Faster way to create a matrix of the unique() of each row of a matrix

Let A be a matrix of relatively big size(A,1), say 2000, and relatively small size(A,2), say 20. I am looking for a faster way to construct a matrix B such that the i-th row of B consists of unique(B(i,:)), padded appropriately, say with 0-s, so that all these rows have the same length (in order to fit together as the matrix B).

I am currently using the following code:

[n,m] = size(A); % A is, say, a non-zero uint8 matrix

A = A.'; % it's better to work with columns in MATLAB?

B = zeros(m,n,'uint8');

t = 0; % will truncate B at t;

for i=1:n % loop through the columns of A.'

col = unique(A(:,i));

s = length(col);

B(1:s,i) = col;

t = max(t,s);

end

B = B.'; % go back to the original form

B = B(:,1:t);

When size(A,1) is big, the long loop is perhaps suboptimal. Moreover, A and B are actually GPU arrays, so the code is supposed to run on the GPU. So is there perhaps a faster/smarter (vectorized) way to do this?

[Marin Genov]

@Bruno Luong: Yes, you are absolutely right, I was hasty, s was meant to be the length of the vector of unique values. I fixed the code in the original post. Thanks!

Answer 1

[Bruno Luong]

% Test example
A=randi(4,10,5)

A = 10×5

1 3 1 4 3 2 1 1 1 4 2 1 3 3 3 3 4 4 3 3 1 2 2 3 1 2 2 1 3 3 1 3 1 3 4 4 2 4 2 4 3 1 4 4 3 3 2 4 1 2

[m,n] = size(A);
As=sort(A,2);
b=[true(m,1),diff(As,1,2)>0];
R=repmat((1:m)',1,n);
C=cumsum(b,2);
r=R(b);
c=C(b);
a=As(b);
B=accumarray([r,c],a)

B = 10×4

1 3 4 0 1 2 4 0 1 2 3 0 3 4 0 0 1 2 3 0 1 2 3 0 1 3 4 0 2 4 0 0 1 3 4 0 1 2 3 4

[Bruno Luong]

% Test example

A small modification to easier find r

A=randi(4,10,5)

A = 10×5

1 3 1 2 2 1 1 3 2 1 4 1 3 2 4 1 3 1 1 4 1 4 2 4 2 3 3 2 1 2 1 4 2 1 4 1 4 3 4 2 3 2 3 4 2 2 1 2 4 3

[m,n] = size(A);
As=sort(A,2);
b=[true(m,1),diff(As,1,2)>0];
C=cumsum(b,2);
[r,~] = find(b); % not need R
c=C(b);
a=As(b);
B=accumarray([r,c],a)

B = 10×4

1 2 3 0 1 2 3 0 1 2 3 4 1 3 4 0 1 2 4 0 1 2 3 0 1 2 4 0 1 2 3 4 2 3 4 0 1 2 3 4

[Marin Genov]

Thanks!