your program seems to be outputting the correct result. However, I felt that I need to refactor your code to improve its readability and remove the bad practices used in it.

The below is the same as your code with a bit of improvement:

#include <iostream>
#include <array>
#include <limits>


int main( )
{
    std::array<int, 1201> arr1; // use std::array instead of raw arrays
    std::array<int, 1201> arr2;

    std::size_t arr1_size { }; // renamed n
    std::size_t arr2_size { }; // renamed m

    std::cin >> arr1_size >> arr2_size;

    if ( arr2_size == 0 ) // this if statement should be here to help end
    {                     // the program early on to prevent the execution
                          // of the for-loops
        std::cout << "There are no missing numbers.\n";
        return 0;
    }

    for ( std::size_t idx { }; idx < arr1_size; ++idx ) // use std::size_t
    {                                                   // for the loop counters
        std::cin >> arr1[ idx ];
    }

    for ( std::size_t idx { }; idx < arr1_size - arr2_size; ++idx )
    {
        std::cin >> arr2[ idx ];
    }

    for ( std::size_t arr1_idx { }; arr1_idx < arr1_size; ++arr1_idx )
    {
        bool isNumberMissing { true }; // this should be of type bool

        for ( std::size_t arr2_idx { }; arr2_idx < arr1_size - arr2_size; ++arr2_idx )
        {
            if ( arr1[ arr1_idx ] == arr2[ arr2_idx ] )
            {
                isNumberMissing = false;

                // this is my trick for solving your code's bug
                arr2[ arr2_idx ] = std::numeric_limits<int>::min( );

                break; // break here to improve performance
            }
        }

        if ( isNumberMissing )
        {
            std::cout << arr1[ arr1_idx ] << " ";
        }
    }

    std::cout << '\n';
}

Sample input/output #1:

7 3
12 34 45 29 100 87 32
100 87 12 34
45 29 32

Sample input/output #2:

7 3
2 6 1 9 3 2 4
4 1 2 3
6 9 2

Note: See Why is "using namespace std;" considered bad practice?