Extract blocks of numbers from array in Matlab

Question

I have a vector and I would like to extract all the blocks from it:

x = [1 1 1 4 4 5 5 4 6 1 2 4 4 4 9 8 4 4 4 4]

so that I will get vectors or a cell containing the blocks:

[1 1 1], [4 4], [5 5], [4], [6], [1], [2], [4 4 4], [9], [8], [4 4 4 4]

Is there an efficient way to do it without using for loops? Thanks!

Answer 1

You can use accumarray with a custom anonymous function:

y = accumarray(cumsum([true; diff(x(:))~=0]), x(:), [], @(x) {x.'}).';

This gives a cell array of vectors. In your example,

x = [1 1 1 4 4 5 5 4 6 1 2 4 4 4 9 8 4 4 4 4];

the result is

y{1} =
     1     1     1
y{2} =
     4     4
y{3} =
     5     5
y{4} =
     4
y{5} =
     6
y{6} =
     1
y{7} =
     2
y{8} =
     4     4     4
y{9} =
     9
y{10} =
     8
y{11} =
     4     4     4     4