[R] indexing in data frames

[David Winsemius]

On Aug 9, 2012, at 2:43 PM, David L Carlson wrote:

> You have not defined a data frame since data frames cannot contain  
> lists,

Not true:

 > dput(a)
structure(list(b = c(1988, 1989),
                c = list(c(1985, 1982, 1984),
                         c(1988, 1980))), .Names = c("b", "c"))

 > ab <- data.frame(a$b)
 > ab
    a.b
1 1988
2 1989
 > ab$cb <- a$c
 > ab
    a.b               cb
1 1988 1985, 1982, 1984
2 1989       1988, 1980
 > str(ab)
'data.frame':	2 obs. of  2 variables:
  $ a.b: num  1988 1989
  $ cb :List of 2
   ..$ : num  1985 1982 1984
   ..$ : num  1988 1980

But it seems unlikely that the OP's "a" object is a dataframe since  
the console eval-print loop would not display a dataframe in that  
manner.

At any rate with the ab dataframe:

 > for( i in 1:NROW(ab) ) print(  ab$a.b[i] - ab$cb[[i]] )
[1] 3 6 4
[1] 1 9

The OP should note the need to use '[[' on a list-object to get  
commensurate classes to pass to the '-' operator.

-- 
david.


> but lists can contain data frames so you are asking about how to  
> process a
> list. I'm changing your object names to a, b, and d because c() is the
> concatenation function and it can cause all kinds of problems to use  
> it as
> an object name.
>
>> a <- list(b=c(1988, 1989), d=list(c(1985, 1982, 1984), c(1988,  
>> 1980)))
>> a
> $b
> [1] 1988 1989
>
> $d
> $d[[1]]
> [1] 1985 1982 1984
>
> $d[[2]]
> [1] 1988 1980
>
>> a$b; a[[1]] # Two ways to refer to the first element of the list
> [1] 1988 1989
> [1] 1988 1989
>
>> a$d; a[[2]] # Two ways to refer to the second element of the list
> [[1]]
> [1] 1985 1982 1984
>
> [[2]]
> [1] 1988 1980
>
> [[1]]
> [1] 1985 1982 1984
>
> [[2]]
> [1] 1988 1980
>
>> a[[2]][[1]]; a$d[[1]] # Two ways to refer to the 1st element of the  
>> 2nd
> element
> [1] 1985 1982 1984
> [1] 1985 1982 1984
>
>> a[[2]][[2]]; a$d[[2]] # Two ways to refer to the 2nd element of the  
>> 2nd
> element
> [1] 1988 1980
> [1] 1988 1980
>
>> a$new <- sapply(1:2, function(i) a$b[i] - a$d[[i]])
>> a$new
> [[1]]
> [1] 3 6 4
>
> [[2]]
> [1] 1 9
>
> You can do all this with a data.frame if you think about it  
> differently:
>
>> a <- data.frame(year = c(1988, 1989), group = c("G1988", "G1989"))
>> b <- data.frame(group = c(rep("G1988", 3), rep("G1989", 2)),
>    d = c(1985, 1982, 1984, 1988, 1980))
>> ab <- merge(a, b)
>> ab <- data.frame(ab, diff=ab$year-ab$d)
>> new <- split(ab$diff, ab$group)
>> new
> $G1988
> [1] 3 6 4
>
> $G1989
> [1] 1 9
>
> ----------------------------------------------
> David L Carlson
> Associate Professor of Anthropology
> Texas A&M University
> College Station, TX 77843-4352
>
>> -----Original Message-----
>> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
>> project.org] On Behalf Of jimi adams
>> Sent: Thursday, August 09, 2012 3:43 PM
>> To: r-help at r-project.org
>> Subject: [R] indexing in data frames
>>
>> I'm still not fully understanding exactly how R is handling data
>> frames, but am getting closer. Any help with this one will likely  
>> go a
>> long way in getting me there. Let's say I have a data frame, let's  
>> call
>> it "a". Within that data frame i have two variables, let's call them
>> "b" and "c", where "b" is a single numeric value per observation,  
>> while
>> "c" is a LIST of numeric values. What I want to be able to do is
>> perform an operation on each element in "c" by the single element in
>> "b".
>>
>> So, for example, if I wanted to subtract each element in "c" from the
>> scalar in "b". For example, if i had
>>
>>> a$b
>> [1] 1988
>> [2] 1989
>> .
>> &
>>> a$c
>> [[1]]
>> [1] 1985 1982 1984
>> [[2]]
>> [1] 1988 1980
>> .
>>
>> I'm looking for a result of:
>> a$new
>> [[1]]
>> [1] 3 6 4
>> [[2]]
>> [1] 1 9
>> .
>>
>> I've tried a few different things, none of which have the desired
>> result. Any help appreciated.
>> thanks!
>>
>> jimi adams
>> Assistant Professor
>> Department of Sociology
>> American University
>> e: jadams at american.edu
>> w: jimiadams.com
>>
>> ______________________________________________
>> 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.

David Winsemius, MD
Alameda, CA, USA