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

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Jon 2023-9-22

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评论： Jon 2023-10-2

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

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Torsten 2023-9-22

Why not renaming a,b,c and d to vecs{1},vecs{2},vecs{3} and vecs{4} ? Should make things easier ...

Bruno Luong 2023-9-22

编辑：Bruno Luong 2023-9-22

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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 2023-9-22

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编辑：Bruno Luong 2023-9-22

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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

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Jon 2023-9-22

Thanks, very much, I will look into this too. Away for a week, no MATLAB, but will review again when I get back

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Walter Roberson 2023-9-22

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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

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Jon 2023-9-22

编辑：Jon 2023-9-22

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@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