How to change Array Data type?

LennoZebra · 10-02-2009, 12:02 PM

Hello, I was wondering if there was a way to change a array full of float values to an array of double values and then reference the new array to the value of a pointer?
Thank You

itCbitC · 10-02-2009, 12:09 PM

Quote:

Originally Posted by LennoZebra

Hello, I was wondering if there was a way to change a array full of float values to an array of double values

Yep! it's called a cast.

Quote:

Originally Posted by LennoZebra

and then reference the new array to the value of a pointer?

Not sure what you mean by the above?

LennoZebra · 10-02-2009, 12:18 PM

I used a FOR loop to cast each variable in the array up to a data type double. But when I try and set the array to the pointer like:
*vrp = hab2;

where hab2 is a array of 300 units, it tells me that they are incompatible data types?
Thank You

nonoob · 10-02-2009, 12:35 PM

No. Since every array element uses 4 byes for float vs. 8 bytes for double, you can not cast the entire array at once.

MK27 · 10-02-2009, 12:45 PM

Do you want a copy of the values in a separate array, or do you want both arrays to point to the same place? If the later, there is no point, since a double and a float are stored differently.

You should post that code that you tried.

Epy · 10-02-2009, 12:47 PM

Quote:

Originally Posted by LennoZebra

Hello, I was wondering if there was a way to change a array full of float values to an array of double values and then reference the new array to the value of a pointer?
Thank You

Technically, an array is a pointer...a pointer to the beginning of the array.
For example, a[2] is the same as *(a+2)

anyway...

Code:

float a[4];
double b[4];
int i;
for (i = 0; i < 4; i++)
     b[i] = (double)a[i];

LennoZebra · 10-02-2009, 01:27 PM

Here is the code that I am having problems with. Notice

Code:

     #include "config.h"
     
     #include "mex.h"

     #include "f77-fcn.h"
     
     extern void F77_FUNC (contrl, CONTRL) 
      (float *phi, float *delta);

     void
     mexFunction (int nlhs, mxArray* plhs[], int nrhs, 
     	     const mxArray* prhs[])
     {
       mwIndex i;
       mwSize n;
       double *vri, *vro;
       double hab1, hab2[2];
       float heg1, heg2[2];
       float guo[300];
       int k;
       
       if (nrhs != 1 || ! mxIsNumeric (prhs[0]))
         mexErrMsgTxt ("expects matrix");
     
       n = mxGetNumberOfElements (prhs[0]);
       plhs[0] = (mxArray *) mxCreateNumericArray 
         (mxGetNumberOfDimensions (prhs[0]),
          mxGetDimensions (prhs[0]), mxGetClassID (prhs[0]),
          mxIsComplex (prhs[0]));
       vri = mxGetPr (prhs[0]);
       vro = mxGetPr (plhs[0]);
     
       if (mxIsComplex (prhs[0]))
         {
           double *vii, *vio;
           vii = mxGetPi (prhs[0]);
           vio = mxGetPi (plhs[0]);
     
           for (i = 0; i < n; i++)
     	{
     	  vro [i] = vri [i] * vri [i] - vii [i] * vii [i];
     	  vio [i] = 2 * vri [i] * vii [i];
     	}
         }
       else
         {

	   hab1 = *vri;
	   heg1 = (float)hab1;

	   F77_FUNC (contrl, CONTRL) (&heg1,(float *)guo);
	   heg2 [1] = guo[13];
	   heg2 [2] = guo[20];
           //***** Important Part*********
	   for (k = 0; k < 3; k++)
	     vro[k] = (double)heg2[k]; //This Line is Having PROBLEMS
          //I create the mex file using Octave and this part does not work
	   
         }
     }

I'm integrating MATLAB, C and FORTRAN together through a Mex function, and thus using API's to get information to and from FORTRAN and MATLAB.

C still has to abide by all the normal rules inside this wrapper. But using the code above or:

Code:

 //***** Important Part*********
	   for (k = 0; k < 3; k++)
	     hab2[k] = (double)heg2[k]; 
          *vro = hab2; //This Line is Having PROBLEMS

still leaves me with problems.

*****I am able to cast the array properly, but I am having problems assigning the array to the value of the pointer*****
Thank you for your help

tabstop · 10-02-2009, 01:37 PM

What kinds of problems are you having? Specifically.

