How do you write a loop for a variable that will list 6 rows but you want the equation to apply for those 6 rows and 30 rows after each of them

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Mariana Guerrero 2022-8-2

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https://ww2.mathworks.cn/matlabcentral/answers/1772915-how-do-you-write-a-loop-for-a-variable-that-will-list-6-rows-but-you-want-the-equation-to-apply-for

回答： dpb 2022-8-3

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the variable "RA4" contains 6 rows that indicate where a perturbation starts.

RA4 = 5721, 51164, 69680, 98014, 118377, 130204.

I am trying to write a loop that will apply the

XError equation -> abs(COMArray(:,1))-abs(New_COM(:,1));

to each of those RA4 rows and the next 30 for each. Meaning (5721:5751), etc. How can I write that?

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David Hill 2022-8-2

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Hard to understand without an example. Not sure what you mean by applying the XError equation to each row.

abs(COMArray(:,1))-abs(New_COM(:,1));%this looks like it is applying to a column 
%RA4(5721,:) would be the row, take that row (I don't know the size of the
%row) and apply it how to the XError equation?

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Answer 1

dpb 2022-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/1772915-how-do-you-write-a-loop-for-a-variable-that-will-list-6-rows-but-you-want-the-equation-to-apply-for#answer_1020685

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nAddLines=30;
for i=1:numel(RA4)
  ix1=RA4(i);
  ix2=ix1+nAddLines;
  XError(?)=abs(COMArray(ix1:ix2,1))-abs(New_COM(ix1:ix2,1)); 
end

You'll have to have some indexing expression to compute a place to put the results -- and how to keep track of which part goes where...that's that the "?" placeholder is for--there's where you need an expression -- or you could just have a cell array each of which cells contains the results for each iteration as the quick 'n dirty way out.

The above applies the calculation only to a first column of some array as in your sample code snippet.