Question

I am using Matlab for one of my projects. I am actually stuck at a point since some time now. Tried searching on google, but, not much success.

I have an array of 0s and 1s. Something like:

A = [0,0,0,1,1,1,1,1,0,0,1,1,1,1,1,1,0,0,0,0,0,1,1,1,0,0,0,0];

I want to extract an array of indicies: [x_1, x_2, x_3, x_4, x_5, ..]

Such that x_1 is the index of start of first range of zeros. x_2 is the index of end of first range of zeros.

x_3 is the index of start of second range of zeros. x_4 is the index of end of second range of zeros.

For the above example:

x_1 = 1, x_2 = 3
x_3 = 9, x_4 = 10 

and so on.

Of course, I can do it by writing a simple loop. I am wondering if there is a more elegant (vectorized) way to solve this problem. I was thinking about something like prefix some, but, no luck as of now.

Thanks,

Anil.

Answer 1

If you want to get your results in a single vector like you described above (i.e. x = [x_1 x_2 x_3 x_4 x_5 ...]), then you can perform a second-order difference using the function DIFF and find the points greater than 0:

x = find(diff([1 A 1],2) > 0);

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EDIT:

The above will work for the case when there are at least 2 zeroes in every string of zeroes. If you will have single zeroes appearing in A, the above can be modified to handle them like so:

diffA = diff([1 A 1],2);
[~,x] = find([diffA > 0; diffA == 2]);

In this case, a single zero value will create repeated indices in x (i.e. if A starts with a single zero, then x(1) and x(2) will both be 1).