R help - Mar 2005 - From FAQ 7.21 to a command like apply(sapply(list(f1,f2,f3),is.na),2,sum)

Dear all,

Last December there was a thread regarding the famous FAQ 7.21 "How can I
turn a string into a variable?" and asking what people want to do with
these strings.
My, certainly trivial application would be as follows:
Assume I have a data.frame containing besides others also the columns f1,
f2, ..., fn and I want to create a command like:
apply(sapply(list(f1,f2,f3),is.na),2,sum)
or
summary(cbind(f1,f2,f3))

Can I start from paste('f',1:3,sep='') to arrive at the
abovementioned
command?
I tried get, parse, as.name, eval in diverse combinations but did not reach
a solution.
More generally my question is, how can I produce a "list" of variables
like
x1 to xn in a convenient way within a command.
I am quite sure that this has been answered several times, but I did not
find one of these answers. So I welcome any hint, where to look.

Heinz T?chler

On Wed, 30 Mar 2005, Heinz Tuechler wrote:
> Dear all,
>
> Last December there was a thread regarding the famous FAQ 7.21 "How
can I
> turn a string into a variable?" and asking what people want to do with
> these strings.
> My, certainly trivial application would be as follows:
> Assume I have a data.frame containing besides others also the columns f1,
> f2, ..., fn and I want to create a command like:
> apply(sapply(list(f1,f2,f3),is.na),2,sum)
> or
> summary(cbind(f1,f2,f3))
>
> Can I start from paste('f',1:3,sep='') to arrive at the
abovementioned
> command?
No parse,as.name or other complications needed. It's all just indexing. 
Suppose your data frame is called dd

fs<-paste('f',1:3,sep='')
apply(sapply(dd[,fs],is.na),2,sum)
summary(dd[,fs])

 	-thomas

At 16:13 29.03.2005 -0800, Thomas Lumley wrote:
>On Wed, 30 Mar 2005, Heinz Tuechler wrote:
>
>> Dear all,
>>
>> Last December there was a thread regarding the famous FAQ 7.21
"How can I
>> turn a string into a variable?" and asking what people want to do
with
>> these strings.
>> My, certainly trivial application would be as follows:
>> Assume I have a data.frame containing besides others also the columns
f1,
>> f2, ..., fn and I want to create a command like:
>> apply(sapply(list(f1,f2,f3),is.na),2,sum)
>> or
>> summary(cbind(f1,f2,f3))
>>
>> Can I start from paste('f',1:3,sep='') to arrive at the
abovementioned
>> command?
>
>No parse,as.name or other complications needed. It's all just indexing. 
>Suppose your data frame is called dd
>
>fs<-paste('f',1:3,sep='')
>apply(sapply(dd[,fs],is.na),2,sum)
>summary(dd[,fs])
>
> 	-thomas
>
>
Thank you, Thomas, for your answer. I was curious if there was a simple way
to do this without referring to the data.frame, so that the resulting
command would correspond in its effect exactly to the abovementioned examples.
It's not urgent, but I will try further.

Many thanks

Heinz

At 09:32 30.03.2005 +0200, Heinz Tuechler wrote:
>At 16:13 29.03.2005 -0800, Thomas Lumley wrote:
>>On Wed, 30 Mar 2005, Heinz Tuechler wrote:
>>
>>> Dear all,
>>>
>>> Last December there was a thread regarding the famous FAQ 7.21
"How can I
>>> turn a string into a variable?" and asking what people want to
do with
>>> these strings.
>>> My, certainly trivial application would be as follows:
>>> Assume I have a data.frame containing besides others also the
columns f1,
>>> f2, ..., fn and I want to create a command like:
>>> apply(sapply(list(f1,f2,f3),is.na),2,sum)
>>> or
>>> summary(cbind(f1,f2,f3))
>>>
>>> Can I start from paste('f',1:3,sep='') to arrive at
the abovementioned
>>> command?
>>
>>No parse,as.name or other complications needed. It's all just
indexing.
>>Suppose your data frame is called dd
>>
>>fs<-paste('f',1:3,sep='')
>>apply(sapply(dd[,fs],is.na),2,sum)
>>summary(dd[,fs])
>>
>> 	-thomas
>>
>>
>Thank you, Thomas, for your answer. I was curious if there was a simple way
>to do this without referring to the data.frame, so that the resulting
>command would correspond in its effect exactly to the abovementioned
examples.
>It's not urgent, but I will try further.
>
>Many thanks
>
>Heinz
>
Continuation:
Maybe not an elegant solution, but it seems to work:
apply(sapply(eval(parse(text=paste('list(',paste('f',1:3,sep='',
collapse=','),')'))) ,is.na),2,sum)
What I missed in my earlier attempts was "collapse=','".

Heinz