Thread: C program multiply matrices

Hey I have a problem here. Maybe someone can figure it out
This is my code and it's not giving me the right output

Code:

#include <stdio.h>


int main()
{
	int dim;
	int i, row, col;
	// inputting matrices dimensions
	int M0, M1, M2;


	// Calculate the product
	scanf("%d", &dim); 


	for (row=1; row<=dim; row++)
	{
		for (col=1; col<=dim; col++)
		{
			//Calculate (row,column) element of the product
			M2 = 0;


				for (i=1; i<=dim; i++)
				{
					// Element in M0 at (row,i)
					M0=(row+1)*(col+1);


					// Element in M1 at (i, column)
					M1=row+col;


					M2+=M0*M1; // The calculation
				}


				// prining an element
				printf("%d\t", M2);
		}
		// Going to the next row
		printf("\n");
	}
	return 0;
}

The output should give me this:
Input
2
Correct Output
2 5
4 10

As a guess: if M0 was based on row+1 and col+1, might not M1 also be based on row+1 and col+1?

Originally Posted by vart

What output it gives you?

And where are the matrices you are planning to multiply?

2
16 36
36 72
Program ended with exit code: 0

this is what it gives me and it should be multiplying
M0[i,j]=(i+1)(j+i)
M1[i,j]=(i+j)

M2=M1*M0

You need to use i in your multiply as well. It's M0[row][i] times M1[i][col].

Originally Posted by vart

Start with something simple

M0 should be matrix not integer

Code:

int M0[2][2];

initialize it using your formula and then print it for testing.

When it works, do the same for M1

When both matrices are initialized correctly and are displayed correctly - start writing function that multiplies them

I can't use arrays

> I can't use arrays
So define a function called M0 that takes two parameter, i and j, and returns the value at that location, per the formula. Do the same for M1. Then, you can calculate M2 for a given (i, j) with a simple loop and some addition, M0(??, ??) * M1(??, ??). I'll let you figure out the ?? parts.

Note, you don't need a function, but it fits very nicely here, and it makes your code more readable.

Originally Posted by laserlight

I'm rather confused: what exactly are you trying to do?

Looking at your sample input, the input is just 2. How on earth do you get an output of 4 numbers when all you entered was the dimension of the matrix? You didn't enter the entries of the matrices, so what's there to multiply?

Furthermore, suppose the input is 10, then what are you going to do? You cannot use arrays? Well, how are you going to store the entries of the 10 by 10 matrices (presumably) to be multiplied?

Threw me for a loop too, for a bit. The answer is in post #4. The elements of the two matrices to multiply are defined by formulae:

Originally Posted by YannB

2
16 36
36 72
Program ended with exit code: 0

this is what it gives me and it should be multiplying
M0[i,j]=(i+1)(j+i)
M1[i,j]=(i+j)

M2=M1*M0