Re: [R] question on graphs and finding area under a curve

2005-08-03 Thread Adaikalavan Ramasamy 
Also see function rocdemo.sca in the ROC package.

The area under the 45 degree line in an ROC curve has an area of 0.5.

Regards, Adai



On Tue, 2005-08-02 at 09:24 -0400, Ravi Varadhan wrote:
 Hi,
 
 To find the area lying between the curve y = y(x) and 45 degree line (which,
 assuming it goes through the origin, is y = x), you can use the following
 function based on trapezoidal rule:
 
 trap.rule - function(x,f) {sum(diff(x)*(f[-1]+f[-length(f)]))/2}
 
 trap.rule(x,f=y-x)
 
 This area will be negative if y(x) is below the 45 degree line.
 
 However, your question is not complete, I think.  You need to specify the
 interval of integration. For this you may need to determine the points of
 intersection of the two curves, which involves the solution of a fixed point
 problem.
 
 Hope this helps,
 Ravi.
 
  -Original Message-
  From: [EMAIL PROTECTED] [mailto:r-help-
  [EMAIL PROTECTED] On Behalf Of Renuka Sane
  Sent: Tuesday, August 02, 2005 8:51 AM
  To: r-help@stat.math.ethz.ch
  Subject: [R] question on graphs and finding area under a curve
  
  Question on graphs:
  
  The default case for drawing a graph in R, is where a little space is left
  on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) -
  -
  there will be some space between the edge of the x-axis and where 0 is
  placed. If I want 0 on the edge, how do I do it in R?
  
  Area under the curve:
  
  I have a 45 degree line and a curve above or below it. Is there a way in R
  to find the area between the two?
  
  Thanks,
  Renuka
  
  --
  Renuka Sane
  http://www.nyx.net/~rsane
  
  [[alternative HTML version deleted]]
  
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Re: [R] question on graphs and finding area under a curve

2005-08-02 Thread Marc Schwartz 
On Tue, 2005-08-02 at 18:20 +0530, Renuka Sane wrote:
 Question on graphs:
 
 The default case for drawing a graph in R, is where a little space is left 
 on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) -- 
 there will be some space between the edge of the x-axis and where 0 is 
 placed. If I want 0 on the edge, how do I do it in R?

See ?par and take note of the 'xaxs' and 'yaxs' parameters. By default,
these are set to 'r', where the axes are extended by 4% in each
direction. Thus, set one or both parameters to 'i' to set the axes to
exactly the ranges of xlim and/or ylim as you require.

 Area under the curve:
 
 I have a 45 degree line and a curve above or below it. Is there a way in R 
 to find the area between the two?

See Frank Harrell's somers2() function in the Hmisc package on CRAN.
Among other things, it outputs a value C, which is the AUC.

HTH,

Marc Schwartz

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Re: [R] question on graphs and finding area under a curve

2005-08-02 Thread Romain Francois 
Le 02.08.2005 14:50, Renuka Sane a écrit :

Question on graphs:

The default case for drawing a graph in R, is where a little space is left 
on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) -- 
there will be some space between the edge of the x-axis and where 0 is 
placed. If I want 0 on the edge, how do I do it in R?

Area under the curve:

I have a 45 degree line and a curve above or below it. Is there a way in R 
to find the area between the two?

  

Hi,

integrate.xy in sfsmisc package might help you.

Romain

Thanks,
Renuka

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visit the R Graph Gallery : http://addictedtor.free.fr/graphiques
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Etudiant  ISUP - CS3 - Industrie et Services   
~~http://www.isup.cicrp.jussieu.fr/  ~~
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~~   http://www.inria.fr/recherche/equipes/select.fr.html~~
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Re: [R] question on graphs and finding area under a curve

2005-08-02 Thread Tuszynski, Jaroslaw W. 
How about:

trapz = function(x, y) 
{ # computes the integral of y with respect to x using trapezoidal
integration. 
  idx = 2:length(x)
  return (as.double( (x[idx] - x[idx-1]) %*% (y[idx] + y[idx-1])) / 2)
} 

Jarek
\===

 Jarek Tuszynski, PhD.   o / \ 
 Science Applications International Corporation  \__,|  
 (703) 676-4192  \
 [EMAIL PROTECTED] `\


-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Renuka Sane
Sent: Tuesday, August 02, 2005 8:51 AM
To: r-help@stat.math.ethz.ch
Subject: [R] question on graphs and finding area under a curve

Question on graphs:

The default case for drawing a graph in R, is where a little space is left
on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) --
there will be some space between the edge of the x-axis and where 0 is
placed. If I want 0 on the edge, how do I do it in R?

Area under the curve:

I have a 45 degree line and a curve above or below it. Is there a way in R
to find the area between the two?

Thanks,
Renuka

--
Renuka Sane
http://www.nyx.net/~rsane

[[alternative HTML version deleted]]

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Re: [R] question on graphs and finding area under a curve

2005-08-02 Thread Ravi Varadhan 
Hi,

To find the area lying between the curve y = y(x) and 45 degree line (which,
assuming it goes through the origin, is y = x), you can use the following
function based on trapezoidal rule:

trap.rule - function(x,f) {sum(diff(x)*(f[-1]+f[-length(f)]))/2}

trap.rule(x,f=y-x)

This area will be negative if y(x) is below the 45 degree line.

However, your question is not complete, I think.  You need to specify the
interval of integration. For this you may need to determine the points of
intersection of the two curves, which involves the solution of a fixed point
problem.

Hope this helps,
Ravi.

 -Original Message-
 From: [EMAIL PROTECTED] [mailto:r-help-
 [EMAIL PROTECTED] On Behalf Of Renuka Sane
 Sent: Tuesday, August 02, 2005 8:51 AM
 To: r-help@stat.math.ethz.ch
 Subject: [R] question on graphs and finding area under a curve
 
 Question on graphs:
 
 The default case for drawing a graph in R, is where a little space is left
 on the x and y axis before the first tick i.e. even if I say xlim=c(0,1) -
 -
 there will be some space between the edge of the x-axis and where 0 is
 placed. If I want 0 on the edge, how do I do it in R?
 
 Area under the curve:
 
 I have a 45 degree line and a curve above or below it. Is there a way in R
 to find the area between the two?
 
 Thanks,
 Renuka
 
 --
 Renuka Sane
 http://www.nyx.net/~rsane
 
   [[alternative HTML version deleted]]
 
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 R-help@stat.math.ethz.ch mailing list
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