Thread: why this code is not working? i wanted to multiply a matrix by itself

What is the test input, expected output and actual output?

Note that your indentation is misleading. It should be:

Code:

#include <stdio.h>
#include<conio.h>

int main()
{
    int i,j;
    int array1[3][3]= {{0,0,0},{0,12,36},{0,32,56}} ;
    int sqr [3][3];
    for(i=1;i<=2;i++)
    {
        for(j=1;j<=2;j++)
            sqr[i][j] = array1[i][j]*array1[i][j];

        printf("%d\t",sqr[i][j]);
    }
    getch();
}

I recommend that you use braces for if statements and loops even when unnecessary.

expected output :
144 1296
1024 3136


actual output : 2686728 0

Your multiplication appears to be correct. You just need to print at the right places. Take my advice concerning the braces first.

By the way, are you sure that your expected output is correct? Given two 3x3 matrices A and B, AB is also a 3x3 matrix.

> for(i=1;i<=2;i++)
I suppose the other point is that subscripts start at 0, not 1

Code:

#include <stdio.h>
#include<conio.h>

int main()
{
    int i,j;
    int array1[3][3]= {{0,0,0},{0,12,36},{0,32,56}} ;
    int sqr [3][3];
    for(i=1;i<=2;i++)
    {
        for(j=1;j<=2;j++)
            sqr[i][j] = array1[i][j]*array1[i][j];

        printf("%d\t",sqr[i][j]);
    }
    getch();
}

To me it all sounds incorrect. The way how to multiply two matrices is to multiply a row of the first matrix with a column in the second matrix, not cell by cell with equal coordinates (that would actually work only for a matrix with one row and one column).

I recommend to check out:

https://en.wikipedia.org/wiki/Matrix_multiplication

int array1[3][3]= {{0,0,0},{0,12,36},{0,32,56}} ;
for(i=1;i<=2;i++)

Another issue, an array in C starts with 0. So the for cycle should go from

Code:

    for(i=0;i<=2;i++)

Here is the code for you:

Code:

#include <stdio.h>
 
int main()
{
    int i,j;
    int array1[3][3]= {{0,0,0},{0,12,36},{0,32,56}} ;
    int sqr[3][3];

    /* multiplication part */
    for(i=0;i<3;i++)
    {
           for(j=0;j<3;j++){
		                      sqr[i][j]=array1[i][0]*array1[0][j]+array1[i][1]*array1[1][j]+array1[i][2]*array1[2][j];
    			            }
    } 



    /* here is the printing part */
    for(i=0;i<3;i++)
    {
	    for(j=0;j<3;j++){
			            printf("%d\t",sqr[i][j]);
			            }
	    printf("\n");
    } 
    return 0;
}

and the output is:

0 0 0
0 1296 2448
0 2176 4288

Remember: row * column!

(what sounds weird to me, if I write there array1[2][2], my compiler returns an error.. so I wrote there 3s and it works now.)

nerio: I think that you should have waited for a reply from XLR8 before proceeding to post a complete working solution. I gave hints in my previous two posts in the hope that XLR8 would figure out how to print correctly, then I ended my second post with an observation that the expected output was wrong, though couched in more placid terms: your assertion that "it all sounds incorrect" would be right once XLR8 is convinced of the problem with the expected output, but until then, it the assertion is not true: the actual output, with newlines correctly added, does match XLR8's expected output, hence by that standard it is correct. Salem's post was an elaboration of that, i.e., to point out that some elements were not being accessed at all.

Furthermore, if you want to post code, then you should post code that is properly formatted.

Well, if his expected output is a 2x2 matrix, as he wrote

expected output :
144 1296
1024 3136

maybe he wants to multiply

Code:

int array1[3][3]= {{12,36},{32,56}};

and I think I am right in this since he also wrote in his code

Code:

for(i=1;i<=2;i++)

It is difficult to help people if they do not state their problem precisely.

EDIT:

matrix {{12,36},{32,56}} to the power of two equals {{1296,2448},{2176,4288}}, so my conclusion is that my previous post is the ultimate reply and his expected output was wrong.