[R] dynamically extract data from a list

[Gabor Grothendieck]

The code I posted does work if you use it as I explained, not
as you changed it.  Executing strings is probably not a very
R-ish thing to do but if that's your aim use eval and parse:

s <- "iris["iris$Sepal.Width > 4,]"
eval(parse(text = s))

On Tue, Aug 12, 2008 at 10:35 AM, Dries Knapen <dries.knapen at gmail.com> wrote:
> Hi,
>
> Thanks for your reply. However, this didn't work exactly as I needed it to
> since the expression is dynamically built as a character vector
>
> i.e. not executed as
> e <- expression(Sepal.Width > 4)
>
> but as
> e <- expression("Sepal.Width > 4")
>
> in which case subset() throws an error (must evaluate to logical).
>
> Fortunately, a good night of sleep resulted in this workaround:
>
> s <- "iris[Sepal.Width > 4,]"
> execute.string <- function(string) {
>  write(string, 'tmp.txt')
>  out <- source('tmp.txt')
>  unlink('tmp.txt')
>  return(out$value)
> }
> execute.string(s)
>
>
> On 12 Aug 2008, at 04:08, Gabor Grothendieck wrote:
>
>> Try this:
>>
>>> e <- expression(Sepal.Width > 4)
>>> subset(iris, eval(e), select = "Sepal.Length")
>>
>>   Sepal.Length
>> 16          5.7
>> 33          5.2
>> 34          5.5
>>>
>>> subset(iris, eval(e))
>>
>>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
>> 16          5.7         4.4          1.5         0.4  setosa
>> 33          5.2         4.1          1.5         0.1  setosa
>> 34          5.5         4.2          1.4         0.2  setosa
>>
>>
>> On Mon, Aug 11, 2008 at 9:36 PM, Dries Knapen <dries.knapen at gmail.com>
>> wrote:
>>>
>>> Hi,
>>>
>>> Based on user input, I wrote a function that creates a list which looks
>>> like:
>>>
>>>> str(list)
>>>
>>> List of 4
>>>  $ varieties: chr [1:12] "temp.26_time.5dagen_biorep.1"
>>> "time.5dagen_temp.26_biorep.2" "temp.18_time.5dagen_biorep.1"
>>> "temp.18_time.5dagen_biorep.2" ...
>>>  $ temp     : Factor w/ 2 levels "18","26": 2 2 1 1 2 2 1 1 1 1 ...
>>>  $ time     : Factor w/ 3 levels "14dagen","28dagen",..: 3 3 3 3 1 1 1 1
>>> 2 2
>>> ...
>>>  $ biorep   : Factor w/ 2 levels "1","2": 1 2 1 2 1 2 1 2 1 2 ...
>>>
>>> Now, based on user input as well, I want to dynamically extract data from
>>> list$varieties. Therefore, I wrote a function which generates a string
>>> containing the data extraction conditions which looks like this:
>>>
>>>> query <- make.contrast.substring(negative.contrast, list)
>>>
>>> Read 1 item
>>> [1]
>>>
>>> "(list$temp=='18')&(list$time=='14dagen'|list$time=='28dagen'|list$time=='5dagen')&(list$biorep=='1'|list$biorep=='2')"
>>>
>>> Now what I want to achieve is to extract data by doing:
>>>
>>> list$varieties[query]
>>>
>>> which doesn't work since "query" is a string and object names are not
>>> expanded...
>>>
>>> Obviously, manually copying the string like so
>>>
>>>
>>> list$varieties[(list$temp=='18')&(list$time=='14dagen'|list$time=='28dagen'|list$time=='5dagen')&(list$biorep=='1'|list$biorep=='2')]
>>>
>>> works perfectly - but I need it to be automated.
>>>
>>> I'm quite new to R and used to programming in PHP, so I may just be
>>> "conceptually" confused about how to do this. Any help would be greatly
>>> appreciated.
>>>
>>> thanks in advance,
>>> Dries Knapen
>>>
>>>
>>>
>>> ********************************************
>>> Dr. Dries Knapen
>>>
>>> University of Antwerp
>>> Department of Biology
>>> Ecophysiology, Biochemistry and Toxicology
>>> Groenenborgerlaan 171 - U711, B-2020 Antwerp
>>> Belgium
>>>
>>> tel ++32 3 265 33 49
>>> fax ++32 3 265 34 97
>>>
>>> ______________________________________________
>>> 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.
>
> ______________________________________________
> 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.
>