Please clarify my doubt on matrix multiplication

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/*Multiplication of two matrices*/
#include<stdio.h>
float product(float[20][20],float[20][20]);
int i,j,k,r1,c1,r2,c2;
main()
{
	/*c1=r2*/
	printf("Enter no.of rows of first matrix:\t");
	scanf("%d",&r1);
	printf("Enter no.of columns of first matrix:\t");
	scanf("%d",&c1);
	printf("Enter no.of rows of second matrix:\t");
	scanf("%d",&r2);
	printf("Enter no.of columns of second matrix:\t");
	scanf("%d",&c2);
	float a[r1][c1];
	float b[r2][c2];

	product(a,b);
	for(i=0;i<r1;i++)
	{
		for(j=0;j<c2;j++)
		{
			printf("%f",c[i][j]);
		}
	}
	
}

float product(float a[20][20],float b[20][20])
{	float c[r1][c2];
	if(c1==r2)
	{
		/*scanning elements*/
		printf("Enter the elements of first matrix:\t");
		for(i=0;i<r1;i++)
		{	for(j=0;j<c1;j++)
			{
				scanf("%f",&a[i][j]);
			}
		}
		printf("Enter the elements of second matrix:\t");
		for(i=0;i<r2;i++)
		{	for(j=0;j<c2;j++)
			{
				scanf("%f",&b[i][j]);
			}
		}
		/*scanning cmpltd*/
		/*multiplication*/
		for(i=0;i<r1;i++)
		{
			for(j=0;j<c2;j++)
			{
				c[i][j]=0;
				for(k=0;k<c1;k++)
				{
					c[i][j]=c[i][j]+(a[i][k]*b[k][j]);
			
				}
				
			}
			printf("\n");
		}
	}
	else
		printf("Please Enter proper matrices");
}

This program takes two matrices as inputs and gives their product. When I have written this program without function I got the solutions. I want to use this operation in other program, that is why I need to write this as a function. But Iam getting this error.

Thnx in advance.

VB

matr.c: In function ‘main’:
matr.c:24: error: ‘c’ undeclared (first use in this function)
matr.c:24: error: (Each undeclared identifier is reported only once
matr.c:24: error: for each function it appears in.)

Important Info from OP copied from comment below:
My sole intention to write this function is, I use it in b/w my program related to Graph theory. where I may use product(product(x,y),z) to get x*y*z.

Posted 9-Oct-12 5:32am

N Shiva

Updated 9-Oct-12 6:29am

Nelek

v3

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Comments

N Shiva 9-Oct-12 11:35am

Iam using array-c in main() even though it is not declared in main(). But even If I declare above main also Iam getting this error.

matr.c:5: error: variably modified ‘c’ at file scope
matr.c:5: error: variably modified ‘c’ at file scope

Sergey Alexandrovich Kryukov 9-Oct-12 16:45pm

Matrix multiplication is one of the simplest things in calculations, but your problem is understanding of the very basics of programming and language. I suggest you learn more of it on more simple examples first.
--SA

3 solutions

Solution 1

C#

printf("%f",c[i][j]);

You have not declared "c" in this statment anywhere. To use c in main you have to declare it just like you did for a and b.

product(a,b);

Does nothing because you are throwing the result away.

Posted 9-Oct-12 5:43am

Wes Aday

Comments

N Shiva 9-Oct-12 11:47am

How can I catch the result in that function. I mean c[i][j].
I just want to store that in array c.

[no name] 9-Oct-12 11:56am

Try declaring c in your main function and pass it to your product function. Might be easier to pass it as a pointer.

ridoy 9-Oct-12 11:48am

right..+5

N Shiva 9-Oct-12 12:06pm

Sorry, Iam not getting u.

[no name] 9-Oct-12 12:14pm

What don't you get? Passing another array to your function? Declaring another array? Pointers?

Nelek 9-Oct-12 12:11pm

"c" do is declared, as local in the function "product"

[no name] 9-Oct-12 12:13pm

Yes and that does not help him when he is trying to print the values from main.

N Shiva 9-Oct-12 12:13pm

Then how can I print this when I want to?

[no name] 9-Oct-12 12:15pm

The same way you are doing now?

N Shiva 9-Oct-12 12:39pm

My sole intention to write this function is, I use it in b/w my program related to Graph theory. where I may use product(product(x,y),z) to get x*y*z.

N Shiva 9-Oct-12 12:17pm

When I declared array-C in main function and I passed it to product function{along with a & b }, I didn't get any error, but Iam getting wrong ans.

Sergey Alexandrovich Kryukov 9-Oct-12 16:52pm

My 5. It takes patience. Sometimes I understand why you don't formally answer, just comment :-)
--SA

[no name] 9-Oct-12 17:05pm

Thanks. Kind of like asking the same guy 3 times what version of VB he is using... :-)

Sergey Alexandrovich Kryukov 10-Oct-12 2:02am

Oh, did you notice?! And how was that? Do you think there is a hope for answer? :-)
--SA

Solution 2

Your main problem is you are declaring the function as float, but you are not returning anything. This is because C is locally declared.

Other point you have to consider is that you are passing two empty arrays to the function, because "a" and "b" have no values, you use them just behind the declaration.

