how to decrease for loop variable counter when deleting rows in a matrix

Question

Gideon Maasz 2018-7-31

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https://ww2.mathworks.cn/matlabcentral/answers/412926-how-to-decrease-for-loop-variable-counter-when-deleting-rows-in-a-matrix

回答： Stephen23 2018-7-31

I have an array A in which I delete rows based n a certain condition within another corresponding matrix which consists of the mean values of A. Each time I delete a row, I want my 'i' counter to jump back 1 iteration in order to not skip rows.

Means_Array=mean(A,2);
for i = 1:size(A,1)
       if Means_Array(i,1)==0
             Means_Array(i,:)=[];
             A(i,:)=[]; 
             i=i-1
     end
end

This does however not work for some reason and the 'i' resets back to the value it would have taken if the i=i-1 was not even there.

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Answer 1

Stephen23 2018-7-31

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Just specify the for loop values to count downwards:

for i = size(A,1):-1:1
    ...
end

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Answer 2

Steven Lord 2018-7-31

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That is correct. In the documentation for the for keyword: "Avoid assigning a value to the index variable within the loop statements. The for statement overrides any changes made to index within the loop."

Instead of deleting rows one by one in the loop, create a logical vector indicating either rows to keep or rows to delete, and use that logical vector after the loop is complete.

M = magic(5)
keep = true(size(M, 1), 1);
toDelete = false(size(M, 1), 1);
for therow = 1:size(M, 1)
  if M(therow, 1) < 11
    keep(therow, 1) = false;
    toDelete(therow, 1) = true;
  end
end
M2 = M(keep, :)
M3 = M;
M3(toDelete, :) = []