Problem using :(colon operator) and ndgrid in Matlab Coder

Hello everyone, i have a problem using a cell array with argument {:} and ndgrid function into Matlab Coder. In detail, in the original code i have:

a matrix:

a=[  5   185 ; 50   230;    95   275;   140   320];

a cell c made in this way:

c={};
for i=1:length(a)
c{1,i}=[a(i,:)];
end

c =
  1×4 cell array
    {1×2 double}    {1×2 double}    {1×2 double}    {1×2 double}
celldisp(c)
c{1} =
     5   185
c{2} =
    50   230
c{3} =
    95   275
c{4} =
   140   320

a matrix B with all combinations of c, made in this way:

[c{:}]=ndgrid(c{:});
n=length(c);
B = reshape(cat(n+1,c{:}),[],n);
B = sort(B,2);

B =
     5    50    95   140
    50    95   140   185
     5    95   140   230
    95   140   185   230
     5    50   140   275
    50   140   185   275
     5   140   230   275
   140   185   230   275
     5    50    95   320
    50    95   185   320
     5    95   230   320
    95   185   230   320
     5    50   275   320
    50   185   275   320
     5   230   275   320
   185   230   275   320

BUT, using Matlab Coder to translate that code, it gives me this (see down) error referred to the colon ":" inside [c{:}]=ndgrid(c{:}); and inside B = reshape(cat(n+1,c{:}),[],n);

"Non-constant expression or empty matrix. This expression must be constant because its value determines the size or class of some expression."

So, i want to obtain B matrix but not using ndgrid (i think about a for-loop istead of it) or any kind of alternative code to do this, specially not using colon ":" except in a for-loop.

Thank you very much :)

Answer 1

[Darshan Ramakant Bhat]

I modified the code a little bit and the code generation was successful

function B = myFunc()
a=[  5   185 ; 50   230;    95   275;   140   320];
c=cell(1,length(a));
for i=1:length(a)
c{1,i}=[a(i,:)];
end
z = cell(1,length(c));
[z{:}]=ndgrid(c{:});
n=length(z);
B = reshape(cat(n+1,z{:}),[],n);
B = sort(B,2);

end

>> codegen myFunc -report

Hope this will be helpful

[giovanni bonciani]

Thank you for your answer Darshan, but Matlab Coder gives me same error at [z{:}]=ndgrid(c{:}); referring to the colon ":" in z and in c:

"Non-constant expression or empty matrix. This expression must be constant because its value determines the size or class of some expression."

So i need a code that can be completely different from what i write upon

[Darshan Ramakant Bhat]

Which version of MATLAB are you using ? It worked for me in R2019b

Answer 2

[Giovanni Bonciani]

If you see down in "release" field, i wrote it, it's 2019b

[Darshan Ramakant Bhat]

I am sorry I missed to look into the "release" field. I tried the "myFunc" code that I had pasted both in R2019b and R2019a, codegen is successful and below is my output from the codegenerated MEX :

>> codegen myFunc.m -report
Code generation successful: View report
>> myFunc_mex()

ans =

     5    50    95   140
    50    95   140   185
     5    95   140   230
    95   140   185   230
     5    50   140   275
    50   140   185   275
     5   140   230   275
   140   185   230   275
     5    50    95   320
    50    95   185   320
     5    95   230   320
    95   185   230   320
     5    50   275   320
    50   185   275   320
     5   230   275   320
   185   230   275   320
   

I am not sure what I am missing here.

[giovanni bonciani]

i tried to generate .mex file, but it failed:

>> codegen myFunc -report
??? Unable to copy file from 'C:\Users\admin\Desktop\codegen\mex\myFunc\myFunc_mex.mexw64' to 'C:\Users\admin\Desktop\myFunc_mex.mexw64'.

Code generation failed: View Error Report
Error using codegen 

moreover, Matlab creates a folder "codegen" in which there are various .c file, so it translates to C language, but i use Matlab Coder tool to translate in C++ language (.cpp).

So, do you have any idea in which way can i write my code?

[Darshan Ramakant Bhat]

From the log it looks like that the MEX creation is successful but it had trouble in copying the generated MEX (may be some permission issue in the folder).

You can change the target language for codegeneration using the below command :

>> cfg = coder.config('lib');
>> cfg.TargetLang = "C++";
>> codegen -config cfg myFunc -d 'myFolder' -report

Please read the below documentation pages to get more insight into the commands :

https://www.mathworks.com/help/coder/ref/coder.codeconfig.html

https://www.mathworks.com/help/coder/ref/codegen.html

Hope this will be helpful for you.