R self reference

Question

In R I find myself doing something like this a lot:

adataframe[adataframe$col==something]<-adataframe[adataframe$col==something)]+1

This way is kind of long and tedious. Is there some way for me
to reference the object I am trying to change such as

adataframe[adataframe$col==something]<-$self+1  

?

Answer 1

Try package data.table and its := operator. It's very fast and very short.

DT[col1==something, col2:=col3+1] 

The first part col1==something is the subset. You can put anything here and use the column names as if they are variables; i.e., no need to use $. Then the second part col2:=col3+1 assigns the RHS to the LHS within that subset, where the column names can be assigned to as if they are variables. := is assignment by reference. No copies of any object are taken, so is faster than <-, =, within and transform.

Also, soon to be implemented in v1.8.1, one end goal of j's syntax allowing := in j like that is combining it with by, see question: when should I use the := operator in data.table.

UDPDATE : That was indeed released (:= by group) in July 2012.

Answer 2

You should be paying more attention to Gabor Grothendeick (and not just in this instance.) The cited inc function on Matt Asher's blog does all of what you are asking:

(And the obvious extension works as well.)

add <- function(x, inc=1) {    eval.parent(substitute(x <- x + inc))  } # Testing the `inc` function behavior 

EDIT: After my temporary annoyance at the lack of approval in the first comment, I took the challenge of adding yet a further function argument. Supplied with one argument of a portion of a dataframe, it would still increment the range of values by one. Up to this point has only been very lightly tested on infix dyadic operators, but I see no reason it wouldn't work with any function which accepts only two arguments:

transfn <- function(x, func="+", inc=1) {    eval.parent(substitute(x <- do.call(func, list(x , inc)))) } 

(Guilty admission: This somehow "feels wrong" from the traditional R perspective of returning values for assignment.) The earlier testing on the inc function is below:

df <- data.frame(a1 =1:10, a2=21:30, b=1:2)  inc <- function(x) {    eval.parent(substitute(x <- x + 1))  }  #---- examples===============>  > inc(df$a1)  # works on whole columns > df    a1 a2 b 1   2 21 1 2   3 22 2 3   4 23 1 4   5 24 2 5   6 25 1 6   7 26 2 7   8 27 1 8   9 28 2 9  10 29 1 10 11 30 2 > inc(df$a1[df$a1>5]) # testing on a restricted range of one column > df    a1 a2 b 1   2 21 1 2   3 22 2 3   4 23 1 4   5 24 2 5   7 25 1 6   8 26 2 7   9 27 1 8  10 28 2 9  11 29 1 10 12 30 2  > inc(df[ df$a1>5, ])  #testing on a range of rows for all columns being transformed > df    a1 a2 b 1   2 21 1 2   3 22 2 3   4 23 1 4   5 24 2 5   8 26 2 6   9 27 3 7  10 28 2 8  11 29 3 9  12 30 2 10 13 31 3 # and even in selected rows and grepped names of columns meeting a criterion > inc(df[ df$a1 <= 3, grep("a", names(df)) ]) > df    a1 a2 b 1   3 22 1 2   4 23 2 3   4 23 1 4   5 24 2 5   8 26 2 6   9 27 3 7  10 28 2 8  11 29 3 9  12 30 2 10 13 31 3