How can I subtract columns for each row by using a for loop

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Jimmy 2014-9-29

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编辑： Andrei Bobrov 2014-9-30

采纳的回答： dpb

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Hi,

I have a matrix like this:

[1.011  1.004  1.054
008  0.998  1.042
984  0.988  1.024
026  1.006  1.016
000  0.996  0.977]

I would like to subtract each column for each row and store these results in a new matrix. How can I do this?

Thanks!

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Joseph Cheng 2014-9-29

can you expand on what you mean by subtract each column for each row? I do not understand what you're subtracting with.

Jimmy 2014-9-29

Ok.

In this example I use 3 columns. I want to subtract the column for each possible combinations for each row. So lets say for row 1, I want subtract col 1 - 2, col 1 - 3, col 2 - 3 and so on. I want for row 2 the same subtractions.

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Answer 1

dpb 2014-9-29

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编辑：dpb 2014-9-29

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nk=nchoosek(1:size(x,2),2);
dx=zeros(size(x,1),size(nk,1));
for i=1:size(nk,1)
  dx=x(:,nk(i,2))-x(:,nk(i,1));
end

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Answer 2

Joseph Cheng 2014-9-29

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编辑：Joseph Cheng 2014-9-29

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you can use combnk() or nchoosek to determine the combination of column subtraction and perform a for loop for each combination.

X = randi(10,4,3);
combin = combnk(1:size(X,2),2);
for ind = 1:size(X,2)
   newX(:,ind) = X(:,combin(ind,1))-X(:,combin(ind,2));
end

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Answer 3

Guillaume 2014-9-29

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Use nchoosek to get all possible combinations of columns, and use that to calculate your differences:

m = [1.011  1.004  1.054
     1.008  0.998  1.042
     0.984  0.988  1.024
     1.026  1.006  1.016
     1.000  0.996  0.977];
colcomb = nchoosek(1:size(m, 2), 2);
coldiff = zeros(size(m, 1), size(colcomb, 1));
for comb = 1:size(colcomb, 1)
  coldiff(:, comb) = diff(m(:, colcomb(comb, :)), 1, 2);
end

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dpb 2014-9-29

For what working definition of small? But, basic idea is one of two choices...

a) go ahead and generate all pairs and then compute the comparison statistic and choose the N smallest of those, or,

b) compute each pair and the statistic at same time; after N replace the largest of those kept with the last if new is less; update the maxValue comparison value.

Jimmy 2014-9-30

I choose for option a. Since the 10 smallest pairs are different for each row (the rows are basically the time line (t).) In the end I want to trade each pair for 125 days on a rolling window. For that reason I want to know which pairs I am trading.

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Answer 4

Andrei Bobrov 2014-9-30

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https://ww2.mathworks.cn/matlabcentral/answers/156670-how-can-i-subtract-columns-for-each-row-by-using-a-for-loop#answer_153356

编辑：Andrei Bobrov 2014-9-30

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X = [1.011  1.004  1.054
     1.008  0.998  1.042
     0.984  0.988  1.024
     1.026  1.006  1.016
     1.000  0.996  0.977];
n = nchoosek(1:size(X,2),2);
out = squeeze(diff(reshape(X(:,n'),[],2,3),1,2));