MATLAB Central Newsreader - unique() Help

unique() Help

Husam Aldahiyat — Mon, 07 Sep 2009 15:58:02 -0400

Hello,
I need a function that outputs the unique elements of each column of my desired matrix.
I can use
sum(~~diff(sort(A)))+1
to get the number of unique elements in each column of a matrix A,
but can't I produce a cell or zero padded matrix for what the unique elements are?
I used a loop and it takes 99% of my calculation time and is killing me!

Help!!!

Re: unique() Help

dpb — Mon, 07 Sep 2009 16:06:41 -0400

Husam Aldahiyat wrote:
> I need a function that outputs the unique elements of each column of my desired matrix.
...

How about (for matrix x)

for i=1:size(x,2), u{i} = unique(x(:,i)); end

--

Re: unique() Help

arun — Mon, 07 Sep 2009 16:16:40 -0400

On Sep 7, 5:58 pm, "Husam Aldahiyat" <numand...@gmail.com> wrote:
> Hello,
> I need a function that outputs the unique elements of each column of my desired matrix.
> I can use
> sum(~~diff(sort(A)))+1
> to get the number of unique elements in each column of a matrix A,
> but can't I produce a cell or zero padded matrix for what the unique elements are?
> I used a loop and it takes 99% of my calculation time and is killing me!
>
> Help!!!

given matrix y (with n columns),
u = arrayfun(@(x) unique(y(:,x)), 1:size(y,2), 'uni', false);
will give a 1*n cell array with each element holding the unique
elements of the column.
best, arun.

Re: unique() Help

Husam Aldahiyat — Mon, 07 Sep 2009 16:44:02 -0400

dpb, thanks for the effort but I already said no loops.

arun, thanks alot!!!!!!!!!!!!! :)

Re: unique() Help

Matt Fig — Tue, 08 Sep 2009 03:43:05 -0400

With a little prep work, the arrayfun solution can be made faster:

%%%
t = sort(y);
d = [true(1,size(y,2)); 0~=(diff(t))]; % Replace call to unique with index into t.
U = arrayfun(@(x) t(d(:,x),x),1:size(y,2),'Un',0); % Slow because of anon. func.
%%%

But this is still NOT as fast as the For loop:

%%%
t = sort(y);
d = [true(1,size(y,2)); 0~=(diff(t))];
U2 = cell(1,size(y,2));

for ii = 1:size(y,2)
U2{ii} = t(d(:,ii),ii); % Replace call to unique with index into t.
end
%%%

For a random array 30-by-2000, the For loop is 4 times faster than the above arrayfun, and 15 times faster than the one line arrayfun approach, at least using 2007b.