Strassen multiplication - c program - c

Question

We had to implement strassen's multiplication in a practical session , the code I wrote is given below , as you can see I have used a lot of intermediate matrices .I wanted to know how to return a 2d array from a function so that the code will look cleaner and more understandable , also it will give me some insights on pointers(a weak area for me )i.e say is use a double pointer as return type of sub function (int **sub(args list))
and since my strassen function has prototype strassen(int , int [],int[]..)
When one argument of strassen function is a result of sub function , I get an error saying int (*)[] expected but returning int **
To resolve this I typecasted the result of sub function with int (*)[] but it does not work as expected
Help please ? Thanks !
#include<stdio.h>
#include<stdlib.h>
void add(int n,int a[n][n],int b[n][n],int result[][n])
{
printf("---add---\n");
int i,j;
//int **result = (int **)malloc(n*sizeof(int *));
/*for(i=0;i<n;i++)
result[i] = (int *)malloc(n*sizeof(int));*/
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
result[i][j] = a[i][j] + b[i][j];
printf("%d\t",result[i][j]);
}
printf("\n");
}
//return result;
}
void sub(int n,int a[n][n],int b[n][n],int result[][n])
{
printf("---sub---\n");
int i,j;
/*int **result = (int **)malloc(n*sizeof(int *));
for(i=0;i<n;i++)
result[i] = (int *)malloc(n*sizeof(int));*/
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
result[i][j] = a[i][j] - b[i][j];
printf("%d\t",result[i][j]);
}
}
}
void divide(int n,int a[n][n],int c[n/2][n/2],int i,int j)
{
int i1,i2,j1,j2;
for(i1=0,i2=i;i1<n/2;i1++,i2++)
{
for(j1=0,j2=j;j1<n/2;j1++,j2++)
{
c[i1][j1] = a[i2][j2];
}
}
}
void join(int n,int a[][n],int c[][n/2],int i,int j)
{
printf("join\n");
int i1,i2,j1,j2;
for(i1=0,i2=i;i1<(n/2);i1++,i2++)
{
for(j1=0,j2=j;j1<(n/2);j1++,j2++)
{
a[i2][j2] = c[i1][j1];
printf("c[%d][%d] %d\n",i1,j1,c[i1][j1]);
}
}
}
void multiply(int n,int a[][n],int b[][n],int result[][n])
{
int i,j;
if(n==2)
{
//partial products
printf("base case\n");
int p1 = (a[0][0]+a[1][1])*(b[0][0]+b[1][1]);
int p2 = (a[1][0]+a[1][1])*b[0][0];
int p3 = a[0][0]*(b[0][1]-b[1][1]);
int p4 = a[1][1]*(b[1][0]-b[0][0]);
int p5 = (a[0][0]+a[0][1])*b[1][1];
int p6 = (a[1][0]-a[0][0])*(b[0][0]+b[0][1]);
int p7 = (a[0][1]-a[1][1])*(b[1][0]+b[1][1]);
int c11 = p1 + p4 - p5 + p7;
int c12 = p3 + p5;
int c21 = p2 + p4;
int c22 = p1 + p3 - p2 + p6;
result[0][0] = c11;
result[0][1] = c12;
result[1][0] = c21;
result[1][1] = c22;
for(i=0;i<2;i++)
{
for(j=0;j<2;j++)
{
printf("%d\t",result[i][j]);
}
printf("\n");
}
}
else
{
int a11[n/2][n/2];
int a12[n/2][n/2];
int a21[n/2][n/2];
int a22[n/2][n/2];
int b11[n/2][n/2];
int b12[n/2][n/2];
int b21[n/2][n/2];
int b22[n/2][n/2];
