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for loop options in Matlab

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Ravi
Ravi 2012-9-24
回答： Shadow 2024-4-2
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

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采纳的回答

Walter Roberson
Walter Roberson 2012-9-24
You would need the more traditional form. There is no way to query which iteration number you are on.

更多回答（2 个）

Matt Fig
Matt Fig 2012-9-24
A = magic(5)
cnt = 1;
for jj = A
disp(['Column #' num2str(cnt)])
disp(jj),
cnt = cnt+1;
end
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Walter Roberson
Walter Roberson 2012-9-24
Oh... yes, you are right, I had forgotten that it went by columns when matrices are used. blush
Ravi
Ravi 2012-9-24
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.

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Shadow
Shadow 2024-4-2
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

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