R help - Sep 2009 - function to solve equations

Hi, 

I am trying to solve this equation prob = exp(-3.33 + 0.0102*x)/(1+exp(-3.33 +
0.0102*x)). I want to write a function where I call the function and enter the
'prob' value and the output should be the 'x'. Im not sure how
to write this. I have a basic structure but im not sure if its correct.

calc <- function(prob){
prob <- exp(-3.33+0.0102*x)/(1+exp(-3.33 + 0.0102*x))
solve(prob)
x
}

Thanks

Yash Gandhi

You don't need an equation solver.  This is basic algebra.

The solution is:

X = (log(prob) - log(1 - prob) + 3.33) / 0.0102


Ravi.
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Ravi Varadhan, Ph.D.

Assistant Professor, The Center on Aging and Health

Division of Geriatric Medicine and Gerontology 

Johns Hopkins University

Ph: (410) 502-2619

Fax: (410) 614-9625

Email: rvaradhan at jhmi.edu

Webpage:
http://www.jhsph.edu/agingandhealth/People/Faculty_personal_pages/Varadhan.h
tml

 

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-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
Behalf Of Yash Gandhi
Sent: Thursday, September 10, 2009 3:35 PM
To: r-help at r-project.org
Subject: [R] function to solve equations

Hi, 

I am trying to solve this equation prob = exp(-3.33 + 0.0102*x)/(1+exp(-3.33
+ 0.0102*x)). I want to write a function where I call the function and enter
the 'prob' value and the output should be the 'x'. Im not sure
how to write
this. I have a basic structure but im not sure if its correct. 

calc <- function(prob){
prob <- exp(-3.33+0.0102*x)/(1+exp(-3.33 + 0.0102*x))
solve(prob)
x
}

Thanks

Yash Gandhi

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