What is a simple way to check if a collection of vectors all have the same number of elements

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Jon
Jon 2023-9-22
评论： Jon 2023-10-2
Ran in:
I am look for a nice simple way to check if a collection of vectors all have the same number of elements. For example this might be used to validate some inputs to a function, or otherwise check compatibility before performing other operations.
Here's my current approach, which works, but I wonder if someone has simpler way of doing the same thing
a = rand(3,1);
b = rand(3,1);
c = rand(4,1);
d = rand(3,1);
vecs = {a,b,c,d};
n = cellfun(@numel,vecs);
if any(n(1)~=n)
error('all vectors must be same length')
end
all vectors must be same length
  2 个评论
Bruno Luong
Bruno Luong 2023-9-22
编辑：Bruno Luong 2023-9-22
I'm half serious but:
The simplest way? IMO just don't do any check, just rely on MATLAB, if any operation on vecs with incompatible sizes will imply error, just let MATLAB throw an error instead of
error('all vectors must be same length')

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采纳的回答

Bruno Luong
Bruno Luong 2023-9-22
编辑：Bruno Luong 2023-9-22
Ran in:
if any(n(1)~=n)
The above will crash if your cell is empty.
IMO
if any(diff(n))
is a more robust test. Thus you can do the test in a single statement
if any(diff(cellfun(@numel,vecs)))
% ...
end
And last but not least, if performance matters, use this
if any(diff(cellfun('prodofsize',vecs)))
% ...
end
instead of @numel.
Here is why
vecs = arrayfun(@(x) zeros(randi(10),1), 1:1e6, 'unif', 0);
timeit(@()cellfun(@numel, vecs),1)
ans = 0.7885
timeit(@()cellfun('prodofsize', vecs),1)
ans = 0.0098

更多回答（1 个）

Walter Roberson
Walter Roberson 2023-9-22
Ran in:
a = rand(3,1);
b = rand(3,1);
c = rand(4,1);
d = rand(3,1);
vecs = {a,b,c,d};
n = cellfun(@numel,vecs,'uniform',0);
if ~isequal(n{:})
error('all vectors must be same length')
end
all vectors must be same length
  6 个评论
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Jon
Jon 2023-9-22
编辑：Jon 2023-9-22
Ran in:
@Walter Roberson That's neat, I didn't realize that isequal could take a variable number of input arguments, e.g. n{:},
I also thought about, using a try catch block something like below. I think my original approach (incorporating Walter's suggested modifications), seems cleaner than the try-catch. Maybe this is just a subjective issue, but wondered whether anyone has any thoughts on comparing the two ways and which approach is "better".
a = rand(3,1);
b = rand(3,1);
c = rand(4,1);
d = rand(3,1);
try
M = horzcat(a(:),b(:),c(:),d(:))
catch
error('all vectors must be same length')
end
all vectors must be same length

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