[R] indexing in data frames

[David L Carlson]

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