Difference between MATLAB's numel and length functions
I know that length(x)
returns max(size(x))
and numel(x)
returns the total number of elements of x, but which is better for a 1 by n array? Does it matter, or are they interchangeable in this case?
EDIT: Just for kicks:
Looks like they're the same performance-开发者_JAVA技巧wise until you get to 100k elements.
For a 1-by-N array, they are essentially the same. For a multidimensional array M
, they can give different results:
numel(M)
is equivalent toprod(size(M))
.length(M)
is equivalent tomax(size(M))
. IfM
is empty (i.e. any dimension is 0), thenlength(M)
is 0.
In that case they return the same and there's no difference. In term of performance, it depends on the inner working of arrays in MATLAB. E.g. if there are metainformations about how many elements are in the array (no matter the shape), then numel is as fast as possible, while max(size(x)) seems to need more work to obtain the same thing (retrieving sizes, and then finding the max among those). I am used to use numel
in that case, but performance speech (hypothetical) apart, I would say they are interchangeable.
As other said they are same for one-dimensional array.
IMHO from code readability viewpoint length
should be used on one-dimensional arrays. It is about "intentional programming", you see the code and understand what programmer had in mind when conceiving his work. So when I see numel
I know it is used on a matrix.
length
vs. numel
was a discussion topic in our team over a number of years. Ex senior developer did not cared about code reability, only about work being done and used only numel
in otherwise not well readable/formatted code. Other guy is a matematician and used length
only on numeric arrays being for him "real" arrays. For cell arrays and struct arrays he used numel
.
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