[R] dynamically extract data from a list

[Gabor Grothendieck]

> eval(parse(text = "iris[iris$Sepal.Width > 4,]"))
   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 Tue, Aug 12, 2008 at 11:00 AM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> 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.
>>
>