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

Question

Ekin 2016-3-19

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链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/274408-how-to-sum-elements-in-a-vector-according-to-the-location-information-from-another-vector

编辑： Stephen23 2016-3-19

采纳的回答： Ahmet Cecen

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!

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Answer 1

Ahmet Cecen 2016-3-19

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https://ww2.mathworks.cn/matlabcentral/answers/274408-how-to-sum-elements-in-a-vector-according-to-the-location-information-from-another-vector#answer_214230

在 MATLAB Online 中打开

sum(V(L))

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Answer 2

Muhammad Usman Saleem 2016-3-19

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/274408-how-to-sum-elements-in-a-vector-according-to-the-location-information-from-another-vector#answer_214229

编辑：Muhammad Usman Saleem 2016-3-19

在 MATLAB Online 中打开

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

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Muhammad Usman Saleem 2016-3-19

my code seem working more efficiently than other. It depend on your own requirement...

Stephen23 2016-3-19

编辑：Stephen23 2016-3-19

在 MATLAB Online 中打开

test.m

@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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