trouble with cumsum?

| Dear R users,
| 
| I am using R version 2.0.1 (2004/11/15) on an i386-pc-mingw32 platform. I
| encounter the following problem while using cumsum:
| 
| > a <- rep(0.01, 100)
| > b <- cumsum(a)
| > sum(a) == 1
| [1] TRUE
| > b[100] == 1
| [1] FALSE
| 
| Am I missing something? 

Yes, FAQ section 7.31 entitled "Why doesn't R think these numbers are equal?".

| Should cumsum have such an outcome?

a <- rep(0.01, 100)
b <- cumsum(a)
sum(a) == 1

[1] FALSE

all.equal(sum(a), 1)

[1] TRUE

all.equal(b[100], 1)

[1] TRUE

On my Linux system with R 2.1.0, the first equality fails whereas it "worked"
for you. Morale: don't forget about floating-point math ...

Hth, Dirk

Statistics: The (futile) attempt to offer certainty about uncertainty.
         -- Roger Koenker, 'Dictionary of Received Ideas of Statistics'

R help document showed us that you should not use "==" for comparing
objects, and you will use "all.equal".

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Makram Talih
Sent: Monday, May 30, 2005 8:17 AM
To: r-help at stat.math.ethz.ch
Subject: [R] trouble with cumsum?

Dear R users,

I am using R version 2.0.1 (2004/11/15) on an i386-pc-mingw32 platform. I
encounter the following problem while using cumsum:

a <- rep(0.01, 100)
b <- cumsum(a)
sum(a) == 1

[1] TRUE

b[100] == 1

[1] FALSE

Am I missing something? Should cumsum have such an outcome?

Thanks in advance for any clarifications any of you can offer.

Regards,

Makram Talih

--
Makram Talih, Ph.D.
Assistant Professor
Department of Mathematics and Statistics Hunter College of the City
University of New York
695 Park Avenue, Room 905 HE
New York, NY 10021

Website: http://stat.hunter.cuny.edu/talih
E-mail: talih at math.hunter.cuny.edu
Tel: 212-772-5308
Fax: 212-772-4858

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