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Re: A problem using IDL? [message #60080 is a reply to message #60079]

Fri, 02 May 2008 09:16

R.G. Stockwell
Messages: 363
Registered: July 1999

Senior Member

<chihuyu@sohu.com> wrote in message
news:337cf67f-424c-4bd8-8340-7ec7d0a121a5@l25g2000prd.google groups.com...
> I used idl to write the following:
>
> window,xsize=800,ysize=400
> min=0
> max=100
> n=200
> x=fltarr(n+1)
> for i=0,n do x[i]=min+double(i)*(max-min)/n
> print,x
> y=sin(2*3.141593*x)
> plot,x,y
>
> When n=100 or n=200,the graph is wrong. When n >=300,it's OK.Can
> someone tell me why?

You are aliased. Definitely read up on it if you are not familiar with
aliasing, as it is quite important in spectral analysis. Also read up on
nyquist frequency.

you can write a sinusoid as

y = sin(2*!Pi*f/length*x)

to explicitly have your frequency in there.

Also, check out this code which explicitly shows the
aliasing.

window,xsize=800,ysize=400
min=0
max=100
n=100
x=fltarr(n+1)
for i=0,n do x[i]=min+double(i)*(max-min)/n
;print,x
y=sin(2*3.141593*x)

!P.multi=[0,1,2]

plot,x,y,psym=-4,xr=[0,10]
x1 = x
y1 = y

n=500
x=fltarr(n+1)
for i=0,n do x[i]=min+double(i)*(max-min)/n
;print,x
y=sin(2*3.141593*x)

plot,x,y,psym=-4,xr=[0,10]
oplot,x1,y1,psym=6,thick=4

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[Message index]

		Re: A problem using IDL? By: R.G. Stockwell on Fri, 02 May 2008 09:20
		Re: A problem using IDL? By: R.G. Stockwell on Fri, 02 May 2008 09:16
		Re: A problem using IDL? By: Vince Hradil on Thu, 01 May 2008 08:41
		Re: A problem using IDL? By: pgrigis on Thu, 01 May 2008 07:49

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