copy array - row-wise

[Soumyadip Rakshit]

I have a 2D Array of Integers A[3][5]. I would like to copy it to another
array B[3][5] taking each row at a time. They are both initialized as
pointers to pointers.

I would like to use something like the following but it doesn't seem to work

for (i=0; i < 3; i++) {
while(*A[i]++ = *B[i]++)
;
}

[Banfa]

In what way does it not work? Unless one of the pointers in each row of B is NULL it will never end because there is no other end condition on the while loop.

Additionally because of operator precedence

*A[i]++ elavulates as *(A[i]++) which is not what you want, you will be writing to and from the wrong locations.

Is there a reason you don't want to memcpy?

memcpy( A, B, sizeof(A) );

or use 2 indices

for (i=0; i < 3; i++) {
for (j=0; j < 5; j++) {
A[i][j] = B[i][j];
}
}

[Grumble]

Re: copy array - row-wise

Soumyadip Rakshit wrote:[color=blue]
> I have a 2D Array of Integers A[3][5]. I would like to copy it to
> another array B[3][5] taking each row at a time. They are both
> initialized as pointers to pointers.
>
> I would like to use something like the following but it doesn't seem
> to work
>
> for (i=0; i < 3; i++) {
> while(*A[i]++ = *B[i]++)
> ;
> }[/color]

Could you provide the definition of A and B?

[Michael Mair]

Re: copy array - row-wise

Soumyadip Rakshit schrieb:[color=blue]
> I have a 2D Array of Integers A[3][5]. I would like to copy it to another
> array B[3][5] taking each row at a time. They are both initialized as
> pointers to pointers.
>
> I would like to use something like the following but it doesn't seem to work
>
> for (i=0; i < 3; i++) {
> while(*A[i]++ = *B[i]++)
> ;
> }[/color]

Please provide a compiling minimal example or your best shot
at it. Your declaration of A and B might be wrong or your
way of testing or ...

Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[stathis gotsis]

Re: copy array - row-wise

"Soumyadip Rakshit" <soumyadipraksh it@gmx.net> wrote in message
news:Iuqoqo.Mx6 @bath.ac.uk...[color=blue]
> I have a 2D Array of Integers A[3][5]. I would like to copy it to another
> array B[3][5] taking each row at a time. They are both initialized as
> pointers to pointers.
>
> I would like to use something like the following but it doesn't seem to[/color]
work[color=blue]
>
> for (i=0; i < 3; i++) {
> while(*A[i]++ = *B[i]++)
> ;
> }[/color]

If you have allocated space for array B and want to copy the integer values
stored in array A to array B, i would recommend you go with the simple:

for (i=0;i<2;i++)
for (j=0;j<3;j++)
B[i][j]=A[i][j];

[Keith Thompson]

Re: copy array - row-wise

"Soumyadip Rakshit" <soumyadipraksh it@gmx.net> writes:[color=blue]
> I have a 2D Array of Integers A[3][5]. I would like to copy it to another
> array B[3][5] taking each row at a time. They are both initialized as
> pointers to pointers.
>
> I would like to use something like the following but it doesn't seem to work
>
> for (i=0; i < 3; i++) {
> while(*A[i]++ = *B[i]++)
> ;
> }[/color]

If they're initialized as pointers to pointers, then you don't have a
2D array. Show us the declarations and initialization code.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Barry Schwarz]

Re: copy array - row-wise

On Wed, 15 Feb 2006 17:32:48 GMT, "Soumyadip Rakshit"
<soumyadipraksh it@gmx.net> wrote:
[color=blue]
>I have a 2D Array of Integers A[3][5]. I would like to copy it to another
>array B[3][5] taking each row at a time. They are both initialized as
>pointers to pointers.[/color]

Arrays are arrays and pointers are pointers. The fact that they use
the same [] syntax for accessing objects pointed to is both a source
of flexibility for the language and a source of confusion for
newcomers.

If your assertion about the definition is correct, then A and B each
point to 3 pointers to integer each. These pointers in turn point to
5 integers each (you did not say int). If you want to copy the 5
integers pointed to by A[0] to the memory pointed to by B[0], you can
use memcpy (or memmove but that is overkill unless the areas overlap)
or a loop.
[color=blue]
>
>I would like to use something like the following but it doesn't seem to work
>
>for (i=0; i < 3; i++) {
> while(*A[i]++ = *B[i]++)
> ;[/color]

There are several problems with this while loop:

You have the assignment backwards. This attempts to copy B to
A, not A to B as your specified.

After the first iteration, A[0] no longer points to the first
of the five integers. Ditto for B[0]. When you exit the while loop,
if it were to execute only five times, A[0] would end up pointing to
the byte just beyond the integer known as A[0][4]. Ditto for B[0].

What makes you think that B[i][4] will always be zero. Strings
are arrays of char that are nul ('\0') terminated. Arrays of integer
in general are under no such constraint. If a zero occurs before the
fifth element, you will not copy the entire array. If none of the
five is zero, you invoke undefined behavior by stepping beyond the
bounds of the five integers A[0] and B[0] point to.
[color=blue]
>}
>[/color]


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