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Fast check if 2 arrays have a common element



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C-Mex: anyEq

This is a simple but fast check, if two arrays contain any common element.
The C-Mex is 25% to 60% faster than the equivalent Matlab expression "any(X(:) == y)" and much faster than "any(ismember(X, Y))" if Y is an array.
If a matching element is found early, this function returns very fast without testing the rest.
In opposite to the Matlab version, the C-mex does not need any temporary arrays.

R = anyEq(X, Y)
X, Y: Arrays of any size. Complex or sparse array are rejected.
R: TRUE is replied if any element of X occurs in Y, FALSE otherwise.

- This is equivalent to:
R = any(X(:) == Y(1)) || any(X(:) == Y(2)) || ...
- This MEX version is faster than the Matlab method, because the creation of
the intermediate logical array is avoided:
Worst case (no matching element): 25% to 60% faster
Best case (first element matches): 99.99% faster for 1e6 elements
(Matlab 2011b/64, MSVC 2008).
- For small LOGICAL arrays (< 5'000 elements) anyEq is up to 65% slower than
any(X) or ~all(X). For larger arrays the speedup depends on the position of
the first match and can reach a factor of 7.
- X and Y are ordered automatically such, that the elements of the smaller
array are searched in the larger one.

anyEq(0:0.1:1, 0.3) % FALSE: Effect of limited precision
anyEq(1:4, [5,2]) % TRUE: 2 is found in 1:4

COMPILATION: See anyEq.c for instructions how to compile the C-file.

TEST: Run uTest_anyEq to check validity and speed of the Mex function.

Tested: Matlab 6.5, 7.7, 7.8, 7.13, WinXP/32, Win7/64
Compiler: LCC2.4/3.8, BCC5.5, OWC1.8, MSVC2008/2010

Suggestion and bugreports by email or in the comment section are appreciated.

Comments and Ratings (5)

Brett Shoelson

Brett Shoelson (view profile)

Hi Jan...just wondering how this compares to



Jan Simon

Jan Simon (view profile)

@William: Thanks for your comments.
I've added a support for logical input now. While anyEq(X,Y) is 25% to 50% faster than ANY(X==Y) for numerical arrays, for LOGICAL input it depends on the application, if this is faster or slower than ANY(X) or ~ALL(X):
x = false(1, 1e6); anyEq(x, true); is 7 times faster than any(x). But anyEq(x, false) needs 65% more processing time than ~all(x). Fortunately the absolute time difference is small even then.

Extremely rapid short-circuiting "any" checking. I am using this to compare large logical 3D arrays, and despite your own comments in the file documentation, it is actually even faster than the built-in any on my data. It is unfortunate this doesn't support logical inputs, because then I have to use find on my logical arrays first, and I could probably see some small speed improvement comparing logical arrays directly.

And yes, it is still faster than the built-in any even with two find calls on equally sized arrays. According to the profiler, its about twice as fast.

Jan Simon

Jan Simon (view profile)

For running the unit-test in Matlab 6.5 a replacement for the newer "cast" function is required:
function y = cast(x, t)
y = feval(t, x)



Accepts LOGICALs also.


Tested under 64 bit.

MATLAB Release
MATLAB 7.13 (R2011b)

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