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

Quote:

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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

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Yep! it's called a cast.

Quote:

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Originally Posted by LennoZebra

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

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Not sure what you mean by the above?

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

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

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.

Quote:

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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

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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:

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

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

Code:

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    #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
         
        }
    }

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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:

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

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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

What kinds of problems are you having? Specifically.

if I use :

Code:

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//***** 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

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Then MATLAB or Octave both Crash while making the mex file


If I use :

Code:

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

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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

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.

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


That line should be:

Code:

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vro = hab2;

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* dereferences the pointer.

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