count of pointer to array question

[Jackie]

Does anyone know how to get a count for a pointer to an array

for example

double** someFnct () {

cout <<"Enter num of row: " <<endl;
cin >> n;

cout <<"Enter num of column: " <<endl;
cin >> m;

double** matrixX = new double[n];
for (int i = 0; i<numOfRows; i++) [
matrixX[i] = new double(m);
}
return matrixX;
}

double** tmpMtrx = someFunct();

int row = sizeof tmpMtrx / sizeof* tmpMtrx; // does not work
int column = // don't even know where to begin with this one


Any insight would be greatly appreciated. Thanks

[Nils O. Selåsdal]

Re: count of pointer to array question

On Sat, 16 Oct 2004 18:08:54 -0700, Jackie wrote:
[color=blue]
> Does anyone know how to get a count for a pointer to an array
>[/color]
....[color=blue]
> int row = sizeof tmpMtrx / sizeof* tmpMtrx; // does not work
> int column = // don't even know where to begin with this one
>
>
> Any insight would be greatly appreciated. Thanks[/color]
basically you have to record the no. of cols/rows.
double** someFnct (int &rows, int &cols);

[Victor Bazarov]

Re: count of pointer to array question

"Jackie" <darkusang@yaho o.com> wrote...[color=blue]
> Does anyone know how to get a count for a pointer to an array[/color]

I don't think you describe it well, but IIUIC, you are trying to
get the size of a dynamically allocated array from the pointer.
That is not possible in C++.
[color=blue]
>
> for example
>
> double** someFnct () {
>
> cout <<"Enter num of row: " <<endl;
> cin >> n;
>
> cout <<"Enter num of column: " <<endl;
> cin >> m;
>
> double** matrixX = new double[n];[/color]

I think you want

double** matrixX = new double*[n];
[color=blue]
> for (int i = 0; i<numOfRows; i++) [[/color]

There is no 'numOfRows'. I believe you called it 'n'. So this
should be

for (int i = 0; i < n; ++i) {
[color=blue]
> matrixX[i] = new double(m);[/color]

And here you probably want

matrixX[i] = new double[m];
[color=blue]
> }
> return matrixX;
> }
>
> double** tmpMtrx = someFunct();
>
> int row = sizeof tmpMtrx / sizeof* tmpMtrx; // does not work
> int column = // don't even know where to begin with this one[/color]

If you want to know what sizes the array was allocated with, pass
addresses of two integers into your 'someFunct' and let it tell
you what sizes it used:

double ** someFunc(int* nRows, int* nCols) {
...
if (nRows) *nRows = n;
if (nCols) *nCols = m;
return matrixX;
}

...

int rows, columns;
double** tmpMtrx = someFunct(&rows , &columns);

Otherwise, ask your questions outside and let someFunct allocate
the memory based on the numbers you pass in.

Victor

[John Harrison]

Re: count of pointer to array question


"Jackie" <darkusang@yaho o.com> wrote in message
news:e78463d9.0 410161708.11b3b 6b9@posting.goo gle.com...[color=blue]
> Does anyone know how to get a count for a pointer to an array
>[/color]

Impossible, use a vector instead. Look up vectors in your favourite C++
book.

john

[Ioannis Vranos]

Re: count of pointer to array question

Jackie wrote:[color=blue]
> Does anyone know how to get a count for a pointer to an array
>
> for example
>
> double** someFnct () {
>
> cout <<"Enter num of row: " <<endl;
> cin >> n;
>
> cout <<"Enter num of column: " <<endl;
> cin >> m;
>
> double** matrixX = new double[n];
> for (int i = 0; i<numOfRows; i++) [
> matrixX[i] = new double(m);
> }
> return matrixX;
> }
>
> double** tmpMtrx = someFunct();
>
> int row = sizeof tmpMtrx / sizeof* tmpMtrx; // does not work
> int column = // don't even know where to begin with this one
>
>
> Any insight would be greatly appreciated. Thanks[/color]


You can't, you will use variables n and m for that.


A better way is to do:


#include <iostream>
#include <vector>

int main()
{
using namespace std;

unsigned m ,n;

cout <<"Enter num of row: " <<endl;
cin >> n;

cout <<"Enter num of column: " <<endl;
cin >> m;

vector<vector<d ouble> >matrixX(n, vector<double>( m));
}



In this matrixX.size() returns the number of rows, and matrixX[i].size()
returns the number of columns.



--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Gianni Mariani]

Re: count of pointer to array question

Jackie wrote:[color=blue]
> Does anyone know how to get a count for a pointer to an array
>
> for example
>
> double** someFnct () {
>
> cout <<"Enter num of row: " <<endl;
> cin >> n;
>
> cout <<"Enter num of column: " <<endl;
> cin >> m;
>
> double** matrixX = new double[n];
> for (int i = 0; i<numOfRows; i++) [
> matrixX[i] = new double(m);
> }
> return matrixX;
> }
>
> double** tmpMtrx = someFunct();
>
> int row = sizeof tmpMtrx / sizeof* tmpMtrx; // does not work
> int column = // don't even know where to begin with this one
>
>
> Any insight would be greatly appreciated. Thanks[/color]

A suggestion: use a matrix class.

http://groups.google.com/groups?selm=cbqpok%24sut%40dispatch.concentric.net

There are plenty of open source matrix libraries.

G

[Jackie]

Re: count of pointer to array question

Thanks everyone,

I appreciate the suggestions and comments, it was enlightening and
helpful. I apologize for the careless code post. I went with vectors
instead. Long time Java/SQL programmer, but first week learning C++.
I'm learning to like C++ more and more.

Thanks again everyone.

Jack