[R] What is this called? lapply(datum,"[[","ColumnName")

[R. Michael Weylandt]

On Tue, Aug 7, 2012 at 2:06 PM, Kevin Chang <kchang01 at uoguelph.ca> wrote:
> Hello R users
>
>
>
> I recently learned how to use this command:
>
>
> lapply(datum,"[[","ColumnName")
>
>
>
> Unfortunately, I don't know how exactly it works, what it's called (in
> particular the "[[" part], and what other things you can do with it
> (retrieve multiple columns?).
>
>
> Given datum is a list of dataframes with the same column, but different
> number of rows,
>
>
>
> lapply(datum,"[[","ColumnName"),
>
>
>
> Returns you a list of a particular column form each of your dataframes from
> datum.
>
>
>
> I think this is a really useful command, and would like to learn more about
> it.
> Unfortunately I don't know what subject or topic it is under and cannot look
> it up in a book.
>
>

To see the help page, type

? `[[`

at your R prompt.

Long story short, when you use syntax like x[3], that's just sugar for

`[`(x, 3)

which tells R to apply the `[` function with arguments "x" and "3". So
when you pass syntax like that to lapply(), it applies the function

`[`( ??, colNames)

 to each element of your list datum in turn putting them in the slot I
marked with double question marks.

It's not so different than something like

lapply(datum, `+`, 3)

to add 3 to each element of datum. This could alternatively be written as

lapply(datum, function(x) x + 3)

and your example could just as easily (and I think more clearly) be written as

lapply(datum, function(x) x[[colName]])


Best,
Michael

>
> Thank you,
>
>
>
> Kevin
>
>
>
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