How to sum a specific range of values in a column?

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Edmundas Povilavicius 2016-11-5

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评论： Guillaume 2016-11-7

Hello,

I'm trying to build a function that would read all the elements in a column matrix "a" then find the same value in column matrix "b" read the exact row and lastly sum all rows from exact row to exact row-30, and make it save all the answers in a new matrix.

This is what I have so far, but something is wrong.

 function [psum, nsum] = datac (a, b, c)
 lngth=length(a);
 for i=1:lngth
    eyear=a(i, 1);
    row=find(b(:,1)==eyear);
    row2=row-30;
    psum(1,i)=sum(c(row2:row, 1) >0);
    nsum(1,i)=sum(c(row2:row, 1) <0);
end

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Edmundas Povilavicius 2016-11-6

With you suggestions, this is what I finally wrote:

 Superset = datenum(datedif);
 Subset = datenum(electiondates);
 [isfound, daterows] = ismember(electiondates, datedif);
 for i=1:numel(daterows)
    if isfound(i)==0
        Subset(i)=Subset(i)-1;
        electiondates(i)=datestr(Subset(i));
        [isfound(i),~] = ismember(electiondates(i), datedif);
        if isfound(i)==0
            Subset(i)=Subset(i)-1;
            electiondates(i)=datestr(Subset(i));
        end
    end
 end
 [~, daterows] = ismember(electiondates, datedif);
 for j = 1:numel(daterows)
    sumdif = difrence(daterows(j) : daterows(j)+29);
    psum(j) = sum(sumdif(sumdif> 0));
    nsum(j) = sum(sumdif(sumdif< 0));
 end

Any suggestions for improvement?

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Answer 1

Guillaume 2016-11-5

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https://ww2.mathworks.cn/matlabcentral/answers/310790-how-to-sum-a-specific-range-of-values-in-a-column#answer_242112

A few notes first. 1) Avoid length, it's a dangerous function. Use numel or size with an explicit dimension. 2) There's no point indexing the singleton dimension with vectors, a(i) is simpler and clearer than a(i, 1) for vectors. 3) Use meaningful variable names.

Assuming that your election dates are stored as datenum or datetime (preferable), then:

%variables:
%electiondates: datenum or datetime vector
%datedif: datenum or datetime vector (or better column of a table)
%difrence: numeric vector (or better column of a table)
[~, daterows] = ismember(electiondates, datedif); %find location of each election date in datedif. Assumes all dates are found
positivesum = zeros(size(electiondates));
negativesum = zeros(size(electiondates));
for row = 1:numel(daterows))
    summationdifrence = difrence(max(1, daterows(row)-29) : daterows(row));  %assume you want 30 elements (hence -29). makes sure you don't go before the first element with max
    positivesum(row) = sum(summationdifrence(summationdifrence> 0));
    negativesum(row) = sum(summationdifrence(summationdifrence< 0));
end

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Edmundas Povilavicius 2016-11-5

This is perfect! Thank you!

Tho I found a problem, it seems some elections happened over the weekend and therefore there are no market data (aka the diference100) because it was closed. How would it be possible to amend the code that if electiondates and datedif would have no match (be 0) the row number would be the closest one below the election date happening over a weekend. (it will be either -1 or -2 lower depending if it's Sunday or Saturday as markets close on Friday).

Thank you

Guillaume 2016-11-7

Well, the simplest thing might be to adjust the electiondates vector to shift the date to friday if it's saturday or sunday.

Another approachis to change the creation of daterows to:

[isexact, daterows] = ismember(electiondates, datedif);
missingdates = electiondates(~isexact);
[isprevious, prevrows] = ismember(missingdates - 1, datedif);
[~, beforerows] = ismember(missingdates(~isprevious) - 1, datedif);
prevrows(~isprevious) = beforerows(~isprevious);
daterows(~isexact) = prevrows(~isexact);

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Answer 2

Image Analyst 2016-11-5

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编辑：Image Analyst 2016-11-5

It's not clear what you want. Can you supply some data and expected output. If you want to just sum in a sliding 30 row window, why don't you simply use conv()? And don't use data for your variable name since that is a built-in functions. And it's not clear why you're using cell arrays instead of simpler double arrays, and why you're putting all the results into the first element of psum instead of the i'th element.

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Image Analyst 2016-11-5

To sum the values of Diference100:

windowSize = 30;
summedValues = movsum(Diference100, windowSize);

Edmundas Povilavicius 2016-11-5

Thank you, however, I don't need to sum up every value because most of the data is useless and I need to skip it. I only need to sum up data 1 to 30 days before a specific date, i.e. election date.

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