How to count the number of consecutive identical elements (in both the directions) in a binary vector ?

Suppose I have a binary vector

X = [0     0     0     0     1     1     1     0     1     1     1     1]

I want to characterize each element with the number of identical elements occurring in consecutive positions in both the directions. For instance, the desired output should look like:

Y = [4 4 4 4 3 3 3 1 4 4 4 4].

I found a similar thread, but it counts only in the forward direction. Thanks in advance for any sort of assistance.

[Guillaume]

I'm not sure I understand the concept of direction for identical consecutive elements. Identity is not directional.

Answer 1

[Jan]

With FEX: RunLength:

X      = [0     0     0     0     1     1     1     0     1     1     1     1];
[B, N] = RunLength(X);
Y      = RunLength(N, N);

If you do not have a C-compiler for the fast C-Mex function, use RunLength_M.

Or with Matlab code:

d = [true, diff(X) ~= 0, true];  % TRUE if values change
n = diff(find(d));               % Number of repetitions
Y = repelem(n, n)

[Image Analyst]

Clever! Voted for.

[Jos (10584)]

the use of repelem is clever indeed!

Answer 2

[Jos (10584)]

Something like this?

X = [0  0  0  0  1  1  1  0  1  1  1  1] % row vector!
numX = numel(X) ;
Q = find([false diff(X)≈0]) ;
I = zeros(1, numX) ;
I(Q) = 1 ;
I = cumsum(I) ;
N = diff([1 Q numX+1]) ;
result = N(I+1)

Answer 3

[Image Analyst]

Here's yet another way:

X = logical([0     0     0     0     1     1     1     0     1     1     1     1])
Y = zeros(1, length(X)); % Initialize output as the same size as X.
props = regionprops(X, 'Area', 'PixelIdxList');
for k = 1 : length(props)
  Y(props(k).PixelIdxList) = props(k).Area;
end
props = regionprops(~X, 'Area', 'PixelIdxList');
for k = 1 : length(props)
  Y(props(k).PixelIdxList) = props(k).Area;
end