Mex mxGetField do loop

Hello,

I'm trying to use a do loop in my mex function that will take the values passed via a struct form from matlab. The matlab struct is as follows:

TurbineComponents.Blade(1).Position = ...
TurbineComponents.Blade(1).Orientation = ...
etc

I try the following as my do loop in the mex file:

              Do J = 1, NumBl

              Blade1_pr = mxGetField(prhs(ArgNum),1,'Blade(J)')
              b1p_pr = mxGetField(Blade1_pr,1,'Position')
              b1p_ptr = mxGetPr(b1p_pr)
              b1o_pr = mxGetField(Blade1_pr,1,'Orientation')
              b1o_ptr = mxGetPr(b1o_pr)
              b1v_pr = mxGetField(Blade1_pr,1,'RotationVel')
              b1v_ptr = mxGetPr(b1v_pr)
              b1t_pr = mxGetField(Blade1_pr,1,'TranslationVel')
              b1t_ptr = mxGetPr(b1t_pr)

             call mxcopyptrtoreal8(b1p_ptr,TurbineComponents%Blade(J)%Position,size2)
             call mxcopyptrtoreal8(b1o_ptr,TurbineComponents%Blade(J)%Orientation,size3)   
             call mxcopyptrtoreal8(b1v_ptr,TurbineComponents%Blade(J)%RotationVel,size2)
             call mxcopyptrtoreal8(b1t_ptr,TurbineComponents%Blade(J)%TranslationVel,size2)

             end Do

The problem is coming from the first line of code, "Blade(J)" I believe. Is there a way to work around the field name?

Answer 1

[James Tursa]

Get Blade using the J as the index. E.g.,

Blade1_pr = mxGetField(prhs(ArgNum),J,'Blade')

[Chris]

Thanks James

[Chris]

Hi James, Changing to J as the index number worked for when J=1 however when it tries to move to the next blade, "TurbineComponents.Blade(2).Position", etc.. then matlab shuts down from a "segmentation error". Is there something wrong with the struct naming? Does some kind of preallocation need to be done?

[James Tursa]

This would be expected if not all elements are pre-allocated. When you create a struct or cell array the actual values for the elements are NULL pointers until you set them to something. So if you are passing a struct to your mex function with only some of the elements set, then you need to check for this condition inside your mex function (actually it is always a good idea to check for this regardless of what you expect). E.g.,

      Blade1_pr = mxGetField(prhs(ArgNum),J,'Blade')
      if( Blade1_pr == 0 ) cycle ! or whatever action is appropriate
      b1p_pr = mxGetField(Blade1_pr,1,'Position')
      if( b1p_pr == 0 ) cycle ! or whatever action is appropriate
      etc.

You will have to decide what action is appropriate when elements are not set. Maybe generate an error or maybe fill in with a default value, etc.

[Chris]

Thanks James