Why am I unable to assign values to matrix blocks like A(10:20,i) inside my PARFOR loop in Parallel Computing Toolbox 4.1 (R2009a) ?

I am attempting to modify a block of elements in my matrix within a PARFOR loop using the code below

a = rand(10);

parfor i = 1:4

   a(2:4,i) =  ones(3,1);   

end

However, when I execute the code above, I get the following error

 ??? Error: The variable A in a parfor cannot be classified.

See Parallel for Loops in MATLAB, "Overview".

Why do I get this error message?

Answer 1

[MathWorks Support Team]

This error message obtained is the expected behavior when using PARFOR loops in Parallel Computing Toolbox 4.1 (R2009a).

PARFOR loops enforce stringent conditions on variables and expressions that appear in the loop body. As per the documentation of PARFOR, the variable A will be treated as a sliced variable as its value can be broken up into segments, each of which can be operated on separately by different workers.

The rules for indexing a sliced variable clearly state that within the list of indices for a sliced variable, one of these indices is of the form i, i+k, i-k, k+i, or k-i, where i is the loop variable and k is a constant or a simple (nonindexed) variable; and every other index is a constant, a simple variable, colon, or end.

Hence using an index of the form

 A(2:4, i)

is not permitted.

If the indices to be extracted are known apriori, the following workaround can be used

A = rand(10, 10);

b = (2:4);

parfor i = 1:4

      A(b, i) = ones(3, 1);

end

The above code snippet adheres to the conditions imposed on sliced variables.