How to count first identical elements without a for cycle?

I have a vector [3 3 3 3 3 4 1 1 0 0 0 5 6 3 3 1 1 1], and I want to count threes at the begining of the vector, so the result has to be 5.

[Rik]

You have now asked over 350 questions. Surely you have been linked to the tutorial multiple times. So you should know that you should show what you have tried so far to achieve this.

I also have a few points for clarification: Should the code measure the run length of the first run of 3s, or should it measure the length of the first run?

Answer 1

[Rik]

In case you want to measure the run length of the first run:

 data=[3 3 3 3 3 4 1 1 0 0 0 5 6 3 3 1 1 1];
 output=find(diff(data),1,'first');

And if you want to measure the run length of the first run of 3:

 
data=[5 3 3 3 3 3 4 1 1 0 0 0 5 6 3 3 1 1 1];
ind=find(data==3,1);
if ind>1
    data(1:(ind-1))=[];
end
output=find(diff(data),1,'first');