Is there a workaround to the 'parfor' command so that it can work for non-consecutive iteration steps?

Currently, the parfor command takes an iteration step of one, and it beats my intention of speeding up the loop calculation.

Answer 1

[Walter Roberson]

If you are using the parfor iteration number as a data parameter, then create a vector with all of the desired parameters and use a sequential parfor index to index that vector. For example instead of

total = 0;
parfor K = [1 2 4 8 16]
  total = total + B.^K;
end

you would use

K_vals = [1 2 4 8 16];
total = 0;
parfor Kidx = 1 : length(K_vals)
  K = K_vals(Kidx);
  total = total + B.^K;
end

This will not work if you are using K to index an array you are writing into.

If you are using the parfor iteration number to iterate in a "random" order, then leave out the re-ordering: parfor does not iterate sequentially anyhow. For example, instead of

parfor K = randperm(123)
  X(K) = ....
end

just use

parfor K = 1 : 123
  X(K) = ...
end

If you are using the parfor iteration number to select a subset of the data, then the work-around is to extract the needed subset of the data and iterate sequentially over the subset, and later write the subset back. For example, instead of

parfor K = [1 2 4 8 16]
  X(K) = X(K) * 2;
end

use

K_vals = [1 2 4 8 16];
X_subset = X(K_vals);
parfor K = 1 : length(K_vals)
    X_subset(K) = X_subset(K) * 2;
end
X(K_vals) = X_subset;