R help - Aug 2012 - What is this called? lapply(datum,"[[","ColumnName")

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.

 

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]]

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
>
>
>
> 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 at 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.

"I... cannot look it up in a book."
Baloney!

Read the R Language Definition manual section on indexing in the
"Evaluation of Expressions" Chapter. Then ?lapply and ?"[["
-- and the
relevant sections in "The Introduction to R" manual.

However, I will admit that it's tricky. The point to grasp --
described at various levels of detail in the above manuals and in
numerous other resources that you can check out on the CRAN website --
is that y[["a"]] is just syntactic sugar for
"[["(y,"a")  . R is a
"functional" language (mostly). So your lapply statement is just
lapplying the "[[" function(extraction operator ) to the list of data
frames.

Cheers,
Bert

On Tue, Aug 7, 2012 at 12:06 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.
>
>
>
> 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 at 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.


-- 

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm