Function defined in scope but the compiler complains that it is out of scope

I tried to implement strassens algorithm for two 2x2 matrices in order to make a recursive matrix multiplication algorithm however the implementation doesn't compile giving me errors like:

"strassen was not declared in this scope" and "unqualified-id"

Here's the code:

#include <iostream>
#include <cstdlib>

using namespace std;

int[][] strassen(int A[][2], int B[][2])
{
    int s1 = B[0][1] - B[1][1];
    int s2 = A[0][0] + A[0][1];
    int s3 = A[1][0] + A[1][1];
    int s4 = B[1][0] - B[0][0];
    int s5 = A[0][0] + A[1][1];
    int s6 = B[0][0] + B[1][1];
    int s7 = A[0][1] - A[1][1];
    int s8 = B[1][0] + B[1][1];
    int s9 = A[0][0] - A[1][0];
    int s10 = B[0][0] + B[0][1];

    int p1 = A[0][0] * s1;
    int p2 = s2 * B[1][1];
    int p3 = s3 * B[0][0];
    int p4 = A[1][1] * s4;
    int p5 = s5 * s6;
    int p6 = s7 * s8;
    int p7 = s9 * s10;

int C[2][2];

C[0][0] = p5 + p4 - p2 + p6;
C[0][1] = p1 + p2;
C[1][0] = p3 + p4;
C[1][1] = p5 + p1 - p3 - p7;

return C[][];
}

int main()
{
    int A[2][2] = {{1,3},{7,5}};
    int B[2][2] = {{6,8},{4,2}};
    int C[][2] = strassen(A,B);
    cout<<C[0][0]<<endl<<C[0][1]<<endl<<C[1][0]<<endl<<C[1][1]<<endl;
    return 0;
}

Could you tell me why I'm getting the compile time errors. I also need to know how to malloc space for a 2D array, as my current implementation of C will go out of scope as soon as the function exits returning garbage values.

There are a couple of reasons why your code doesn't compile: You are getting the error of function out of scope because the function strassen is not compiling, and it is not compiling because you are returning an array declared inside the function.

A good rule of thumb is to never return arrays nor pass them as arguments, use references instead, it saves memory and time.

Heres is a solution without using dynamic memory (although I think it would be easier to do it that way)

#include <iostream>

using namespace std;

void strassen(int (&A)[2][2], int (&B)[2][2], int (&C)[2][2])
{
    int s1 = B[0][1] - B[1][1];
    int s2 = A[0][0] + A[0][1];
    int s3 = A[1][0] + A[1][1];
    int s4 = B[1][0] - B[0][0];
    int s5 = A[0][0] + A[1][1];
    int s6 = B[0][0] + B[1][1];
    int s7 = A[0][1] - A[1][1];
    int s8 = B[1][0] + B[1][1];
    int s9 = A[0][0] - A[1][0];
    int s10 = B[0][0] + B[0][1];

    int p1 = A[0][0] * s1;
    int p2 = s2 * B[1][1];
    int p3 = s3 * B[0][0];
    int p4 = A[1][1] * s4;
    int p5 = s5 * s6;
    int p6 = s7 * s8;
    int p7 = s9 * s10;

    C[0][0] = p5 + p4 - p2 + p6;
    C[0][1] = p1 + p2;
    C[1][0] = p3 + p4;
    C[1][1] = p5 + p1 - p3 - p7;

}

int main()
{
    int A[2][2] = {{1,3},{7,5}};
    int B[2][2] = {{6,8},{4,2}};
    int C[2][2];

    strassen(A,B,C);

    cout<<C[0][0]<<endl<<C[0][1]<<endl<<C[1][0]<<endl<<C[1][1]<<endl;

    return 0;
}

Note that you are passing C as a reference to the function so the changes that you make to it inside the function will also affect it outside the function