MATLAB Central Newsreader - Greatest commmon divisor

Greatest commmon divisor

Feng — Fri, 12 Dec 2008 21:21:02 -0500

Hi, any one know how can get the gcd for an array. The function gcd() in matlabe will give result for a array.
for example I want to get 2 for the array A= [10 2 10 -4 2 0];
Is there any function to do that? or how can I do that?

Thanks

Re: Greatest commmon divisor

Feng — Fri, 12 Dec 2008 21:29:02 -0500

"Feng" <nclxin@hotmail.com> wrote in message <ghukju$ksu$1@fred.mathworks.com>...
> Hi, any one know how can get the gcd for an array. The function gcd() in matlabe > will give result for two scalar numbers.
> for example I want to get 2 for the array A= [10 2 10 -4 2 0];
> Is there any function to do that? or how can I do that?
>
> Thanks

Re: Greatest commmon divisor

Roger Stafford — Fri, 12 Dec 2008 21:47:02 -0500

"Feng" <nclxin@hotmail.com> wrote in message <ghul2u$rfp$1@fred.mathworks.com>...
> "Feng" <nclxin@hotmail.com> wrote in message <ghukju$ksu$1@fred.mathworks.com>...
> > Hi, any one know how can get the gcd for an array. The function gcd() in matlabe > will give result for two scalar numbers.
> > for example I want to get 2 for the array A= [10 2 10 -4 2 0];
> > Is there any function to do that? or how can I do that?
> >
> > Thanks

You could always use an old-fashioned for-loop:

c = A(1);
for k = 2:length(A)
c = gcd(c,A(k));
end

Roger Stafford

Re: Greatest commmon divisor

Loren Shure — Tue, 16 Dec 2008 01:38:23 -0500

In article <ghukju$ksu$1@fred.mathworks.com>, nclxin@hotmail.com says...
> Hi, any one know how can get the gcd for an array. The function gcd() in matlabe will give result for a array.
> for example I want to get 2 for the array A= [10 2 10 -4 2 0];
> Is there any function to do that? or how can I do that?
>
> Thanks
>

Check out the function arrayfun

--
Loren
http://blogs.mathworks.com/loren

Re: Greatest commmon divisor

Bruno Luong — Tue, 16 Dec 2008 05:51:02 -0500

Loren Shure <loren@mathworks.com> wrote in message <MPG.23b0d09e505fb8f4989907@news.mathworks.com>...
> >
>
> Check out the function arrayfun
>

I can't see how arrayfun would be used here. I rather stick with Roger's for-loop solution.

Bruno

Re: Greatest commmon divisor

Jos — Tue, 16 Dec 2008 07:54:05 -0500

"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <ghum4l$d95$1@fred.mathworks.com>...
> "Feng" <nclxin@hotmail.com> wrote in message <ghul2u$rfp$1@fred.mathworks.com>...
> > "Feng" <nclxin@hotmail.com> wrote in message <ghukju$ksu$1@fred.mathworks.com>...
> > > Hi, any one know how can get the gcd for an array. The function gcd() in matlabe > will give result for two scalar numbers.
> > > for example I want to get 2 for the array A= [10 2 10 -4 2 0];
> > > Is there any function to do that? or how can I do that?
> > >
> > > Thanks
>
> You could always use an old-fashioned for-loop:
>
> c = A(1);
> for k = 2:length(A)
> c = gcd(c,A(k));
> end
>
> Roger Stafford

What about

min(gcd(A(1:end-1),A(2:end)))

Jos

Re: Greatest commmon divisor

Jos — Tue, 16 Dec 2008 07:59:07 -0500

"Jos " <#10584@fileexchange.com> wrote in message <gi7mqt$87b$1@fred.mathworks.com>...
> "Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <ghum4l$d95$1@fred.mathworks.com>...
> > "Feng" <nclxin@hotmail.com> wrote in message <ghul2u$rfp$1@fred.mathworks.com>...
> > > "Feng" <nclxin@hotmail.com> wrote in message <ghukju$ksu$1@fred.mathworks.com>...
> > > > Hi, any one know how can get the gcd for an array. The function gcd() in matlabe > will give result for two scalar numbers.
> > > > for example I want to get 2 for the array A= [10 2 10 -4 2 0];
> > > > Is there any function to do that? or how can I do that?
> > > >
> > > > Thanks
> >
> > You could always use an old-fashioned for-loop:
> >
> > c = A(1);
> > for k = 2:length(A)
> > c = gcd(c,A(k));
> > end
> >
> > Roger Stafford
>
>
> What about
>
> min(gcd(A(1:end-1),A(2:end)))
>
> Jos

On second thoughts, you have to remove the zeros first, otherwise something like
A = [x 0 0 y]
will always give 0, instead of gcd(x,y)

Jos

Re: Greatest commmon divisor

Bruno Luong — Tue, 16 Dec 2008 08:09:06 -0500

>
> min(gcd(A(1:end-1),A(2:end)))
>

Jos, this does not work

A=[15 30 20]

gcd is 5, not 10

Bruno

Re: Greatest commmon divisor

Roger Stafford — Tue, 16 Dec 2008 08:16:05 -0500

"Jos " <#10584@fileexchange.com> wrote in message <gi7mqt$87b$1@fred.mathworks.com>...
> What about
>
> min(gcd(A(1:end-1),A(2:end)))
>
> Jos

I don't think that would work, Jos, even with zeros eliminated. Let A = [15,21,35]. Then gcd(A(1:end-1),A(2:end)) would yield [3,7] but the minimum of these is not the gcd of the three combined quantities. The greatest common factor of all three is simply 1, not this minimum.

Roger Stafford

Re: Greatest commmon divisor

Jos — Tue, 16 Dec 2008 10:04:03 -0500

"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <gi7o45$297$1@fred.mathworks.com>...
> "Jos " <#10584@fileexchange.com> wrote in message <gi7mqt$87b$1@fred.mathworks.com>...
> > What about
> >
> > min(gcd(A(1:end-1),A(2:end)))
> >
> > Jos
>
> I don't think that would work, Jos, even with zeros eliminated. Let A = [15,21,35]. Then gcd(A(1:end-1),A(2:end)) would yield [3,7] but the minimum of these is not the gcd of the three combined quantities. The greatest common factor of all three is simply 1, not this minimum.
>
> Roger Stafford

Yep, you and Bruno are right, of course. I was just wondering why Loren suggested arrayfun, and so I was looking for a function that could do this.

Jos

Re: Greatest commmon divisor

Loren Shure — Tue, 16 Dec 2008 15:16:09 -0500

In article <gi7uei$22l$1@fred.mathworks.com>, #10584@fileexchange.com
says...
> "Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <gi7o45$297$1@fred.mathworks.com>...
> > "Jos " <#10584@fileexchange.com> wrote in message <gi7mqt$87b$1@fred.mathworks.com>...
> > > What about
> > >
> > > min(gcd(A(1:end-1),A(2:end)))
> > >
> > > Jos
> >
> > I don't think that would work, Jos, even with zeros eliminated. Let A = [15,21,35]. Then gcd(A(1:end-1),A(2:end)) would yield [3,7] but the minimum of these is not the gcd of the three combined quantities. The greatest common factor of all three is simply 1, not this minimum.
> >
> > Roger Stafford
>
> Yep, you and Bruno are right, of course. I was just wondering why Loren suggested arrayfun, and so I was looking for a function that could do this.
>
> Jos
>
>

Perhaps I misread the problem. I thought the OP wanted GCD of each
"pair" of points in 2 arrays. Probably read too fast...

--
Loren
http://blogs.mathworks.com/loren