For loop with vertical vs horizontal array

Is the below difference intended functionality or a bug?

x = [1, 2, 3, 4];
for y = x
    disp( 'Printing' );
    disp( y );
end

Result:

Printing
     1
Printing
     2
Printing
     3
Printing
     4

Whereas if I transpose x, the loop only runs once with y = [1; 2; 3; 4].

x = [1, 2, 3, 4]';
for y = x
    disp( 'Printing' );
    disp( y );
end

Result:

Printing
     1
     2
     3
     4

Perhaps I'm being thick, but I can't find this behavior documented anywhere and I would have expected it to loop through the elements in the matrix/array as if it were one dimensional (linear indexing).

Answer 1

[Image Analyst]

I agree it's confusing at first. However it is documented in the "for" section:

for index = valArray

creates 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 for 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 string, cell array, or struct.

In your sexcond case, n is 1 - one column so the loop executes only once. In your first case, n was 4 - four columns so the loop executed 4 times.

[Chris]

Ok, fair enough. I guess no smart way of making sure I get a linear indexing effect regardless of the valArray other than something like:

for index =  reshape( 1, numel(valArray) )

in which case I probably prefer:

for i = 1:numel(valArray)
    element = vallArray(i)

[Walter Roberson]

If you are using indices then use

for index = 1 : numel(valArray)

If you are using array values and do not know the orientation then

for index = valArray(:).'

[Chris]

Thanks, that is much nicer. Is the . in the .' necessary? If so what does it do differently than a simple '

[Walter Roberson]

The simple ' is conjugate transpose, in which the signs of the complex part of values are reversed. The .' operator is plain transpose.

[Chris]

got it, thanks

Answer 2

[Walter Roberson]

It is deliberate behavior.

http://www.mathworks.com/help/matlab/ref/for.html

valarray : creates 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 for 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 string, cell array, or struct.