R help - Mar 2010 - how to deal with vector[0]?

Hi,

I have  a vector with 4 elements, e.g., tau_i=c(100,200,300,400), but
potentially tau_i[0]=0. In a "for" loop,

tau_i=c(100,200,300,400)
m=4
tau_i[0]=0  # <------- ?
P_i=1
	for(i in 2:m)
	{
		P_i = P_i*(tau_i[i-1]-tau_i[i-2])
	}


Error in P_i = P_i * (tau_i[k - 1] - tau_i[k - 2]): 
  replacement has length zero


Unfortunately, I can add this potential element into "tau_i" because
it has
been defined initially and used mainly throughout all procedures.

When I calculate 
> k=2
> tau_i[k-1]-tau_i[k-2]
umeric(0)

But, it should be "100". Is there anyone can help me solve this
problem?


Best,

Jin
NREL, Golden CO
(303) 275-4642
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Jin wrote:
> 
> Hi,
> 
> I have  a vector with 4 elements, e.g., tau_i=c(100,200,300,400), but
> potentially tau_i[0]=0. In a "for" loop,
> 
> tau_i=c(100,200,300,400)
> m=4
> tau_i[0]=0  # <------- ?
> P_i=1
> 	for(i in 2:m)
> 	{
> 		P_i = P_i*(tau_i[i-1]-tau_i[i-2])
> 	}
> 
> 
> Error in P_i = P_i * (tau_i[k - 1] - tau_i[k - 2]): 
>   replacement has length zero
> 
> 
> Unfortunately, I can add this potential element into "tau_i"
because it
> has been defined initially and used mainly throughout all procedures.
> 
> When I calculate 
>> k=2
>> tau_i[k-1]-tau_i[k-2]
> umeric(0)
> 
> But, it should be "100". Is there anyone can help me solve this
problem?
> 
> 
> Best,
> 
> Jin
> NREL, Golden CO
> (303) 275-4642
> 
I'm a little confused on exactly what you're trying to achieve here--
vector
indexes in R start at 1, not 0, and so trying to assign any value to tau_i[
0 ] is meaningless.

Using tau_i[ 0 ] in a calculation is equivalent to using numeric( 0 ) --
i.e. a numeric vector of *length* zero, which contains no values at all.

Does this help?

-Charlie

-----
Charlie Sharpsteen
Undergraduate-- Environmental Resources Engineering
Humboldt State University
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