Basic question about loops

Very basic question: why is this not working?

instead of

Idx2=Idx(:,2)
Idx3=Idx(:,3)
Idx4=Idx(:,4)
Idx5=Idx(:,5)

I wrote

for i=2:5
    Idx(i)=Idx(:,i)
end

The error is: Unable to perform assignment because the left and right sides have a different number of elements.

[millercommamatt]

Let me make sure I understand what you are trying to achieve.

You want to break out each column - or some subset of columns - of a 2-D array into their own variables?

[Stephen23]

"Very basic question: why is this not working?"

Because you chose a complex and indirect way to write your code.

Numbering variables like that is a sign that you are doing something wrong.

Forcing meta-data (e.g. pseudo-indices) into variable names is one way that beginners force themselves into writing slow, complex, inefficient, obfuscated, buggy code that is difficult to debug.

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Your task could be solved simply and much more efficiently using indexing.

[Pelajar UM]

yes, i have seen this thread before but can't quite understand how indexing works.

Can you take a line of my code and re-write it?

[Stephen23]

"but can't quite understand how indexing works. "

You are using indexing already on the RHS. That is how indexing works.

Splitting the data up into numbered variables is most likely a superfluous step: why can't you just access the data using that indexing? Or if splitting is required, just use a cell array (e.g. via NUM2CELL) rather than lots of variables.

[Pelajar UM]

Ok. I think I'm starting to understand what you mean, but how do I output the results to a single array?

Like here centroid is a 10x3 array, so for i=2:10, I expect to get 10x(3x8) array but I still get a 10x3 array.

for i=2:10
    [sortedcentroid]=centroid(Idx(:,i),:);
end

[Stephen23]

"...how do I output the results to a single array?"

Use indexing on the LHS, just like you are already doing on the RHS.

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Preallocate the ND array using nan(..) and then index into it. If the number of elements returned on each iteration are different then you will have to use a cell array instead:

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

[Pelajar UM]

I have tried but it keeps saying the number of elements are different on reach side. How should I index the LHS?

Idx is 11446x10 and centroid is 11446x3.

I have added the preallocation:

sortedcentroid = nan(size(centroid));

[Stephen23]

The size of each RHS depends on the indices, which you have told us nothing about. So I can only presume that they are different on each iteration. In that case, you could use a cell array:

C = cell(1,10);
for k = 2:10
    C{k} = centroid(Idx(:,k),:);
end

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

[Pelajar UM]

Perfect! Thank you so much.

Small error: your code mixes k and i.

[Stephen23]

"Small error: your code mixes k and i"

Fixed, thank you!

If the content of C are of suitable sizes then after the loop you could concatenate them back into one array, e.g.:

A = cat(3,C{:})

Answer 1

[millercommamatt]

You could make use of genvarname, but this would work too.

for i = 2:5
    eval(['Idx' num2str(i) ' = Idx(:,' num2str(i) ');']);
end

When you start using functions like genvarname and eval, it's usually a sign that you should be approaching your task in a different way.

[Pelajar UM]

Yes, this is exactly what I wanted. Thanks. But what happens if I'm transposing a variable. Something like this:

angle2 = transpose (rad2deg (atan2(vecnorm(cross(N',sortedN2')), dot(N',sortedN2'))))
angle3 = transpose (rad2deg (atan2(vecnorm(cross(N',sortedN3')), dot(N',sortedN3'))))
angle4 = transpose (rad2deg (atan2(vecnorm(cross(N',sortedN4')), dot(N',sortedN4'))))
angle5 = transpose (rad2deg (atan2(vecnorm(cross(N',sortedN5')), dot(N',sortedN5'))))

[millercommamatt]

Something like this. Study how to manipulate strings.

for i = 2:5
    eval(['angle' num2str(i) ' = transpose (rad2deg (atan2(vecnorm(cross(N'',sortedN' num2str(i) ''')), dot(N'',sortedN' num2str(i) '''))));']);
end

[Pelajar UM]

Wonderful. Thanks a lot.

[Stephen23]

"Yes, this is exactly what I wanted"

and is a slow, complex, inefficient, obfuscated approach this task.