splitting output of a loop into multiple parts

Question

tjerk Verweij 2023-9-29

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评论： Dyuman Joshi 2023-9-29

Hello!

Im new to programming, but i need to do it a bit for my work. yesterday someone already helped me with importing multiple files with a loop. now i have the following problem.

p=0;

for b=1:30

T=A{1,b}.data;

for i=1:length(T(:,1))-1

p=p+1;

R2=T(:,1);

R1=T(:,9);

R(:,p)=(R1(i)*(R2(i+1)-R2(i))+R1(i+1)*(R2(i+1)-R2(i)))/2;

%plot(T(:,[1]),T(:,[9]))

end

Q=sum(R);

end

This is de code i have written to far. I have 30 documents imported (b:1:30) and want to put them all trough a calculation, R. the problem i have currently is that all the calculated values all get into 1 array, instead of a separate array for every document, so basically 30 different ones. I have tried multiple things yesterday but couldnt figure out how to do it.

Hopefully someone here can help me out, if feels like there should be an easy fix for this problem.

Thanks in advance!

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Answer 1

Dyuman Joshi 2023-9-29

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Dynamically growing arrays is not a good coding practice.

Instead, Preallocate a cell array and vectorize the inner for lop.

Read more here - Preallocating Arrays and Vectorization

p=0;
%As it is not known if the number of rows of T is same for all values of b
%Therefore a cell array is chosen for preallocation
R = cell(1,30);
for b=1:30
    T=A{1,b}.data;
    i=1:size(T,1)-1;
    R2=T(i,1);
    R1=T(i,9);
    %Use element-wise operations for vectorization
    vec=(R1(i).*(R2(i+1)-R2(i))+R1(i+1).*(R2(i+1)-R2(i)))/2;
    Q(b)=sum(vec);
    R{b}=vec;
end

You can preallocate Q as well. If the number of rows of T is same for all values of b, then prealloace a zeros array for R.

It would be better if you could attach the data you are working with.

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tjerk Verweij 2023-9-29

hello!

Thanks, for the answer, this worked for me. Had to change one thing, but you coudlnt have known that since you didnt have the data. i will read more about the preallocation and vectorization for in the future :)

Thanks again!

Dyuman Joshi 2023-9-29

You are welcome!

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Answer 2

Katy 2023-9-29

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在 MATLAB Online 中打开

Hey tjerk,

Let me know if this addresses your question. Hopefully "R" ends up being a 30x1 cell array, with each cell containing the results of the R calculation for each document.

Let me know if you get any errors or this doesn't work.

p=0;
for b=1:30
T=A{1,b}.data;
    for i=1:length(T(:,1))-1
        p=p+1;
        R2=T(:,1);
        R1=T(:,9);
        %R(:,p)=(R1(i)*(R2(i+1)-R2(i))+R1(i+1)*(R2(i+1)-R2(i)))/2;
        R{b,1} = [R{b,1} (R1(i)*(R2(i+1)-R2(i))+R1(i+1)*(R2(i+1)-R2(i)))/2];
        %plot(T(:,[1]),T(:,[9]))
end
Q=sum(R);       
end

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tjerk Verweij 2023-9-29

hey,

Thanks for replying

Im getting this message now:

Index in position 1 exceeds array bounds. Index must not exceed 1.

I also forgot to mention that the lenght or R for each document should be the same as the amount of columns in the document, which is different for a few documents.

Thanks!

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