How to sum elements in a vector according to the location information from another vector?

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Hello,
I have a vector lets say V=[3 5 3 7 1 7 9 3 9 2 0 3 9]
and another vector that is indicating the which elements that are needed to be summed. Lets say L=[1 4 5 6 8]
So my output should be the sum of first,fourth,fifth,sixth and eighth elements in V. I.e. sum=3+7+1+7+3=21
How can this code be to get the sum value?
Thanks in advance!

Risposta accettata

Ahmet Cecen
Ahmet Cecen il 19 Mar 2016
sum(V(L))

Più risposte (1)

Muhammad Usman Saleem
Muhammad Usman Saleem il 19 Mar 2016
Modificato: Muhammad Usman Saleem il 19 Mar 2016
I have convert this question to a function. Please check it and tell me if not working
function sum1=runsum(V,L)
runsum=0;
for i=1:length(L)
runsum=runsum+V(L);
end
sum1= runsum;
end
  3 Commenti
Stephen23
Stephen23 il 19 Mar 2016
Modificato: Stephen23 il 19 Mar 2016
@Muhammad Usman Saleem: not only is your code ten times slower than Ahmet Cecen's, your code also does not give the correct answer:
>> [A,B] = test(1e5)
Elapsed time is 0.183132 seconds. % Ahmet's code (fast)
Elapsed time is 1.775014 seconds. % Your code (slow)
A =
21 % Ahmet's code (correct)
B =
15 35 5 35 15 % Your code (incorrect)
Your code has one obvious bug (keyword: indexing), and the fact that its output is obviously incorrect shows that you never tested your code. It is also pointless to reinvent perfectly good inbuilt functionality.
The test file is here, so you can try it yourself (I used R2012b):

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