How to get the longest consecutive values in a column vector and the position at which it starts

Hello,

Suppose i have a single column vector A'=[0 0 0 1 0 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1]

I only want the longest consecutive values of 1's and display only that.

I'd really appreciate any help!

Answer 1

[Bruno Luong]

A=[0 0 0 1 0 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1]'

i=reshape(find(diff([0;A;0])~=0),2,[]);
[lgtmax,jmax]=max(diff(i));
istart=i(1,jmax);

lgtmax % length of the longest sequence of 1s
istart % where it starts

[Meghana Balasubramanian]

This helps!

But as an amateur who just started learning matlab, it's quite difficult to understand the terminologies: lgtmax and jmax. I was unable to find the theory behind this. Could you please explain how this works?

Thank you

[Bruno Luong]

The terminologies are mine it doesn't matter for MATLAB syntax.

Actually my code is compact but difficult to understand, even for people who are initiated.

If you want to understand, you can split into multiple small and basic commands to analyze it and read the doc of corresponding command and experiment with your own examples.

[0;A;0]

Pad 0s at the header and trailer of A.

The command

diff([0;A;0])~=0

returns a logical array with TRUE whene there is a transitions (0->1) or (1->0).

And the TRUE positions must come in pairs: alternate in order of 0->1, 1->0, 0->, 1->0, etc ... since I take care to pad the arrays with 0s in both ends.

So by reshaping

i=reshape(... ,2,[]);

I'll get two-row array, the first row contains index positions of 0->1, and the second row of 1->0. You'll need to know about MATLAB major-column storage scheme for array to fully understand such trick.

Therefore the difference of the two columns is the the length of the consecutive 1s in A with this command:

diff(i);

By doing

[lgtmax,jmax]=max(diff(i));

I search for the longest sequence of 1s, and the number of this sequence is in retreived in jmax.

The last command

istart=i(1,jmax)

just maps back the starting position of this sequence in A (since I pad 0 in the head, it's actually the position of the first 1).

[Laurent Haret]

nice !

Answer 2

[Walter Roberson]

https://www.mathworks.com/matlabcentral/fileexchange/41813-runlength

[Meghana Balasubramanian]

Thank you, but I was looking for a matlab code to help me out rather than a downloadable function, even if it does make my job easier.

Answer 3

[Benjamin Hezrony]

It can also be done in one line:

B = bwareafilt(A,1)

A classic function in segmentation!

https://www.mathworks.com/help/images/ref/bwareafilt.html#d126e42061