for loop value array must be a row vector(?)

Hi all,

The next-to-simplest matlab code is approximately:

idxs = [1,2,3];
for k = idxs
k
end

k = 1

k = 2

k = 3

But if the idxs array is a column vector, the results are rather different:

idxs = [1;2;3];
for k = idxs
k
end

k = 3×1

1 2 3

That is, you get one value of k equal to the whole column vector. I did not know that. I've been using Matlab for almost 30 years. How did I miss that?

[Stephen23]

This is an ancient MATLAB feature that no one uses, no one likes, and only causes bugs:

https://www.mathworks.com/matlabcentral/answers/468253-can-you-explain-this-behaviour

https://www.mathworks.com/matlabcentral/answers/1728680-is-it-possible-to-go-through-the-elements-of-an-array-without-resorting-to-length-in-a-for-loop#comment_2183070

https://www.mathworks.com/matlabcentral/answers/367676-outputting-multiple-matrix-of-a-function-within-a-for-loop#answer_291759

I for one would love to be able to write:

for k = find(..)

without the ugly rigmarole and obfuscation that is currently required.

No doubt some at TMW worry about "breaking existing code", but that argument does not hold much water:

• no one uses this syntax
• it causes far more bugs than it being used as intended,
• TMW have broken other fairly fundamental syntaxes (e.g. input/output argument order of some basic functions, changed default first/last value returned, changes to scoping rules, etc. etc.) without causing the end of the world.

Just like IF accepting non-scalar values, this is something that should have been retired a long time ago.

[Walter Roberson]

@Stephen23

I have occasionally taken advantage of this aspect of for. Not very much at all.

Answer 1

[Cris LaPierre]

I didn't realize that either. It does appear to be documented.

• valArray — Create a column vector, index, from subsequent columns of array valArray on each iteration. For example, on the first iteration, index = valArray(:,1). The loop executes a maximum of n times, where n is the number of columns of valArray, given by numel(valArray(1,:)). The input valArray can be of any MATLAB® data type, including a character vector, cell array, or struct.

So it is the number of columns that determines how many times your loop runs, and index is all values in the nth column.