error while executing a matrix multiplication program

[girishc13]

i have written the below program for matrix mult using pointers and functions.
the code is:

#include<stdio.h>
int M = 3,N = 2;
main()
{
int mat1[M][N],mat2[N][M],i,j,res[M][M];
void matmul(int *a,int *b,int *c);
printf("\n enter the matrix elements row wise the size is %dx%d \n",M,N);
for (i = 0;i < M;i++)
for (j = 0;j < N;j++)
scanf("%d",&mat1[i][j]);
printf("\n enter the second matrix elements of size %dx%d \n",N,M);
for (i = 0;i < N;i++)
for (j = 0;j < M;j++)
scanf("%d",&mat2[i][j]);
matmul(&mat1[0][0],&mat2[0][0],&res[0][0]);
printf("\n Matrix 1 = \n");
for (i = 0;i < M;i++)
{
for (j = 0;j < N;j++)
printf("%d \t",mat1[i][j]);
printf("\n");
}
printf("\n Matrix 2 = \n");
for (i = 0;i < N;i++)
{
for (j = 0;j < M;j++)
printf("%d \t",mat2[i][j]);
printf("\n");
}
printf("\n Result = \n");
for (i = 0;i < M;i++)
{
for (j = 0;j < M;j++)
printf("%d \t",res[i][j]);
printf("\n");
}
}
 
void matmul(int *a,int *b,int *c)
{
int i,j,k,d = sizeof(int);
printf("\n size of int = %d \n",d);
for (i = 0;i < M;i++)
{
for (j = 0;j < M;i++)
{
*(c + i * d + j) = 0;
//*(*(c + i) + j) = 0;
for (k = 0;k < N;k++)
*(c + i * d + j) += (*(a + i * d + k)) * (*(b + k * d + j));
//*(*(c + i) + j) += (*(*(a + i) + k)) * (*(*(b + k) + j));
}
}
}
 

im starting pointer exercises, and executing the pgm in fedora 12.
if i execute the above pgm, i get segmentation fault.
if i execute using the commented lines in the func, i get the foll error:
ex28.c: In function ‘matmul’:
ex28.c:49: error: invalid type argument of ‘unary *’ (have ‘int’)
ex28.c:52: error: invalid type argument of ‘unary *’ (have ‘int’)
ex28.c:52: error: invalid type argument of ‘unary *’ (have ‘int’)
ex28.c:52: error: invalid type argument of ‘unary *’ (have ‘int’)

help in debugging the errors or mistakes

[weaknessforcats]

The calling aruments of your matmul function are int*. This is not a 2D array but is only the address of a single int.

There is a correct way to pass the address of a 2D array to a function and that is to pass the address of element 0. In a 2D array element 0 is itself an array so you need to pass the address of the array that is the second dimension.

This array:

int arr[3][4];

is passed to a function using the argument of a pointer to an array of 4 int:

void MyFunc(int (*arg)[4]), int num);

You also need to pass in the number of elements. In this case the number of 4 int arrays.


Read this: http://bytes.com/topic/c/insights/77...rrays-revealed

[girishc13]

#include<stdio.h>
int M = 3,N = 2;
main()
{
int mat1[M][N],mat2[N][M],i,j,res[M][M];
void matmul(int (*a)[N],int (*b)[M],int (*c)[M]);
printf("\n enter the matrix elements row wise the size is %dx%d \n",M,N);
for (i = 0;i < M;i++)
for (j = 0;j < N;j++)
scanf("%d",&mat1[i][j]);
printf("\n enter the second matrix elements of size %dx%d \n",N,M);
for (i = 0;i < N;i++)
for (j = 0;j < M;j++)
scanf("%d",&mat2[i][j]);
matmul(mat1,mat2,res);
printf("\n Matrix 1 = \n");
for (i = 0;i < M;i++)
{
for (j = 0;j < N;j++)
printf("%d \t",mat1[i][j]);
printf("\n");
}
printf("\n Matrix 2 = \n");
for (i = 0;i < N;i++)
{
for (j = 0;j < M;j++)
printf("%d \t",mat2[i][j]);
printf("\n");
}
printf("\n Result = \n");
for (i = 0;i < M;i++)
{
for (j = 0;j < M;j++)
printf("%d \t",res[i][j]);
printf("\n");
}
}
 
void matmul(int (*a)[N],int (*b)[M],int (*c)[M])
{
int i,j,k,d = sizeof(int);
printf("\n size of int = %d \n",d);
or (i = 0;i < N;i++)
for (j = 0;j < M;j++)
{
printf("%d \t",(*(a + i))[j]);
}
for (i = 0;i < M;i++)
{
for (j = 0;j < M;i++)
{
(*(c + i))[j] = 0;
for (k = 0;k < N;k++)
(*(c + i))[j] += (*(a + i))[k] * (*(b + k))[j];
}
}
}
 

ok i changed the pgm for passing the array parameters.but i still get segmentation fault. and cant print the array in the func using pointers

[newb16]

and cant print the array in the func using pointers
?
If you can't see the output it's because crash occurs before the output stream is flushed. As for seg fault itself, the reason may be an array index out of bound - then icorrect pointer is fetched and when it's accessed, crash occurs. Did you localize it?

[weaknessforcats]

Your code compiled and executed OK on Visual Studio.NET 2008. I didn;t check that the answer was correct but therewas no crash.

I did have to make M and N const and I did have to make main() return an int.

[newb16]

It crashes because 'i' runs out of range.

[girishc13]

its working now thanx for ur replies,heres the working pgm:

#include<stdio. h>
int M = 3,N = 2;
void main()
{
int mat1[M][N],mat2[N][M],i,j,res[M][M];
void matmul(int ar1[][N],int ar2[][M],int ar3[][M]);
printf("\n enter the matrix elements row wise the size is %dx%d \n",M,N);
for (i = 0;i < M;i++)
for (j = 0;j < N;j++)
scanf("%d",&mat 1[i][j]);
printf("\n enter the second matrix elements of size %dx%d \n",N,M);
for (i = 0;i < N;i++)
for (j = 0;j < M;j++)
scanf("%d",&mat 2[i][j]);
matmul(mat1,mat 2,res);
printf("\n Matrix 1 = \n");
for (i = 0;i < M;i++)
{
for (j = 0;j < N;j++)
printf("%d \t",mat1[i][j]);
printf("\n");
}
printf("\n Matrix 2 = \n");
for (i = 0;i < N;i++)
{
for (j = 0;j < M;j++)
printf("%d \t",mat2[i][j]);
printf("\n");
}
printf("\n Result = \n");
for (i = 0;i < M;i++)
{
for (j = 0;j < M;j++)
printf("%d \t",res[i][j]);
printf("\n");
}
}

void matmul(int ar1[][N],int ar2[][M],int ar3[][M])
{
int i = 0,j = 0,k = 0,d = sizeof(int),(*a )[N] = ar1,(*b)[M] = ar2,(*c)[M] = ar3;
printf("\n size of int = %d \n",d);
printf("\n size of int = %d \n",d);
for (i = 0;i < M;i++)
{
for (j = 0;j < M;j++)
{
(*(c + i))[j] = 0;
printf("%d \t",(*(c + i))[j]);
}
}
for (i = 0;i < M;i++)
{
for (j = 0;j < M;j++)
{
(*(c + i))[j] = 0;
for (k = 0;k < N;k++)
{
(*(c + i))[j] += (*(a + i))[k] * (*(b + k))[j];
}
}
}
return;
}

'i' was going out of range cos i was incremented in the j loop also. newbie mistake.the lesson was very good for arrays to function parameter passing.