[R] Using the value of a variable as a variable

[Henrik Bengtsson]

On Jan 30, 2008 1:15 PM, Ted Harding <Ted.Harding at manchester.ac.uk> wrote:
> On 30-Jan-08 19:47:55, Ramon Hidalgo wrote:
> > Hello,
> >
> > How can I make the following expressions are equivalent
> > datos$Col1 and datos$var when I define var <- "Col1"?
> >
> > I am trying to get the same result with
> >> datos$Col1
> >  [1] 0 1 1 0 1 0 1 1 0
> >
> > And
> >> datos$var
> > NULL

This one must the most frequently asked question among all FAQs.

>
> It will work if you use [], though I'm not sure that
> this is the best or orthodox way to do it:
>
>   datos<-data.frame(Col1=c(0,1,1,0,1,0,1,1,0),Col2=c(1,2,2,1,2,1,2,2,1))
>   var<-"Col1"
>   datos[var]
> ##  Col1
> ##1    0
> ##2    1
> ##3    1
> ##4    0
> ##5    1
> ##6    0
> ##7    1
> ##8    1
> ##9    0

In order to get a similar return value as operator $() you want to use
[[(), i.e.

datos[[var]]

or alternatively

datos[,var]

In the first case you treat the data frame as if it is a list
structure, and in the second case you treat it as if it is a matrix,
which will drop the dimensions if you ask for a single column, cf.

datos[,var]
datos[,var, drop=TRUE]
datos[,var, drop=FALSE]

/Henrik

>
> Hoping this helps,
> Ted.
>
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> Date: 30-Jan-08                                       Time: 21:15:07
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