i param in "for" loop does not takes zeros?

From: Rich FitzJohn <rich.fitzjohn at gmail.com>
Reply-To: Rich FitzJohn <rich.fitzjohn at gmail.com>
To: "Francisco J. Zagmutt" <gerifalte28 at hotmail.com>
CC: R-help at stat.math.ethz.ch
Subject: Re: [R] i param in "for" loop does not takes zeros?
Date: Thu, 14 Apr 2005 09:36:21 +1200

The for loop is not ignoring the zero at all, but the assignment is,
since R indexes starting at 1, not zero.

sim <- c()
sim[0] <- 1
sim

numeric(0)

To run this loop this way, you need to add one to the index:
for ( i in 0:5 )
  sim[i+1] <- dbinom(i, 5, p)

However, you'd be better off passing your vector of values directly to
dbinom():

dbinom(0:5, 5, p)

[1] 0.32768 0.40960 0.20480 0.05120 0.00640 0.00032

all(dbinom(0:5, 5, p) == sim)

[1] TRUE

Cheers,
Rich

On 4/14/05, Francisco J. Zagmutt <gerifalte28 at hotmail.com> wrote:

Hi all

Is there any reason why the parameter i in a "for" loop ignores a value 

of

zero?  For example

sim=c()
p=.2
for(i in 0:5)
 {sim[i]=dbinom(i,5,p)
 }

sim
[1] 0.40960 0.20480 0.05120 0.00640 0.00032

In this example the quantile i= 0 was ignored since
dbinom(0,5,p)
[1] 0.32768

The same behaviour occurs if I use a while loop to perform the same
calculation:
sim=c()
p=.2
i=0
while(i <6)
 {sim[i]=dbinom(i,5,p)
 i=i+1
 }
sim
[1] 0.40960 0.20480 0.05120 0.00640 0.00032

How can I perform a loop passing a zero value parameter?  I know I can 

use

an if statement for i<=0 but I was wondering why the loop is ignoring 

the

zero value.

Many thanks!

Francisco