strange array shift

**Richard Heathfield** · Sep 19 '08, 08:25 AM

Re: strange array shift

MN said:

Hi,
I was writing a program that must dynamically allocate memory to a 2-
dimensions array, initialize it to 0, then place the result of
multiplication of two 1-dimension arrays as described in the next code
example.
My code is:
>
#include<stdio. h>
#include<stdlib .h>
>
int main ()
{
int A[7] = {1,1,1,1,1,1,1} ;
int B[4] = {2,2,2,2};

Okay, we have an array of seven ints, and an array of four ints. For matrix
multiplication, the number of columns in A has to match the number of rows
in B. So there are only two ways for this to make sense. One of them is:

1
1
1
[ 1 ] * [ 2 2 2 2 ]
1
1
1

which multiplies out to

2 2 2 2
2 2 2 2
2 2 2 2
[ 2 2 2 2 ]
2 2 2 2
2 2 2 2
2 2 2 2

and the other is the same deal only rotated 90 degrees.

>
int i = 0;
int ii = 0;
>
int j = 0;
int jj = 0;

Indices, presumably.

>
int k = 0;
>
int deg_a = sizeof(A)/sizeof(A[0]) - 1;
int deg_b = sizeof(B)/sizeof(B[0]) - 1;

deg_a is 6, and deg_b is 4. These look like "max index" values to me.

int C_dimension = deg_a+deg_b+1;

It is no longer obvious to me what you are actually trying to do.

>
int** C_1 = NULL;
>
>
if (C_1 != NULL)
free(C_1);

How can it not be NULL? You only just set it to NULL.

C_1 = (int**)malloc(s izeof(int**) * (deg_a+1));

That's wrong. C_1 has type int **, so you're using it to point at a bunch
of int *, so you need space for a bunch of int *, not a bunch of int **.
You can get this right every time using the following template:

p = malloc(n * sizeof *p);

Thus, in your case:

C_1 = malloc((deg_a + 1) * sizeof *C_1);

Okay, so what do you do with this memory?

for (i = 0; i< (deg_a+1); i++)
C_1[i]= malloc(C_dimens ion);

Here's your problem right here. Well, /a/ problem. You're not allocating
enough storage (unless ints are only 1 byte wide on your system, which can
happen, but isn't typical on desktop machines).

>
// Initialize all values of C_1 to 0
printf("C_1 is initialized to 0:\n");
>
for(j = 0; j < (deg_a+1); j++)
{
for (k = 0; k < C_dimension; k++)
{
C_1[j][k] = 0;
// For verification
printf("%3d ", C_1[j][k]);
}
printf("\n");
}

Well, okay, you could simplify this a lot while you're fixing the bug:

if(C_1 == NULL)
{
take evasive action
}
else
{
for(i = 0; i < deg_a + 1; i++)
{
C_1[i] = calloc(C_dimens ion, sizeof *C_1[i]);
if(C_1[i] == NULL)
{
take evasive action

That's probably the issue, anyway - insufficient storage for your array.
But since it's not obvious to me what you're trying to do, it may well be
that there are other issues too that I didn't spot.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

MN · Sep 19 '08, 11:15 AM

Re: strange array shift

>

That's probably the issue, anyway - insufficient storage for your array.
But since it's not obvious to me what you're trying to do, it may well be
that there are other issues too that I didn't spot.

What I want to get is a program that can do a polynomial
multiplication, so I supposed that each polynomial can be represeteb
by an array where:
array[0] represents the coeffcient of x^0
array[1] represents the coeffcient of x^1
array[2] represents the coeffcient of x^2
..... and so on.

After I can multiply each coefficient of the first array with all
coefficients of the second array and put the result in a new row of an
new array (in this case called C_1). At the end I can easily do the
sum of all elements that are in the column in C_1 and this will give
the final result of polynomial multiplication.
As I think (may be I'm wrong!) this is the simple approach to code
polynomial multiplication in c language. Perhaps there is another and
simple way to do this.

**gw7rib@aol.com** · Sep 19 '08, 11:35 AM

Re: strange array shift

On 19 Sep, 12:05, MN <mazouz.nezh... @gmail.comwrote :

That's probably the issue, anyway - insufficient storage for your array.
But since it's not obvious to me what you're trying to do, it may well be
that there are other issues too that I didn't spot.

>
What I want to get is a program that can do a polynomial
multiplication, so I supposed that each polynomial can be represeteb
by an array where:
array[0] represents the coeffcient of x^0
array[1] represents the coeffcient of x^1
array[2] represents the coeffcient of x^2
.... and so on.
>
After I can multiply each coefficient of the first array with all
coefficients of the second array and put the result in a new row of an
new array (in this case called C_1). At the end I can easily do the
sum of all elements that are in the column in C_1 and this will give
the final result of polynomial multiplication.
As I think (may be I'm wrong!) this is the simple approach to code
polynomial multiplication in c language. Perhaps there is another and
simple way to do this.

If that's what you're trying to do, you don't need a two-dimensional
array at all. Given the difficulties of two-dimensional arrays in C,
an approach which does not use them should be simpler. What's wrong
with:

(allocate C to the right size, and initialise all the values to zero)

for (ii = 0; ii <(deg_a+1); ii++)
{
for(jj = 0; jj <(deg_b+1) ; jj++)
{
C[ii+jj] += A[ii]*B[jj];
}
}

Hope that helps.
Paul.

MN · Sep 19 '08, 12:15 PM

Re: strange array shift

Paul,
Thanks for help!! program is working !

**Keith Thompson** · Sep 19 '08, 04:15 PM

Re: strange array shift

MN <mazouz.nezhate @gmail.comwrite s:
[...]

My code is:
>
#include<stdio. h>
#include<stdlib .h>
>
int main ()
{
int A[7] = {1,1,1,1,1,1,1} ;
int B[4] = {2,2,2,2};
>
int i = 0;
int ii = 0;
>
int j = 0;
int jj = 0;
>
int k = 0;
>
int deg_a = sizeof(A)/sizeof(A[0]) - 1;
int deg_b = sizeof(B)/sizeof(B[0]) - 1;
>
int C_dimension = deg_a+deg_b+1;
>
int** C_1 = NULL;
>
>
if (C_1 != NULL)
free(C_1);
>
C_1 = (int**)malloc(s izeof(int**) * (deg_a+1));
>
for (i = 0; i< (deg_a+1); i++)
C_1[i]= malloc(C_dimens ion);
>
// Initialize all values of C_1 to 0
printf("C_1 is initialized to 0:\n");
>
for(j = 0; j < (deg_a+1); j++)
{
for (k = 0; k < C_dimension; k++)
{
C_1[j][k] = 0;
// For verification
printf("%3d ", C_1[j][k]);
}
printf("\n");
}
>
// Do multiplication of A[deg_a+1] and B[deg_b+1]
for (ii = 0; ii <(deg_a+1); ii++)
{
for(jj = 0; jj <(deg_b+1) ; jj++)
{
C_1[ii][ii+jj] = A[ii]*B[jj];
}
}
>
printf("\n");
// For verification
printf("After multiplication of A[%d] and B[%d], C_1 is:\n", deg_a
+1, deg_b+1);
for(j = 0; j < (deg_a+1); j++)
{
for (k = 0; k < C_dimension; k++)
{
printf("%3d ", C_1[j][k]);
}
printf("\n");
}
return 0;
>
}

[...]

Among other oddities in your code, you compute deg_a and deg_b as 1
less than the lengths of the A and B arrays, but then you almost
always refer to deg_a+1 and deg_b+1, which are simply the lengths of A
and B. Why not just compute and use their lengths?

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

strange array shift

strange array shift

Comment

Comment

Comment

Comment

Comment