Why do I get an "undeclared identifier" error when using MEX with the option -D in MATLAB 7.7(R2008b)?

While using the following code :

var1 = var1_GUI; % value set from the GUI script

outputName = sprintf('file%d', var1);

mex -DVAR1=var1 -output outputName file.c

I get the following error messages:

 Error ...file.c: <lineNumber> undeclared identifier `var1'

Answer 1

[MathWorks Support Team]

The MEX function does not evaluate MATLAB variables from the workspace. In order to evaluate workspace variables, replace the following code:

mex -DVAR1=var1 file.c

with:

eval(['mex -DVAR1=' var1 ' file.c])'

This will make sure that the value of var1 is printed into a string and therefore evaluated.