Why is my variable not available inside a parfor loop?

I've run into some behavior that I can't understand, In the following piece of code:

exist('srRays','var')

    parfor ii=1:nr 

               exist('srRays','var')

                str=etags{ii};
                tf_localEvent = (str(1)=='L'); 

                if tf_localEvent
                    [x{ii} y{ii} z{ii} d{ii} ttm(ii) itm(ii)] = fetchrayLocal(srGeometry, srModel, srRays, data.evt, iprec, etags{ii}, xsta, ysta);
                else
                    [x{ii} y{ii} z{ii} d{ii} ttm(ii) itm(ii)] = fetchray(srtimes, srindex, etags(ii), BT2, iprec, srModel, xsta, ysta);
                end

            end

The variable 'srRays' is on the workspace before going into the parfor loop, but then inside the loop Matlab can't find it. So that in the line before the loop

exist('srRays','var')

results in

ans = 1

in the line inside the loop, it results in

ans = 0

and then I get an error when it tries to use it as an input to the function 'fetchrayLocal'. Does anybody know why my variable is disappearing? if it helps, srRays is a (1x1) structure, so are srGeometry and srModel.

Thanks,

Max

[Walter Roberson]

Is srRays a global variable? Global variables are not communicated to workers.

[Shayan Modiri]

This is true, you shouldn't use Global Variables in parfor loops. You can check this before parfor:

whos m

If you see Global in Attributes, it means this is a Global variable. You can easily copy this variable before your parfor to tmp_srRays and use this new variable in your parfor.

[Jan]

Slightly faster and nicer:

tf = strncmp(etags, 'L', 1);
parfor ii = 1:nr 
  if tf(ii)
  ...

Answer 1

[Adel H]

We had a similar situation for variables we loaded from a .mat file.

As recommended by the troubleshooter, https://uk.mathworks.com/help/matlab/import_export/troubleshooting-loading-variables-within-a-function.html , we tried explicitly loading variable names.

However, that did not help.

What did fix the issue, was re-assigning these variables to themselves before the parfor loop. e.g:

subDegRes = subDegRes;
a2PLimit = a2PLimit;
a2P = a2P;