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Multi-Dimension Array Question

The input to a function of a 3rd party library I want to use requires a
double**, which is a multi-dimension array of doubles.

I have looked on the net etc and seen several ways of supposedly doing
this, but I don't seem to be able to get them to work. I was wondering
if anybody can tell me what I am doing wrong.

int rows = 10 ;
int cols = 10 ;

double** data ;

data = new (double*)[rows] ;

for (int i = 0 ; i < rows ; i++) {

data[i] = new double[cols];

}

main.cpp(291) : error C2143: syntax error : missing ';' before '['
main.cpp(291) : error C2337: 'rows' : attribute not found; it is neither
a built-in nor a custom attribute that is accessible in the current
namespace

both the error point to the data = new (double*)[rows] line
May I don't need to create the multi-dimension array this way, I don't
know. As mentioned above, I have a 3rd party library function that
requests double** data.

One point is that I won't know the row and column sizes until runtime.

Any suggestions would be much appreciated,

Adam
Jul 23 '05 #1
6 2231
ben
Looks like a Visual C++ bug. The code compiled fine with gcc. Try this:

typedef double* ptr_to_double;

int rows = 10;
int cols =10;

double** data = new ptr_to_double[rows];

//...

delete[] data;
ben

"Adam Hartshorne" <or********@yah oo.com> wrote in message
news:d8******** **@wisteria.csv .warwick.ac.uk. ..
The input to a function of a 3rd party library I want to use requires a
double**, which is a multi-dimension array of doubles.

I have looked on the net etc and seen several ways of supposedly doing
this, but I don't seem to be able to get them to work. I was wondering
if anybody can tell me what I am doing wrong.

int rows = 10 ;
int cols = 10 ;

double** data ;

data = new (double*)[rows] ;

for (int i = 0 ; i < rows ; i++) {

data[i] = new double[cols];

}

main.cpp(291) : error C2143: syntax error : missing ';' before '['
main.cpp(291) : error C2337: 'rows' : attribute not found; it is neither
a built-in nor a custom attribute that is accessible in the current
namespace

both the error point to the data = new (double*)[rows] line
May I don't need to create the multi-dimension array this way, I don't
know. As mentioned above, I have a 3rd party library function that
requests double** data.

One point is that I won't know the row and column sizes until runtime.

Any suggestions would be much appreciated,

Adam

Jul 23 '05 #2
ben wrote:
Looks like a Visual C++ bug. The code compiled fine with gcc. Try this:

typedef double* ptr_to_double;

int rows = 10;
int cols =10;

double** data = new ptr_to_double[rows];

//...

delete[] data;
ben

"Adam Hartshorne" <or********@yah oo.com> wrote in message
news:d8******** **@wisteria.csv .warwick.ac.uk. ..
The input to a function of a 3rd party library I want to use requires a
double**, which is a multi-dimension array of doubles.

I have looked on the net etc and seen several ways of supposedly doing
this, but I don't seem to be able to get them to work. I was wondering
if anybody can tell me what I am doing wrong.

int rows = 10 ;
int cols = 10 ;

double** data ;

data = new (double*)[rows] ;

for (int i = 0 ; i < rows ; i++) {

data[i] = new double[cols];

}

main.cpp(29 1) : error C2143: syntax error : missing ';' before '['
main.cpp(29 1) : error C2337: 'rows' : attribute not found; it is neither
a built-in nor a custom attribute that is accessible in the current
namespace

both the error point to the data = new (double*)[rows] line
May I don't need to create the multi-dimension array this way, I don't
know. As mentioned above, I have a 3rd party library function that
requests double** data.

One point is that I won't know the row and column sizes until runtime.

Any suggestions would be much appreciated,

Adam



Thanks that did the trick! Any idea why that is the case?

Adam
Jul 23 '05 #3
Adam Hartshorne wrote:
The input to a function of a 3rd party library I want to use requires
a double**, which is a multi-dimension array of doubles.

I have looked on the net etc and seen several ways of supposedly doing
this, but I don't seem to be able to get them to work. I was wondering
if anybody can tell me what I am doing wrong.

int rows = 10 ;
int cols = 10 ;

double** data ;

data = new (double*)[rows] ;

for (int i = 0 ; i < rows ; i++) {

data[i] = new double[cols];

}

main.cpp(291) : error C2143: syntax error : missing ';' before '['


Drop the parentheses.

V
Jul 23 '05 #4
Adam Hartshorne wrote:
The input to a function of a 3rd party library I want to use requires a
double**, which is a multi-dimension array of doubles.
Actually it is a pointer to pointer to double.
It can be confusing to call a pointer-to-pointer a
multi-dimension array. The term 'multi-dimension array' usually
suggests an array of arrays, eg:

double x[10][10];

Of course, this sort of array is not suitable for your function.
double** data ;
data = new (double*)[rows] ;
'new (double *)[rows]' means '(new (double *))[rows]'.

In other words, it allocates one pointer to double, and then
immediately dereferences a pointer to that (causing undefined
behaviour).

To allocate an array of pointers-to-double:

data = new double *[rows];
for (int i = 0 ; i < rows ; i++) {
data[i] = new double[cols];
}

main.cpp(291) : error C2143: syntax error : missing ';' before '['


Good that your compiler won't compile such trash (it's certainly
an error), but bad because it means the compiler is not standards-
compliant :)

Jul 23 '05 #5
ben
A parser bug perhaps. VC++ might be a bit lazy and it expects

new XXX[nnn];

without parenthesis around XXX.

ben
Thanks that did the trick! Any idea why that is the case?

Adam

Jul 23 '05 #6
ben
Yeah that's a better solution!

ben

Drop the parentheses.

V

Jul 23 '05 #7

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