MATLAB: Vectorized method to find the number of consecutive 1s from the left of an array

arraylabeling

Hi, I want to find the number of consecutive 1s from the left of an array (a binary array). For example, if I have A = [1 1 1 0 1 0 0 0 1 1] then the result would be 3 because there are 3 consecutive 1s from the left. Another example: A = [1 0 1 1 1 1 1] then the result would be 1. Or A = [0 1 1 1 0 0 1 0 1], the result would be 0.

I can write this in for loop, however I do not know how to vectorize this part. I really need it for my code so that it might run faster.

Please help me, thank you very much.

Best Answer

idx=find(diff(A));
if A(1)==0
Result=0
elseif all(A)
Result=numel(A)
else
Result=idx(1)
end

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MATLAB: Count the occurence of a number in between other numbers

This is simpler and actually works for all horizontal vectors (unlike the accepted answer):

>> x = [ 1 0 0 1 1 1 0 1 1 0 0 0 0 0 1 1 0 1 0];
>> D = diff([false,x==0,false]);
>> find(D<0)-find(D>0)
ans =
     2     1     5     1     1

For a slightly faster version you can call find once:

>> F = find(diff([false,x==0,false]));
>> F(2:2:end)-F(1:2:end)
ans =
     2     1     5     1     1

EDIT: uses Jan's logical vector suggestion.

MATLAB: How to find the longest time an array has a zero value in a row

You said "find" but then I guess you want the length, not the location. So to find the longest length of zeros, if you have the Image Processing Toolbox, you can do it in one line of code:

vec = [0 0 1 1 1 0 0 0 1 1 1] % Create sample data.
longestZeroLength = sum(bwareafilt(vec==0, 1))  % returns 3.

Best Answer

Related Solutions

MATLAB: Count the occurence of a number in between other numbers

MATLAB: How to find the longest time an array has a zero value in a row

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