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version (12.8 KB) by Jan
Fast linear indexing

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Updated 05 Feb 2015

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CopyMask - Logical indexing
Matlab's linear indexing is fast, much faster than using FIND usually.
But it can be more than twice as fast using a C-Mex function.
Y = CopyMask(X, Mask, Dim)
X: Array of type: DOUBLE, SINGLE, (U)INT8/16/32/64), LOGICAL, CHAR.
X can be complex.
Mask: Mask as LOGICAL vector.
Dim: Specify dimension for masking. If omitted or the empty matrix []
linear indexing is applied. Optional, default: [].
Y: Array of same type as X. For linear indexing Y is a [N x 1] vector.

- Equivalent Matlab code: Y = X(Mask)
- Difference to Matlab's logical indexing:
* 2 to 3 times faster for linear indexing.
* A column vector is replied in every case.
* Mask cannot be longer than the array, while Matlab allows additional
trailing values, when they are FALSE.
- If Dim is specified and X is not small, this function is only some percent
faster than the equivalent Matlab code, e.g.: Y = X(:, Mask, :)

X = rand(2,3,4);
Y = CopyMask(X, X > 0.2); % Matlab: X(X > 0.2), remarkably faster

M = [true, false, true];
Z = CopyMask(X, M, 2); % Matlab: X(:, M, :), a little bit faster

I could test this under Matlab 2009a and 2011b only. Perhaps the speedup vanishes in modern Matlab versions. Please feel free to send me the output of the unit-test function to the address given in the code.

The C-file must be compiled at first. If "mex -setup" was performed before, calling CopyMask without inputs starts an automatic compilation. If compiling fails, you can download pre-compiled files at:

Cite As

Jan (2021). CopyMask (, MATLAB Central File Exchange. Retrieved .

Comments and Ratings (2)

Peter Xu

Thank you

Rob Campbell

Yup, also a good speedup on 2014a.
@TMW, is it possible to incorporate these methods?

>> X = rand(8000,8000);
>> tic, CopyMask(X, X > 0.2); toc
Elapsed time is 1.696195 seconds.
>> tic, X(X > 0.2); toc
Elapsed time is 4.824547 seconds.

MATLAB Release Compatibility
Created with R2011b
Compatible with any release
Platform Compatibility
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