Is there a foreach in MATLAB? If so, how does it behave if the underlying data changes?

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)?

Solution 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

Solution 2

Zach is correct about the direct answer to the question.

An interesting side note is that the following two loops do not execute the same:

for i=1:10000
  % do something
end
for i=[1:10000]
  % do something
end

The first loop creates a variable i that is a scalar and it iterates it like a C for loop. Note that if you modify i in the loop body, the modified value will be ignored, as Zach says. In the second case, Matlab creates a 10k-element array, then it walks all elements of the array.

What this means is that

for i=1:inf
  % do something
end

works, but

for i=[1:inf]
  % do something
end

does not (because this one would require allocating infinite memory). See Loren's blog for details.

Also note that you can iterate over cell arrays.

Solution 3

The MATLAB for loop basically allows huge flexibility, including the foreach functionality. Here some examples:

1) Define start, increment and end index

for test = 1:3:9
   test
end

2) Loop over vector

for test = [1, 3, 4]
   test
end

3) Loop over string

for test = 'hello'
   test
end

4) Loop over a one-dimensional cell array

for test = {'hello', 42, datestr(now) ,1:3}
   test
end

5) Loop over a two-dimensional cell array

for test = {'hello',42,datestr(now) ; 'world',43,datestr(now+1)}
   test(1)   
   test(2)
   disp('---')
end

6) Use fieldnames of structure arrays

s.a = 1:3 ; s.b = 10  ; 
for test = fieldnames(s)'
   s.(cell2mat(test))
end

Solution 4

If you are trying to loop over a cell array and apply something to each element in the cell, check out cellfun. There's also arrayfun, bsxfun, and structfun which may simplify your program.

Solution 5

ooh! neat question.

Matlab's for loop takes a matrix as input and iterates over its columns. Matlab also handles practically everything by value (no pass-by-reference) so I would expect that it takes a snapshot of the for-loop's input so it's immutable.

here's an example which may help illustrate:

>> A = zeros(4); A(:) = 1:16

A =

     1     5     9    13
     2     6    10    14
     3     7    11    15
     4     8    12    16

>> i = 1; for col = A; disp(col'); A(:,i) = i; i = i + 1; end;
     1     2     3     4

     5     6     7     8

     9    10    11    12

    13    14    15    16

>> A

A =

     1     2     3     4
     1     2     3     4
     1     2     3     4
     1     2     3     4

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Comments

[Mr Fooz]

Yeah, I was surprised about this when I ran into it. This optimization of arrays actually takes place in many places. If you use bracket notation, sometimes you'll see performance warnings in the Matlab editor telling you it thinks it can optimize out the array allocation if you let it.

[Kleist]

That just prints 10 since numel(array) is the number of elements in array. perhaps you meant 1:numel(array)?

[Kleist]

If B is undefined, your first example doesn't print 1-5. It prints Undefined function or variable 'B'.

[yuk]

For the 1st example make sure that A is a row vector, not column vector. If A is a matrix, each k will be a column vector from that matrix. So, transpose(A') or vectorize (A(:)') if needed.

[Admin]

though, from experience I would say that their performance is equal or worst to writing a for-loop, better looking though, and who knows they might improve in the future.

[bobobobo]

-1 I do not think Java-like code should be your first choice way to work with Matlab in .m files.

[Oriol]

Wouldn't for i = -1:0.1:10; disp(i); end; be better ?

[Evgeni Sergeev]

With the cell array, take note that it will iterate over columns of the cell array.

[Dmytro]

greetings from the future; we come with plenty of solutions to iterator invalidation problem.

[Dmytro]

I hear Matlab has lazy evaluation now. If not, we do have the technology to implement them.