Stéphane Cruveiller wrote:
> Dear R users,
>
> I have a simple question on variable manipulation.
> Imagine I have an object "OBJ" that has "toto" as one of its variables.
> I would like to understand why if I do
>
> > varname <- "toto"
>
> >OBJ$varname returns no results
>
> whereas
>
> > OBJ[varname] returns the column entitled
> "toto"
>
>
> Thanks for your help.
>
> Stéphane.
>
because if the value of `varname' is substituted in the expressions, in
the first case that yields
OBJ$"toto" and in the second
OBJ["toto"]
the latter is valid, the former is not (you'd need `OBJ$toto' there),
read ` ?"$" ':
"...Both '[[' and '$' select a single element of the list. The main
difference is that '$' does not allow computed indices, whereas
'[[' does. 'x$name' is equivalent to 'x[["name"]]'..."
not, too, the difference between `[' (sublist) and `[[' (single element
extraction)
joerg
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