[R] Efficient lookup on a two-dimensional table

[Gabor Grothendieck]

Try this (shown for stated problem but generalizes by just adding
additional arguments):

mapply("[", list(x), ltrs, mnths)


On Thu, Jun 25, 2009 at 10:24 AM, Rama Ramakrishnan<rama at alum.mit.edu> wrote:
> Follow-on question: is there a way to do this for higher-dimensional (i.e.
> more than 2 dimensions) arrays?
>
>
> On Thu, Jun 25, 2009 at 10:17 AM, Rama Ramakrishnan <rama at alum.mit.edu>wrote:
>
>> That works!! Very nice way to do it! Thank you, Henrique!
>> Rama Ramakrishnan
>>
>>
>> On Thu, Jun 25, 2009 at 10:11 AM, Henrique Dallazuanna <wwwhsd at gmail.com>wrote:
>>
>>> Try this:
>>>
>>> y$values <- diag(x[y$ltrs, y$mnths])
>>>
>>> On Thu, Jun 25, 2009 at 11:02 AM, Rama Ramakrishnan <rama at alum.mit.edu>wrote:
>>>
>>>> Dear R-Users,
>>>> I need to lookup values from a 2-d table using the row names and column
>>>> names as indices. I was wondering if there's a way to do this without an
>>>> explicit loop.
>>>>
>>>> Example:
>>>> #x is the 2-d table that holds the values
>>>>
>>>> x <- matrix(rnorm(26*12),nrow=26)
>>>>
>>>> rownames(x) <- letters
>>>>
>>>> colnames(x) <- month.name
>>>>
>>>>
>>>> #y is a data frame that has the "keys" I want to use as indices into x
>>>>
>>>> y <- data.frame(ltrs=sample(letters,5),mnths=sample(month.name
>>>> ,5),values=0)
>>>>
>>>>
>>>> #I want to fill in the "values" column using the "ltrs" and "mnths"
>>>> columns
>>>> as keys to look up
>>>>
>>>> # the associated value from x
>>>>
>>>> #One way to do this is with a FOR loop
>>>>
>>>> for (i in 1:nrow(y)) {y$val[i] <- x[y$ltrs[i],y$mnths[i]]}
>>>>
>>>>
>>>> My question: Is there a more efficient way (e.g., one without using an
>>>> explicit loop) to do this?
>>>>
>>>>
>>>> Thanks in advance!
>>>>
>>>>
>>>> -Rama Ramakrishnan
>>>>
>>>>        [[alternative HTML version deleted]]
>>>>
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide
>>>> http://www.R-project.org/posting-guide.html
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>>>>
>>>
>>>
>>>
>>> --
>>> Henrique Dallazuanna
>>> Curitiba-Paraná-Brasil
>>> 25° 25' 40" S 49° 16' 22" O
>>>
>>
>>
>
>        [[alternative HTML version deleted]]
>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>