Question

Is there a foreach structure in MATLAB? If so, what happens if the underlying data changes (i.e. if objects are added to the set)?

Answer 1

MATLAB’s FOR loop is static in nature; you cannot modify the loop variable between iterations, unlike the for(initialization;condition;increment) loop structure in other languages. This means that the following code always prints 1, 2, 3, 4, 5 regardless of the value of B.

A = 1:5;  for i = A     A = B;     disp(i); end 

If you want to be able to respond to changes in the data structure during iterations, a WHILE loop may be more appropriate — you’ll be able to test the loop condition at every iteration, and set the value of the loop variable(s) as you wish:

n = 10; f = n; while n > 1     n = n-1;     f = f*n; end disp(['n! = ' num2str(f)]) 

Btw, the for-each loop in Java (and possibly other languages) produces unspecified behavior when the data structure is modified during iteration. If you need to modify the data structure, you should use an appropriate Iterator instance which allows the addition and removal of elements in the collection you are iterating. The good news is that MATLAB supports Java objects, so you can do something like this:

A = java.util.ArrayList(); A.add(1); A.add(2); A.add(3); A.add(4); A.add(5);  itr = A.listIterator();  while itr.hasNext()      k = itr.next();     disp(k);      % modify data structure while iterating     itr.remove();     itr.add(k);  end