How to randomly generate one non-zero element of each row from a matrix

How to randomly generate one non-zero element of each row from a matrix and then create a new matrix?

For A=[1 5 6 7 0; 2 0 0 4 2; 0 0 3 4 0]; I want to have a random vector B where each element in B (non-zero) is randomly generated from A. e.g., B=[1,2,3] or [5 4 3] or [7 2 3]...

Answer 1

[Rik]

There's probably a faster way, but the code below should work.

A=[1 5 6 7 0; 2 0 0 4 2; 0 0 3 4 0];
A_cell=cellfun(@(x) x(x~=0),...
    mat2cell(A,ones(1,size(A,1)),size(A,2)),...
    'UniformOutput',false);
B=arrayfun(@(x) x{1}(randi(numel(x{1}))),A_cell);

[Chaoyang Jiang]

Thank you very much. This line might in a loop that will be repeated 10,000 times. I am hoping there will be an efficient way.

[Dennis]

A=[1 5 6 7 0; 2 0 0 4 2; 0 0 3 4 0];
tic
for i=1:10000

   [rows,~]=size(A);
   B=zeros(rows,1);
   for k=1:rows
       le=find(A(k,:));
       le=le(randi(length(le)));
       B(k)=A(k,le);
   end
  end
  toc

Elapsed time is 0.114470 seconds.

[Stephen23]

"This line might in a loop that will be repeated 10,000 times. I am hoping there will be an efficient way."

@Chaoyang Jiang: why not just generate all random selection at once? Is a loop strictly required?

[Chaoyang Jiang]

Yes, as each loop is independent and has totally different A.

Answer 2

[Stephen23]

In two lines:

>> A = [1,5,6,7,0;2,0,0,4,2;0,0,3,4,0];
>> [R,C] = find(A);
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   5
   2
   3
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   7
   2
   3
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   5
   2
   3
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   6
   4
   3
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   7
   2
   3
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   6
   2
   4
>> accumarray(R,A(A~=0),[],@(v)v(randi(numel(v))))
ans =
   5
   2
   3

Or slightly more clearly:

[R,~] = find(A);
fun = @(v)v(randi(numel(v)));
B = accumarray(R,A(A~=0),[],fun)