//divide matrices A & B into four parts
divide(n,a,a11,0,0);
divide(n,a,a12,0,n/2);
divide(n,a,a21,n/2,0);
divide(n,a,a22,n/2,n/2);
divide(n,b,b11,0,0);
divide(n,b,b12,0,n/2);
divide(n,b,b21,n/2,0);
divide(n,b,b22,n/2,n/2);
//partial products
int p1[n/2][n/2],p2[n/2][n/2],p3[n/2][n/2],p4[n/2][n/2],p5[n/2][n/2],p6[n/2][n/2],p7[n/2][n/2];
int c11[n/2][n/2],c12[n/2][n/2],c21[n/2][n/2],c22[n/2][n/2];
int i1[n/2][n/2],i2[n/2][n/2];
add(n/2,a11,a22,i1);
add(n/2,b11,b22,i2);
multiply(n/2,i1,i2,p1);
int i3[n/2][n/2];
add(n/2,a21,a22,i3);
multiply(n/2,i3,b11,p2);
int i4[n/2][n/2];
sub(n/2,b12,b22,i4);
multiply(n/2,a11,i4,p3);
int i5[n/2][n/2];
sub(n/2,b21,b11,i5);
multiply(n/2,a22,i5,p4);
int i6[n/2][n/2];
add(n/2,a11,a12,i6);
multiply(n/2,i6,b22,p5);
int i7[n/2][n/2];
int i8[n/2][n/2];
sub(n/2,a21,a11,i7);
add(n/2,b11,b12,i8);
multiply(n/2,i7,i8,p6);
int i9[n/2][n/2];
int i10[n/2][n/2];
sub(n/2,a12,a22,i9);
add(n/2,b21,b22,i10);
multiply(n/2,i9,i10,p7);
//for c11
int r1[n/2][n/2];
int r2[n/2][n/2];
add(n/2,p1,p4,r1); //sub operation
sub(n/2,r1,p5,r2); //sub operation
add(n/2,r2,p7,c11); //main operation
//for c12
add(n/2,p3,p5,c12);
//for c21
add(n/2,p2,p4,c21);
//for c22
int r3[n/2][n/2];
int r4[n/2][n/2];
add(n/2,p1,p3,r3); //sub operation
sub(n/2,r3,p2,r4); //sub operation
add(n/2,r4,p6,c22); //main operation
join(n,result,c11,0,0);
join(n,result,c12,0,n/2);
join(n,result,c21,n/2,0);
join(n,result,c22,n/2,n/2);
printf("---c11---\n");
for(i=0;i<n/2;i++)
{
for(j=0;j<n/2;j++)
{
printf("%d\t",c11[i][j]);
}
printf("\n");
}
printf("---c12---\n");
for(i=0;i<n/2;i++)
{
for(j=0;j<n/2;j++)
{
printf("%d\t",c12[i][j]);
}
printf("\n");
}
printf("---c21---\n");
for(i=0;i<n/2;i++)
{
for(j=0;j<n/2;j++)
{
printf("%d\t",c21[i][j]);
}
printf("\n");
}
printf("---c22---\n");
for(i=0;i<n/2;i++)
{
for(j=0;j<n/2;j++)
{
printf("%d\t",c22[i][j]);
}
printf("\n");
}
/*for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
printf("%d\t",result[i][j]);
}
printf("\n");
}*/
}
}
int main()
{
int n;
printf("Enter the order of the matrices(power of 2)\n");
scanf("%d",&n);
int i,j;
int a[n][n],b[n][n];
printf("Enter first matrix\n");
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
scanf("%d",&a[i][j]);
}
}
printf("Enter second matrix\n");
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
scanf("%d",&b[i][j]);
}
}
printf("First matrix is \n");
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
printf("%d\t",a[i][j]);
}
printf("\n");
}
printf("Second matrix is \n");
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
printf("%d\t",b[i][j]);
}
printf("\n");
}
int r[n][n];
multiply(n,a,b,r);
printf("---RESULT OF MULTIPLICATION---\n");
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
printf("%d\t",r[i][j]);
}
printf("\n");
}
return 0;
}