Finding index in array

Question

A = [011100111100]

Hi, I want to find the index of start and end 1 and make that into a sub array. I am parsing a file so I have to do this programmatically. I am programming in Matlab.

A = [011100111100]
      ^ ^ B = [1 1 1]
      2 4 B_index = [2 4] 
A = [011100111100]
           ^  ^ C = [1 1 1 1]
           7  10C_index = [7 10]

My try

for i=1:length(A)
   % Added to stop the loop froming looping more than i.
   if i == A(end)
       break; % stops loop
   end;
   if A(i) == 1 && A(i+1) == 0
       start_index = i;
   end;
end;

This works to find the start index but I want to find the end as well.

possible duplicate of Finding islands of zeros in a sequence — Luis Mendo
– Luis Mendo, Commented Mar 28, 2014 at 16:38
@Luis Mendo Wow that is a lot like my situation, i look through it. — Arsalan
– Arsalan, Commented Mar 28, 2014 at 16:46
@Arsalan Actually it's simpler here. I think it deserves a separate answer. I've removed my duplicate vote — Luis Mendo
– Luis Mendo, Commented Mar 28, 2014 at 16:47

Luis Mendo · Accepted Answer · 2014-03-28 16:46:51Z

1

A = [ 0 1 1 1 0 0 1 1 1 1 0 0 ]; %// example data
ind = diff([0 A 0]); %// detect changes
start_index = find(ind==1); %// a start is a positive change
end_index = find(ind==-1)-1; %// an end is (just before) a negative change

answered Mar 28, 2014 at 16:46

Luis Mendo

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2 Comments

Arsalan Over a year ago

i will try it and let you know.

Luis Mendo Over a year ago

@Arsalan It tells you the positions where ind equals 1, that is, where a change from value 0 to 1has ocurred. That corresponds to the beginning of each run of 1 values.

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