Which is faster, logical indexing vs for loop

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Aly Elhefny 2020-3-19

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https://ww2.mathworks.cn/matlabcentral/answers/511772-which-is-faster-logical-indexing-vs-for-loop

评论： Raynier Suresh 2020-3-24

Hello everyone,

I'm currently constructing a code that would run for extremly big data arrays so I'm currently working on minimizing my code's computational time to minimum. And my question is: Which of the following will be faster for matlab

To run a foor loop for the arrays element by element and applying if conditions at each cycle to give an output.
or use logical indexing and apply the different if conditions to groups following same criteria. (ex, r = find ( a>8)).

In other words, is it faster to run a for loop along the array or to logical index scan it about 10 times ?

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Stephen23 2020-3-19

编辑：Stephen23 2020-3-19

Exactly how big are your "extremly big data arrays" ?

It is possible that creating a large logical array via logical indexing could be slower than a loop. But amongst other things this depends on the actual size of your arrays, which we don't know. We also don't know what your system specifications are.

Star Strider 2020-3-19

@Aly Elhefny — My pleasure!

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Answer 1

Raynier Suresh 2020-3-23

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https://ww2.mathworks.cn/matlabcentral/answers/511772-which-is-faster-logical-indexing-vs-for-loop#answer_421446

在 MATLAB Online 中打开

To find the time taken by a certain piece of code you could use the “tic toc” or “timeit” or “cputime”. The below code is an example which can tell you whether the logical indexing or “find” command is faster.

a = 10:20;  
tic  
r = find(a>14); 
toc % Time taken by find command to find index of elements greater than 14  
r1 = []; 
tic 
for i=1:numel(a) 
    if a(i)>14 
       r1 = [r1 i]; 
    end 
end 
toc %Time taken to find index of elements greater than 14 by indexing 