SIG on Teaching Statistics using R

AbouEl-Makarim Aboueissa | 16 Feb 2012 15:13

Shading the solution of quadratic inequality

Dear all R Users:

How can I shade the solution (-Inf,-2) or (2,Inf) of the inequality -x^2+4<0 within the curve of f(x)=-x^2+4.

and 

the solution (-Inf,-2)or(2,Inf) of the inequality x^2-4>0 within the curve of 
f(x)=x^2-4.

 
with many thanks

abou

 
==========================
AbouEl-Makarim Aboueissa, Ph.D.
Associate Professor of Statistics
Graduate Program Coordinator
Department of Mathematics & Statistics
University of Southern Maine
96 Falmouth Street
P.O. Box 9300
Portland, ME 04104-9300
USA

Tel: (207) 228-8389
Fax: (207) 780-5607
Email: aaboueissa <at> usm.maine.edu
          aboueiss <at> yahoo.com

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R. Michael Weylandt | 16 Feb 2012 15:42

Re: Shading the solution of quadratic inequality

You should probably say more about which graphics engine you are
using, but here's a start:

curve(x^2-4, from = -3, to = 3)
x <- seq(-3, 3, length.out = 200)
polygon(c(x, rev(x)), c(x^2-4, rep(Inf, length(x))), col = 2)

The red is a little "oppressive" so I'd try to add transparency if
your device supports it. Adding a shade above/below argument to curve
based on the same principles might be a fun exercise too...

Michael

On Thu, Feb 16, 2012 at 9:13 AM, AbouEl-Makarim Aboueissa
<aaboueissa <at> usm.maine.edu> wrote:
> Dear all R Users:
>
> How can I shade the solution (-Inf,-2) or (2,Inf) of the inequality -x^2+4<0 within the curve of f(x)=-x^2+4.
>
> and
>
> the solution (-Inf,-2)or(2,Inf) of the inequality x^2-4>0 within the curve of
> f(x)=x^2-4.
>
>
> with many thanks
>
> abou
>
>

(Continue reading)

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Randall Pruim | 16 Feb 2012 16:07

Re: Shading the solution of quadratic inequality

I'm not sure exactly what sort of plot was desired, but I'll offer 1.5  
possibilities.

Currently you can do something like this:

require(mosaic)
plotFun( x^2 - 4 ~ x , xlim=c(-6,6), col='blue' )
plotFun( (x^2 - 4)/as.numeric(x^2 -4 > 0) ~ x , xlim=c(-6,-2),  
add=TRUE, col='blue', lwd=4 )

It is a bit of kludge since I'm using division by 0 to hide part of  
the function.  You can adjust colors, line thickness, and alpha to  
your particular sensitivities.  And you can use type='h' if you want a  
"filled in version".

Soon, you should be able to do

plofFun( x^2 - 4 ~ x, groups=(x^2-4>0) )

If you find yourself wanting to plot "mathematical" functions like  
this, I encourage you to give plotFun() a try and let us know how you  
like it.

---rjp

On Feb 16, 2012, at 9:42 AM, R. Michael Weylandt wrote:

> You should probably say more about which graphics engine you are
> using, but here's a start:
>

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Randall Pruim | 16 Feb 2012 16:11

Re: Shading the solution of quadratic inequality


The xlim=c(-6,-2) is extraneous in the code below.  It snuck in from  
something else I was doing.  It doesn't hurt any, but it doesn't do  
anything either since the plotting limits are inferred from the  
surrounding plot when add=TRUE is used.

On Feb 16, 2012, at 10:07 AM, Randall Pruim wrote:

> I'm not sure exactly what sort of plot was desired, but I'll offer  
> 1.5 possibilities.
>
> Currently you can do something like this:
>
> require(mosaic)
> plotFun( x^2 - 4 ~ x , xlim=c(-6,6), col='blue' )
> plotFun( (x^2 - 4)/as.numeric(x^2 -4 > 0) ~ x , xlim=c(-6,-2),  
> add=TRUE, col='blue', lwd=4 )
>
> It is a bit of kludge since I'm using division by 0 to hide part of  
> the function.  You can adjust colors, line thickness, and alpha to  
> your particular sensitivities.  And you can use type='h' if you want  
> a "filled in version".
>
> Soon, you should be able to do
>
> plofFun( x^2 - 4 ~ x, groups=(x^2-4>0) )
>
> If you find yourself wanting to plot "mathematical" functions like  
> this, I encourage you to give plotFun() a try and let us know how  
> you like it.

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