Iterating an empty matrix using a for loop

Question

I always assumed that iterating an empty vector using a for loop was the same as not having the loop at all. However, I stumbled upon this strange behavior:

for t = []          %// Iterate an empty 0x0 matrix
    1
end
for t = ones(1, 0)  %// Iterate an empty 1x0 matrix
    2
end
for t = ones(0, 1)  %// Iterate an empty 0x1 matrix
    3
end

The result is:

ans =
    3

Does it make sense, or is this a bug?

Dennis Jaheruddin · Accepted Answer · 2013-07-30T15:44:42.067

22

The for loop runs over all columns of its input. Since a 0x1 matrix has one (empty) column, the loop will simply go over that. No exception is mentioned for empty matrices, so here t will simply be the empty matrix as seen from:

for t = ones(0, 1) %// Iterate over an empty 0x1 matrix
    size(t) % t is a 0x1 matrix
end

Is it a bug? Probably not.
Does it make sense? Well, I think I would prefer the loop not to execute if the input is empty, but probably there are advantages to this as well.

At least it is definitely something to be alert of!

edited Jul 30 '13 at 15:44

answered Jul 30 '13 at 15:16

Dennis Jaheruddin

21,208
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do you mean `for t = t'` ? – Buck Thorn Jul 30 '13 at 15:43
@TryHard My question was edited and someone removed the `v`. I have updated the answer to match. – Dennis Jaheruddin Jul 30 '13 at 15:45
@DennisJaheruddin - This was something I wasn't aware about, and this makes total sense. Thanks for this tidbit of information! – rayryeng Jul 14 '14 at 18:23
Hi @DennisJaheruddin, would you please accept the answer to mark the question resolved? (Unless, of course, you're expecting other answers...) – Dev-iL Oct 23 '17 at 10:59

Iterating an empty matrix using a for loop

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