[R] Accessing a field in a data fram

[Marc Schwartz]

on 07/02/2008 02:10 PM R_Learner wrote:
> raw <- read.csv(file=filename, head=TRUE,sep=",") 
> 
> I've read in a csv file, and I'm looking to access a column whose name is
> held in a string.
> 
> For example, I want to access raw$one or raw$two, but this will depending on
> the string. Let's say that the string is decision<-"one". How would I access
> raw$one?
> 
> The following doesn't work:
> numbers<-raw$decision
> 
> Please help me!
> Thanks!


 > str(iris)
'data.frame':	150 obs. of  5 variables:
  $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
  $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
  $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
  $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
  $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 
1 1 1 1 1 ...


MyCol <- "Petal.Length"


# Use column indexing
 > iris[, MyCol]
   [1] 1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 1.5 1.6 1.4 1.1 1.2 1.5
  [17] 1.3 1.4 1.7 1.5 1.7 1.5 1.0 1.7 1.9 1.6 1.6 1.5 1.4 1.6 1.6 1.5
...

# Use list indexing. A data frame is a list...
 > iris[[MyCol]]
   [1] 1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 1.5 1.6 1.4 1.1 1.2 1.5
  [17] 1.3 1.4 1.7 1.5 1.7 1.5 1.0 1.7 1.9 1.6 1.6 1.5 1.4 1.6 1.6 1.5


# Use subset()
 > subset(iris, select = MyCol)
     Petal.Length
1            1.4
2            1.4
3            1.3
4            1.5
5            1.4
...


See ?"[.data.frame" and ?subset


HTH,

Marc Schwartz