LennoZebra · 10-02-2009, 02:09 PM

if I use :

Code:

 //***** Important Part*********
	   for (k = 0; k < 3; k++)
	     vro[k] = (double)heg2[k]; //This Line is Having PROBLEMS
          //I create the mex file using Octave and this part does not work

Then MATLAB or Octave both Crash while making the mex file

If I use :

Code:

//***** Important Part*********
	   for (k = 0; k < 3; k++)
	     hab2[k] = (double)heg2[k]; 
          *vro = hab2; //This Line is Having PROBLEMS

The programs tell me: error: incompatible types in assignment

The wrapper can send out a single value very well, but I need a array to come out of the wrapper.

Thank You

tabstop · 10-02-2009, 02:27 PM

So having looked up all these mxfunctions, if you want an array of doubles, shouldn't you create an array of doubles with your plhs[0] thing? It looks like you are not.

MK27 · 10-02-2009, 03:17 PM

*vro = hab2; //This Line is Having PROBLEMS

That line should be:

Code:

vro = hab2;

* dereferences the pointer.

LennoZebra · 10-02-2009, 03:46 PM

Awesome! Thanks Guys. No crashing anymore and the function is now outputting a array of zeros. I'm happier :-). I think there may be another problem with how I am declaring the mxCreateDoubleMatrix, but thats not a C issue.

Thank You so Much for all your help! Hope everyone have a nice weekend

10-02-2009, 12:02 PM	#1
LennoZebra Registered User Join Date: Oct 2009 Posts: 5	How to change Array Data type? Hello, I was wondering if there was a way to change a array full of float values to an array of double values and then reference the new array to the value of a pointer? Thank You
LennoZebra is offline

10-02-2009, 12:18 PM	#3
LennoZebra Registered User Join Date: Oct 2009 Posts: 5	I used a FOR loop to cast each variable in the array up to a data type double. But when I try and set the array to the pointer like: *vrp = hab2; where hab2 is a array of 300 units, it tells me that they are incompatible data types? Thank You
LennoZebra is offline

10-02-2009, 12:35 PM	#4
nonoob Registered User Join Date: Sep 2008 Location: Toronto, Canada Posts: 507	No. Since every array element uses 4 byes for float vs. 8 bytes for double, you can not cast the entire array at once.
nonoob is offline

10-02-2009, 12:45 PM	#5
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	Do you want a copy of the values in a separate array, or do you want both arrays to point to the same place? If the later, there is no point, since a double and a float are stored differently. You should post that code that you tried. __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
MK27 is offline

10-02-2009, 01:37 PM	#8
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	What kinds of problems are you having? Specifically.
tabstop is offline

10-02-2009, 02:09 PM	#9
LennoZebra Registered User Join Date: Oct 2009 Posts: 5	if I use : Code: //*** Important Part***** for (k = 0; k < 3; k++) vro[k] = (double)heg2[k]; //This Line is Having PROBLEMS //I create the mex file using Octave and this part does not work Then MATLAB or Octave both Crash while making the mex file If I use : Code: //* Important Part******* for (k = 0; k < 3; k++) hab2[k] = (double)heg2[k]; *vro = hab2; //This Line is Having PROBLEMS The programs tell me: error: incompatible types in assignment The wrapper can send out a single value very well, but I need a array to come out of the wrapper. Thank You
LennoZebra is offline

10-02-2009, 02:27 PM	#10
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	So having looked up all these mxfunctions, if you want an array of doubles, shouldn't you create an array of doubles with your plhs[0] thing? It looks like you are not.
tabstop is offline

10-02-2009, 03:17 PM	#11
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	vro = hab2; //This Line is Having PROBLEMS That line should be: Code: vro = hab2; dereferences the pointer. __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
MK27 is offline

10-02-2009, 03:46 PM	#12
LennoZebra Registered User Join Date: Oct 2009 Posts: 5	Awesome! Thanks Guys. No crashing anymore and the function is now outputting a array of zeros. I'm happier :-). I think there may be another problem with how I am declaring the mxCreateDoubleMatrix, but thats not a C issue. Thank You so Much for all your help! Hope everyone have a nice weekend
LennoZebra is offline

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