[R] string substitution for argument in function

[Prof. Dr. Pedro Martinez Arbizu]

Me again,
what I really dont understand is the behaivour of R here.
notice, elem is not written in "brackets", why it is interpreted as 
string and not as a variable?
I would expect an error "elem not found" because it is not defined as 
variable, but in contrary it is accepted as column name, see below:

 > a = c('one','two','three')
 > data =c()
 > data=cbind(elem = 2,data)
 > data
      elem
[1,]    2

 > data=cbind('elem' = 2,data)
 > data
      elem elem
[1,]    2    2

 > elem
Fehler: Objekt 'elem' nicht gefunden

pedro








On 26.03.2012 17:22, Prof. Dr. Pedro Martinez Arbizu wrote:
> HI, thanks Weidong, Henrique,
> this works for the example (may be a bad example, but it should be as 
> simple as possible),
>
> This is however only a workaround to name the columns of a dataframe, 
> but is not addressing the problem itself, that is, how can I 
> substitute a string (used as argument in a function) within a function 
> call in a for-loop??
>
> I dont know if I have explain myself,
>
> something like this works well:
>
> > a = c('one','two','three')
>
>>  data= c()
>>  for(elem in a){data=cbind('title' = elem,data)}
>>  data
>      title   title title
> [1,] "three" "two" "one"
>
>
> So I can use 'title' = elem within the function and it will substitute 
> elem by a name, every loop by a different name of a list.
>
> ...but I cannot use the contrary:   elem = 'something' within the 
> function call.
>
> My real life problem is using randomForest() I want to balance the 
> class frequency with the argument sampsize = c( 'absent' = 23, elem = 23)
> where elem is everytime a different name taken from a list in a for-loop.
>
> Regards Pedro
>
>
>
> On 26.03.2012 02:17, Weidong Gu wrote:
>> Hi,
>>
>> This may help
>>
>>   a = c('one','two','three')
>> data.frame(eval(substitute(rbind(var,2),list(var=a))))
>>
>> ?substitute
>> ?eval
>>
>> Weidong Gu
>>
>>
>> On Sun, Mar 25, 2012 at 5:22 PM, Pedro Martinez
>> <Pedro.Martinez at senckenberg.de>  wrote:
>>> hello,
>>> I want to iterate through a list of names and use each element as an
>>> argument in a function. For instance:
>>>
>>>> a = c('one','two','three')
>>>> data= c()
>>>> for(elem in a){data=cbind(elem = 2,data)}
>>>> data
>>>      elem elem elem
>>> [1,]    2    2    2
>>>
>>> instead I want 'elem' to be substituted by the string in the list. 
>>> Doing
>>> it by hand would be:
>>>> data = cbind('one'=2,data)
>>>> data = cbind('two'=2,data)
>>>> data = cbind('three'=2,data)
>>>> data
>>>      'one' 'two' 'three'
>>> [1,]    2    2    2
>>>
>>> I guess that the clue would be in sub(),gsub(), paste() or similar 
>>> but I
>>> didnt get it to work.
>>>
>>> I am comming from python were we woudl do something like:
>>>> a = ['one','two','three']
>>>> data = {}
>>>> for elem in a:
>>>>        data[elem] = 2
>>>> data
>>> {'three': 2, 'two': 2, 'one': 2}
>>>
>>> Thanks, Pedro
>>>
>>>
>>> -
>>> Prof. Dr. P. Martinez Arbizu
>>> DZMB-Forschungsinstitut Senckenberg
>>>
>>> Suedstrand 44
>>> D-26382 Wilhelmshaven
>>> Germany
>>>
>>> Tel: +49 (0)4421 9475-100
>>> Fax: +49 (0)4421 9475-111
>>>
>>> Email: pmartinez at senckenberg.de
>>>
>>> Senckenberg Gesellschaft für Naturforschung
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>>> Direktorium: Prof. Dr. Dr. h.c. Volker Mosbrugger, Prof. Dr. Michael
>>> Türkay, Dr. Johannes Heilmann, Prof. Dr. Pedro Martinez Arbizu, Prof.
>>> Dr. Georg Zizka, Prof. Dr. Uwe Fritz
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>
>


-- 
---------------------------------------------------------
Prof. Dr. Pedro Martinez Arbizu

Deutsches Zentrum für Marine Biodiversitätsforschung
Senckenberg am Meer
Südstrand 44
26382 Wilhelmshaven
Tel +49 (0)4421 9475 100
Fax +49 (0)4421 9475 111
Email: pmartinez at senckenberg.de
http://www.senckenberg.de/root/index.php?page_id=2141

und

C.v.O. Universität Oldenburg
FK V, Institut für Biologie u. Umweltwissenschaften
Ag Marine Biodiversität

**********************************************
Senckenberg Gesellschaft für Naturforschung
Rechtsfähiger Verein gemäß § 22 BGB
Senckenberganlage 25
60325 Frankfurt
Direktorium: Prof. Dr. Dr. h.c. Volker Mosbrugger, Prof. Dr. Michael Türkay, Dr. Johannes Heilmann, Prof. Dr. Pedro Martinez Arbizu, Prof. Dr. Georg Zizka, Prof. Dr. Uwe Fritz
Vorsitzender des Präsidiums: Dietmar Schmid
Aufsichtsbehörde: Magistrat der Stadt Frankfurt am Main (Ordnungsamt)