[R] summarize a vector

[David Winsemius]

On Aug 10, 2012, at 12:57 PM, Bert Gunter wrote:

> ... or perhaps even simpler:
>
>> sz <- function(x,k)tapply(x,(seq_along(x)-1)%/%k, sum)
>> sz(1:10,3)
> 0  1  2  3
> 6 15 24 10
>
> Note that this works for k>n, where the previous solution does not.
>> sz(1:10,15)
> 0
> 55

I agree that it is more elegant, but I do not get an error or an  
unexpected result with my method.

 > N=10
 > k=15
 > w <- tapply( v ,rep(1:(N/k +1), each=k, len=N ) , sum)
 > w
  1
55

A different label but the same result. I'm protected from the typical  
1:0 problem that seq_along solves by including +1 in the second  
argument to ":"/seq(). Unless, of course, you set N to a negative  
number, but that wouldn't make much sense would it, and you get an  
error from rep() anyway.

Best;
David.

>
> -- Bert
>
> On Fri, Aug 10, 2012 at 12:37 PM, David Winsemius
> <dwinsemius at comcast.net> wrote:
>>
>> On Aug 10, 2012, at 12:20 PM, Sam Steingold wrote:
>>
>>> I have a long numeric vector v (length N) and I want create a  
>>> shorter
>>> vector of length N/k consisting of sums of k-subsequences of v:
>>>
>>> v <- c(1,2,3,4,5,6,7,8,9,10)
>>>
>>> N=10, k=3
>>> ===> [6,15,24,10]
>>>
>>> I can, of course, iterate:
>>>
>>>> w <- vector(mode="numeric",length=ceiling(N/k))
>>>> for (i in 1:length(w)) w[i] <- sum(v(i*k:(i+1)*k))
>>>
>>>
>>> (modulo boundary conditions)
>>> but I wonder if there is a better way.
>>
>>
>> Well, using v with parentheses instead of square-brackets might not  
>> be the
>> right way, since v is not a function.
>>
>> Consider this alternate (no need to pre-allocate 'w'):
>>
>>> w <- tapply( v ,rep(1:(N/k +1), each=k, len=N ) , sum)
>>> w
>> 1  2  3  4
>> 6 15 24 10
>>
>> --
>>
>> David Winsemius, MD
>> Alameda, CA, USA
>>
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>> and provide commented, minimal, self-contained, reproducible code.
>
>
>
> -- 
>
> Bert Gunter
> Genentech Nonclinical Biostatistics
>
> Internal Contact Info:
> Phone: 467-7374
> Website:
> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

David Winsemius, MD
Alameda, CA, USA