Question

If you have a dataframe like this

mydf <- data.frame(firstcol = c(1,2,1), secondcol = c(3,4,5))

Why would

mydf[mydf$firstcol,]

work but

mydf[firstcol,]

wouldn’t?

Answer 1

You can do this:

mydf[,"firstcol"]

Remember that the column goes second, not first.

In your example, to see what mydf[mydf$firstcol,] gives you, let’s break it down:

> mydf$firstcol
[1] 1 2 1

So really mydf[mydf$firstcol,] is the same as

> mydf[c(1,2,1),]
    firstcol secondcol
1          1         3
2          2         4
1.1        1         3

So you are asking for rows 1, 2, and 1. That is, you are asking for your row one to be the same as row 1 of mydf, your row 2 to be the same as row 2 of mydf and your row 3 to be the same as row 1 of mydf; and you are asking for both columns.

Another question is why the following doesn’t work:

> mydf[,firstcol]
Error in `[.data.frame`(mydf, , firstcol) : object 'firstcol' not found

That is, why do you have to put quotes around the column name when you ask for it like that but not when you do mydf$firstcol. The answer is just that the operators you are using require different types of arguments. You can look at '$' to see the form x$name and thus the second argument can be a name, which is not quoted. You can then look up ?'[', which will actually lead you to the same help page. And there you will find the following, which explains it. Note that a “character” vector needs to have quoted entries (that is how you enter a character vector in R (and many other languages).

i, j, ...: indices specifying elements to extract or replace.  Indices
      are ‘numeric’ or ‘character’ vectors or empty (missing) or
      ‘NULL’.  Numeric values are coerced to integer as by
      ‘as.integer’ (and hence truncated towards zero).  Character
      vectors will be matched to the ‘names’ of the object (or for
      matrices/arrays, the ‘dimnames’): see ‘Character indices’
      below for further details.