Rowfun and sum of array elements based on start and stop position from another array

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Michel Rathé
Michel Rathé on 30 Oct 2020
Edited: Matt J on 31 Oct 2020
Hi everybody,
I'm trying to sum values of a timetable based on start and endposition from another array. The results will be inserted in the timetable in a new column.
It works with loops but the size of my table makes it quite slow and since I use vectorization for other purposes, and it is quite fast, I only assume that there is a way to achive it with rowfun for example. But I tried several scenarios but it does not work.
Here is what I try to achieve through an example .
start position array : Start=[1;3;6]
stop position array : Stop=[2;5;8]
Values from timetable: Values=[10;20;10;30;50;40;80;20;10]
I want an output vector with the sum based on start and stop position, so a 3X1.
For example:
First result: sum of Values from position 1 to 2: 10+20=30
Second result: sum of Values from position 3 to 5: 10+30+50=90
Third result: sum of Values from position 6 to 8: 40+80+20=140
So, how to achieve that simple operation with rowfun?
Thanks.
Michel Rathé

Accepted Answer

Matt J
Matt J on 30 Oct 2020
Edited: Matt J on 30 Oct 2020
It works with loops but the size of my table makes it quite slow
It shouldn't be. Even with several thousand table columns, a loop should still run in well less than 1 sec.
T=array2table(rand(1e4,3e3));
[M,N]=size(T);
Start=ones(1,N);
Stop=M*Start; %Worst case
matrix=table2array(T);
result=nan(1,N);
tic
for i=1:N
result(i)=sum(matrix(Start(i):Stop(i),i));
end
toc
Elapsed time is 0.068844 seconds.
  1 Comment
Michel Rathé
Michel Rathé on 31 Oct 2020
Thanks so much Matt to provide me 2 solutions that covers new grouds, especially the fisrt one.
I found the culprit in the loop that I had:
Work(ind_start(i),3)=num2cell(sum(Work.Rend(ind_start(i):ind_stop(i)-1)));
To prevent an error message, I converted with numcell at each row!!!
Now I use a temp array and convert ourside the loop and insert it in .008 seconds.
a_temp=zeros(size(ind_start,1),1);
for i=1:size(ind_start,1)
a_temp(i)=sum(Work.Rend(ind_start(i):ind_stop(i)-1));
end
Work(ind_start,3)=array2table(a_temp);
Thanks again,
Michel

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More Answers (1)

Matt J
Matt J on 30 Oct 2020
Edited: Matt J on 30 Oct 2020
So, how to achieve that simple operation with rowfun?
I don't see rowfun as being part of the solution. Just pull the values out into matrices as you have done and use matrix operations,
Start=[1;3;6];
Stop=[2;5;8];
Values=[10;20;10;30;50;40;80;20;10];
e=(1:size(Values,1)).';
result=sum( (e<=Stop(:).' & e>=Start(:).').*Values ,1)
result = 1×3
30 90 140

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