how to do sum of element of multiple number in matlab?

Hello everyone.

If I have

m=[3 3 4 3 2 4 3]

c=[3 4]

then I want to calculate the sum of m(c) at the same time

I tried

b=1:m(c)

b=sum(b)

but only give me one number. I want it to be 2 numbers. How to do this?

Thanks in advance

[Timo Dietz]

Could you please explain in more detail? Does c contain the indices up to which you like to sum up m - starting always from 1? Or why do you expect two numbers as result?

[Putri Basenda Tarigan]

this means that for c=3, I want to sum m from 1 to 3, so it's 10. and for c=4, it's sum of m from 1 to 4, so it's 13.

then I want to achieve these two numbers to compare them in the next step

Answer 1

[Rik]

b=cumsum(m);
b=b(c);

[KALYAN ACHARJYA]

Yes sir, simplest

[Timo Dietz]

Uh, I wasn't aware of the cumsum function - nice. But for very large arrays it may be faster just to calculate the numbers needed -with arrayfun- instead of doing the cumulative sum over all members.

[Rik]

There is a trade-off here: arrayfun also has an overhead, as does the creation of the new array that has to be summed. I suspect cumsum will be faster if there are many elements in c, especially if they are towards numel(m). If there are few elements in c and they occur early in m, that might make arrafun faster (or actually: a for-loop).

[Timo Dietz]

That's intersting. Many thanks for the insights.

[Putri Basenda Tarigan]

Thank you Sir

Answer 2

[Timo Dietz]

Not sure whether I got your issue. But in case you want to summarize all members of m starting at index 1, up to each member of c (containing indices of m) individually, you can do this:

b=arrayfun(@(x) sum(m(1:x)), c)

[Rik]

Functions like arrayfun and cellfun tend to be slower than using a for loop. The exception appears to be the legacy calls of cellfun (so those with the char input, instead of a function handle).

[Timo Dietz]

I see. Thanks a lot for the details.