Have a look to http://www.tutorialspoint.com/cplusplus/cpp_return_arrays_from_functions.htm[^]. The example will show you what we mean. Hope it helps

Posted 9-Oct-12 6:46am

Nelek

Updated 9-Oct-12 6:48am

v2

Comments

Sergey Alexandrovich Kryukov 9-Oct-12 16:53pm

Sure, a 5.
--SA

Nelek 9-Oct-12 18:10pm

Thank you

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CPallini · Accepted Answer · 2012-10-09T07:07:00

You should write something similar to:

C

// a, b are IN parameters, while c is the OUT parameter
int product(float a[20][20],float b[20][20], float c[20][20])
{
//... 
}

and then , in the main function:

C

//..
float a[r1][c1];
float b[r2][c2];
float c[r2][c2];
product(a,b,c); // c = a x b
//..

[UPDATE]

N Shiva wrote:
float a[r1][c1];
float b[r2][c2];

These statements are evil wrong (at compile time r1, c1, .. values are indefinite)!

A working (although still ugly) version of your program:

C

/*Multiplication of two matrices*/
#include<stdio.h>
int product(float a[20][20], float b[20][20],  float c[20][20]);
int i,j,k,r1,c1,r2,c2;

int main()
{
  /*c1=r2*/
  printf("Enter no.of rows of first matrix:\t");
  scanf("%d",&r1);
  printf("Enter no.of columns of first matrix:\t");
  scanf("%d",&c1);
  printf("Enter no.of rows of second matrix:\t");
  scanf("%d",&r2);
  printf("Enter no.of columns of second matrix:\t");
  scanf("%d",&c2);
  float a[20][20];
  float b[20][20];
  float c[20][20];

  if ( product(a,b,c) ) return -1;

  for(i=0;i<r1;i++)
  {
    for(j=0;j<c2;j++)
    {
      printf("%f ",c[i][j]);
    }
    printf("\n");
  }
  return 0;
}

int product(float a[20][20], float b[20][20],  float c[20][20])
{
  if(c1==r2)
  {
    /*scanning elements*/
    printf("Enter the elements of first matrix:\t");
    for(i=0;i<r1;i++)
    { for(j=0;j<c1;j++)
      {
        scanf("%f",&a[i][j]);
      }
    }
    printf("Enter the elements of second matrix:\t");
    for(i=0;i<r2;i++)
    { for(j=0;j<c2;j++)
      {
        scanf("%f",&b[i][j]);
      }
    }
    /*scanning cmpltd*/
    /*multiplication*/
    for(i=0;i<r1;i++)
    {
      for(j=0;j<c2;j++)
      {
        c[i][j]=0;
        for(k=0;k<c1;k++)
        {
          c[i][j]=c[i][j]+(a[i][k]*b[k][j]);
        }
      }
      printf("\n");
    }
  }
  else
  {
    printf("Please Enter proper matrices");
    return -1;
  }
  return 0;
}

[/UPDATE]

[UPDATE 2]

N Shiva wrote:
float a[r1][c1];
float b[r2][c2];

Actually (I didn't know that) some compilers (e.g. GCC) by default implement this C99 extension (arrays of variable length[^]). However, you should use them properly, try:

C

/*Multiplication of two matrices*/
#include<stdio.h>
int product(int r1, int c1, float a[r1][c1], int r2, int c2, float b[r2][c2],  float c[r1][c2]);

int main()
{
  int i,j,r1,c1,r2,c2;
  /*c1=r2*/
  printf("Enter no.of rows of first matrix:\t");
  scanf("%d",&r1);
  printf("Enter no.of columns of first matrix:\t");
  scanf("%d",&c1);
  printf("Enter no.of rows of second matrix:\t");
  scanf("%d",&r2);
  printf("Enter no.of columns of second matrix:\t");
  scanf("%d",&c2);
  float a[r1][c1];
  float b[r2][c2];
  float c[r1][c2];

  if ( product(r1, c1, a, r2, c2, b,c) ) return -1;

  for(i=0;i<r1;i++)
  {
    for(j=0;j<c2;j++)
    {
      printf("%f ",c[i][j]);
    }
    printf("\n");
  }
  return 0;
}

int product(int r1, int c1, float a[r1][c1], int r2, int c2, float b[r2][c2],  float c[r1][c2])
{
  int i, j, k;
  if(c1==r2)
  {
    /*scanning elements*/
    printf("Enter the elements of first matrix:\t");
    for(i=0;i<r1;i++)
    { for(j=0;j<c1;j++)
      {
        scanf("%f",&a[i][j]);
      }
    }
    printf("Enter the elements of second matrix:\t");
    for(i=0;i<r2;i++)
    { for(j=0;j<c2;j++)
      {
        scanf("%f",&b[i][j]);
      }
    }
    /*scanning cmpltd*/
    /*multiplication*/
    for(i=0;i<r1;i++)
    {
      for(j=0;j<c2;j++)
      {
        c[i][j]=0;
        for(k=0;k<c1;k++)
        {
          c[i][j]=c[i][j]+(a[i][k]*b[k][j]);
        }
      }
      printf("\n");
    }
  }
  else
  {
    printf("Please Enter proper matrices");
    return -1;
  }
  return 0;
}

[/UPDATE 2]

Please clarify my doubt on matrix multiplication

3 solutions

Solution 1

Solution 2

Solution 3

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