index must be a positive integer or logical error

I have defined a 3x443 matrix q in a quasi-1D CFD solver written as a Level II MATLAB S-Function. At iteration 53726 the Simulink simulation stops and I get the following error message:

Error evaluating registered method 'Outputs' of MATLAB S-Function 'File_S2' in 'File_SM_V1/Nozzle/Level-2 MATLAB S-Function'. The following is the MATLAB call stack (file names and line numbers) that produced this error: ['/home/APSE/Simulink/File_S2.m'] [245]

Caused by: Attempted to access q(2,1); index must be a positive integer or logical.

I don't understand the error since '2' and '1' are both positive integers and q(2,1) exists. Any advice on what could be causing the error would be appreciated.

Answer 1

[Walter Roberson]

It could be that the 2 or 1 are the printable representations of numbers very close to 2 or 1.

Put a breakpoint in at that line, and check the indices, subtracting the value that it appears to be from the actual variable holding the index. Make sure you are using

format long g

to get the proper printout.