Which is faster, logical indexing vs for loop

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 ?

[Star Strider]

Without knowing more, it is quite likely that logical indexing (without using find unless you need to know the numerical indices) would be faster.

[Aly Elhefny]

That was helpful. Thank you!

[Stephen23]

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]

@Aly Elhefny — My pleasure!

Answer 1

[Raynier Suresh]

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 

Refer to the links below for more details:

timeit : https://www.mathworks.com/help/matlab/ref/timeit.html

tictoc : https://www.mathworks.com/help/matlab/ref/tic.html

Cputime : https://www.mathworks.com/help/matlab/ref/cputime.html

[Stephen23]

"The below code is an example which can tell you whether the logical indexing or “find” command is faster."

The code does not use logical indexing.

Logical indexing is explained in the MATLAB documentation, blogs etc.:

https://www.mathworks.com/help/matlab/math/array-indexing.html

https://blogs.mathworks.com/loren/2013/02/20/logical-indexing-multiple-conditions/

Logical indexing is where a logical array is used as an index, e.g.:

idx = [some logical array]
B = A(idx)

There is no code given in this answer that matches the definition of logical indexing.

"%Time taken to find index of elements greater than 14 by indexing "

This loop timing will be distorted by the lack of array preallocation:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

[Raynier Suresh]

yes, you are right