Series of consecutive numbers (different lengths)

Question

I would appreciate if someone showed me an easy way to do this. Let's say I have a vector in MATLAB like

d = [3 2 4 2 2 2 3 5 1 1 2 1 2 2 2 2 2 9 2]

I want to find the series of consecutive number "twos" and the lengths of those series.

Number twos can easily be found by x=find(d==2). But what I want is to get a vector which contains the lengths of all series of consecutive number twos, which means that my result in this case would be a vector like this:

[1 3 1 5 1].

Anyone who could help me?

Answer 1

This seems to work:

q = diff([0 d 0] == 2);
v = find(q == -1) - find(q == 1);

gives

v =

   1   3   1   5   1

for me

Answer 2

This is called run length encoding. There is a good m-file available for it at http://www.mathworks.com/matlabcentral/fileexchange/4955-rle-deencoding . This method is generally faster than the previously posted diff/find way.

tic
d_rle = rle(d==2);
d_rle{2}(d_rle{1}==1);
toc

Elapsed time is 0.002632 seconds.

tic
q = [0 diff([0 d 0] == 2)];
find(q == -1) - find(q == 1);
toc

Elapsed time is 0.003061 seconds.

Answer 3

What if we want the indices of the original matrix where the consecutive values are located? Further, what if we want a matrix of the same size as the original matrix, where the number of consecutive values are stored in the indices of the consecutive values? For example:

  original_matrix = [1 1 1;2 2 3; 1 2 3];

  output_matrix = [3 3 3;2 2 0;0 0 0];

This problem has relevance for meteorological data quality control. For example, if I have a matrix of temperature data from a number of sensors, and I want to know what days had constant consecutive values, and how many days were constant, so I can then flag the data as possibly faulty.

temperature matrix is number of days x number of stations and I want an output matrix that is also number of days x number of stations, where the consecutive values are flagged as described above.