Re: detecting a sign change in a vector

[Bob Walton]

dirac wrote
> ...
> I have a vector, V, which basically contains the values of a cos/sin
> function. The signs of the elements in the vector change from + to -
> occasionally. I would like to find these points without using a loop as I
> have done in the past. Is there a way to do this?
> ...
> MA

Here is some code that gives the subscripts of the sign changes:

t=linspace(0,20,1001)';
x=sin(t.^2);
z=x>=0;
c=conv(z,[-1 1]);
st=find(c<-.5); %subscript of the start of a run of non-negatives 
en=find(c>.5)-1; %subscript of the end of a run of non-negatives
runs=[st en]; %start and end of runs of non-negatives

Result:  

runs =

     1   178
   252   307
   356   397
   436   469
   503   532
   562   588
   615   640
   665   687
   710   731
   753   773
   794   813
   833   851
   870   887
   905   921
   939   955
   972   987

-- 
Bob Walton





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