How to substract a loop from the first 10 cell values of the loop?

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Wolfgang McCormack 2021-3-11

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https://ww2.mathworks.cn/matlabcentral/answers/770223-how-to-substract-a-loop-from-the-first-10-cell-values-of-the-loop

评论： Wolfgang McCormack 2021-3-12

采纳的回答： Jorg Woehl

Hi everyone,

I have a loop which reads an excel file and calculates some stuff and read a value to cells in a row

For n= 1:200

X(1,n) = C(1,2)

X(2,n) = X(1,n)- [ X(1,n) I want this tem remain constant in pattern of 10. For instance, X(1,101)-X(1,1), X(1,102) - X(1,2) up to ten and again X(1,111)-X(1,1), X(1,112)-X(1,2) ....)

end of n

Could you please help me to get this done. Thank you!

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Answer 1

Jorg Woehl 2021-3-11

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https://ww2.mathworks.cn/matlabcentral/answers/770223-how-to-substract-a-loop-from-the-first-10-cell-values-of-the-loop#answer_645558

在 MATLAB Online 中打开

Would this work?

X(2,n) = X(1,n) - X(1,mod(n-1,10)+1)

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Jorg Woehl 2021-3-12

在 MATLAB Online 中打开

Hi Wolfgang, my pleasure :)... And yes, you can reduce the interval to every 4 simply by replacing the 10 by 4. To understand what the statement does, let's do a smaller loop:

for n=1:20
    mod(n-1,10)+1
end

This gives the series 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 (and so on for larger n).

The mod function is the remainder (modulo) of the first argument (n-1) divided by the second (10). I first tried

mod(n,10)

but that yields the series 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0. Which doesn't work here, because we don't want 0 as an index into X. And we also don't want to stop at index 9, but at 10.

Which means that we need to add 1 to the mod result, but then the series starts with 2 3 4..., while we want it to start with 1 2 3.... The solution to that is to subtract 1 before taking the remainder: mod(n-1,10) + 1.

So, the modulo function gives you the repeating blocks that you want, and after that it's a little bit of playing around until everything is in place.

Wolfgang McCormack 2021-3-12