On Tue, Aug 7, 2012 at 2:06 PM, Kevin Chang <kchan...@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 > > > > Master of Science Student > > Department of Food, Resource and Agricultural Economics > > University of Guelph > Office: 519-824-4120 ext. 58528 > > Mobile: 226-979-2813 > > <http://fare.uoguelph.ca/users/kchang01> > http://fare.uoguelph.ca/users/kchang01 > > > > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
