Documentation

A temporary variable is any variable that is the target of a direct, nonindexed assignment, but is not a reduction variable. In the following parfor-loop, a and d are temporary variables:

In contrast to the behavior of a for-loop, MATLAB^® effectively clears any temporary variables before each iteration of a parfor-loop. To help ensure the independence of iterations, the values of temporary variables cannot be passed from one iteration of the loop to another. Therefore, temporary variables must be set inside the body of a parfor-loop, so that their values are defined separately for each iteration.

MATLAB does not send temporary variables back to the client. A temporary variable in the context of the parfor statement has no effect on a variable with the same name that exists outside the loop, again in contrast to ordinary for-loops.

Uninitialized Temporaries

Because temporary variables are cleared at the beginning of every iteration, MATLAB can detect certain cases in which any iteration through the loop uses the temporary variable before it is set in that iteration. In this case, MATLAB issues a static error rather than a run-time error, because there is little point in allowing execution to proceed if a run-time error is guaranteed to occur. This kind of error often arises because of confusion between for and parfor, especially regarding the rules of classification of variables. For example, suppose you write

  b = true;
  parfor i = 1:n
     if b && some_condition(i)
        do_something(i);
        b = false;
     end
     ...
  end

This loop is acceptable as an ordinary for-loop, but as a parfor-loop, b is a temporary variable because it occurs directly as the target of an assignment inside the loop. Therefore it is cleared at the start of each iteration, so its use in the condition of the if is guaranteed to be uninitialized. (If you change parfor to for, the value of b assumes sequential execution of the loop, so that do_something(i) is executed for only the lower values of i until b is set false.)

Temporary Variables Intended as Reduction Variables

Another common cause of uninitialized temporaries can arise when you have a variable that you intended to be a reduction variable, but you use it elsewhere in the loop, causing it technically to be classified as a temporary variable. For example:

s = 0;
parfor i = 1:n
   s = s + f(i);
   ...
   if (s > whatever)
      ...
   end
end

If the only occurrences of s were the two in the first statement of the body, it would be classified as a reduction variable. But in this example, s is not a reduction variable because it has a use outside of reduction assignments in the line s > whatever. Because s is the target of an assignment (in the first statement), it is a temporary, so MATLAB issues an error about this fact, but points out the possible connection with reduction.

Note that if you change parfor to for, the use of s outside the reduction assignment relies on the iterations being performed in a particular order. The point here is that in a parfor-loop, it matters that the loop "does not care" about the value of a reduction variable as it goes along. It is only after the loop that the reduction value becomes usable.