for loop options in Matlab

I just learned about Matlab's ability to use matrices as arguments to a for loop;

This displays the columns of the matrix A

A = magic(5)
for n = A
disp(n)
end

I was wondering if it is possible to obtain a sequential index into a loop like this. ie. if I want to print the column number along with the column itself, can I continue with the form above or would I need to revert to a more typical form like

A = magic(5)
for jj = 1:length(A)
    disp(['Column #',num2str(jj)])
    disp(A(:,jj))
end

Answer 1

[Walter Roberson]

You would need the more traditional form. There is no way to query which iteration number you are on.

Answer 2

[Matt Fig]

A = magic(5)
cnt = 1; 
for jj = A 
  disp(['Column #' num2str(cnt)])
  disp(jj),
  cnt = cnt+1; 
end

[Ravi]

Thanks for this. Unfortunately, it looks like I can only mark one answer as accepted...

[Matt Fig]

Walter, did you look closely at the code? I ran it, and it does exactly what Ravi asked for...

Column #1
    17
    23
     4
    10
    11

Column #2
    24
     5
     6
    12
    18

Column #3
     1
     7
    13
    19
    25

Column #4
     8
    14
    20
    21
     2

Column #5
    15
    16
    22
     3
     9

[Walter Roberson]

Oh... yes, you are right, I had forgotten that it went by columns when matrices are used. blush

[Ravi]

I see... Backstory is that I was thinking about rewriting some existing code with for loops using the matrix approach for potential performance improvements. I did some quick tests and both methods take almost identical times. I can see the matrix approach being helpful down the line. Kind of like having a 'foreach' command.

Answer 3

[Shadow]

enumer = @(my_array) cell2mat(arrayfun(@(x,idx) struct("cargo",x,"idx",idx), my_array(:).', 1:numel(my_array(:).'),UniformOutput=false));

data = rand(5,4)
s=size(data);
for elem = enumer(data)
    [row,col] = ind2sub(s,elem.idx);
    disp("Element " + string(elem.cargo) + " with index " + string(elem.idx) + " at location " + string(row) + "," + string(col))
end