Thread Subject:

Find quantity of sign changes in a vector

i have a vector :
a=[1 -2 3 4 -4 -2 4]
now i want to find the number of sign changes in above vector? In this vector the sign has been changed for 4 times
thanks

On 7/1/2012 11:06 AM, mukim wrote:
> i have a vector :
> a=[1 -2 3 4 -4 -2 4]
> now i want to find the number of sign changes in above vector? In this
> vector the sign has been changed for 4 times
> thanks

 > a=[1 -2 3 4 -4 -2 4];
 >> b=a<0; b=diff(b);b(b==0)=[];
 >> nsign=length(b)
nsign =
      4
 >>

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On 7/1/2012 12:05 PM, dpb wrote:
> On 7/1/2012 11:06 AM, mukim wrote:
>> i have a vector :
>> a=[1 -2 3 4 -4 -2 4]
>> now i want to find the number of sign changes in above vector? In this
>> vector the sign has been changed for 4 times
>> thanks
>
> > a=[1 -2 3 4 -4 -2 4];
> >> b=a<0; b=diff(b);b(b==0)=[];
> >> nsign=length(b)
> nsign =
> 4
> >>

Or, implemented more concisely...

nsign=sum(abs(diff(a<0)));

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A very slight variation:

sum(diff(sign(a)) ~= 0)

On 7/1/2012 12:28 PM, james bejon wrote:
> A very slight variation:
>
> sum(diff(sign(a)) ~= 0)

Or yet another w/o an explicit test...

sum(abs(diff(sign(a)./2)))

Skinning cats, and all that... :)

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