R help - Apr 2005 - i param in "for" loop does not takes zeros?

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

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
> 
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-- 
Rich FitzJohn
rich.fitzjohn <at> gmail.com   |   
http://homepages.paradise.net.nz/richa183
                      You are in a maze of twisty little functions, all alike