[R] summarize a vector

[Bert Gunter]

Oh yes, I stand corrected. I didn't look at your code carefully enough.

-- Bert

On Fri, Aug 10, 2012 at 3:07 PM, David Winsemius <dwinsemius at comcast.net> wrote:
>
> 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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>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> 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
>



-- 

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