Multi-Dimension Array Question

[Adam Hartshorne]

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 = 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

 

[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]

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

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 = 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

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

Adam

 

[Victor Bazarov]

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 = new double[cols];

}

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

Drop the parentheses.

V

 

[Old Wolf]

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 = 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

 

[ben]

A parser bug perhaps. VC++ might be a bit lazy and it expects

new XXX[nnn];

without parenthesis around XXX.

ben

 

[ben]

Yeah that's a better solution!

ben