Partial matching in atomic vectors

Question

Is there a way to achieve partial matching in atomic vectors? I know that the $ operator does not work on atomic vectors but I thought it would be possible by using [["i", exact = FALSE]].

x <- c("a1", "a2", "a3", "b1", "b2")
names(x) <- x

x$a

just returns "$ operator is invalid for atomic vectors"

x[["a", exact=FALSE]]

returns "Object "a" not found".

Is there a way to do partial matching with atomic vectors like this?

Cheers, Zuup

Answer 1

I don't know of something that does exactly what you want. The following is a bit kludgy but may work for you:

x[grep("^a", names(x))]
#   a1   a2   a3 
# "a1" "a2" "a3" 

Additionally, partial matching can be done, but it only works if there is only one index entry that matches your partial index. So for example:

y <- 1:5
names(y) <- paste(letters[1:5], 1:5)
y[["a", exact=F]]
# [1] 1
names(y)[[2]] <- "a 2"  # add a second index that starts with "a"
y[["a", exact=F]]
# Error in y[["a", exact = F]] : subscript out of bounds

Finally, note that you need to quote the character string inside [[, which you didn't do in your example.

Answer 2

Basically, through overloading of operators you can get any behaviour you want. With the following code the brackets are overloaded to behave the way you want them to behave:

# overload the brackets for reading out
"[.pmatch_vec" = function(obj,idx){
  origclass <- setdiff(class(obj),"pmatch_vec")
  if (length(origclass)==0) origclass <- ""
  class(obj) <- origclass
  if (!is.character(idx)) 
    return(obj[idx])
  else
    return(obj[grep(paste("^",idx,sep=""),names(obj))])
}

# overload the assignment operator []<- 
"[<-.pmatch_vec" = function(obj,idx,value){
  saveclass <- class(obj)
  origclass <- setdiff(class(obj),"pmatch_vec")
  if (length(origclass)==0) origclass <- ""
  class(obj) <- origclass
  if (!is.character(idx)) 
    obj[idx] <- value
  else
    obj[grep(paste("^",idx,sep=""),names(obj))] <- value
  class(obj) <- saveclass
  return(obj)
}

Since it is dangerous to overload the brackets in general, they were overloaded only for the defined class "pmatch_vec". Additionally note, that within these functions "pmatch_vec" is temporarily removed from the class attribute in order to avoid infinite recursion.

Here are some examples for the behaviour of atomic vectors defined to be of class "pmatch_vec":

# create some vector
A = 1:6
names(A) <- c(paste("a",1:3,sep=""),paste("b",1:3,sep=""))
# set the class
class(A) = c("pmatch_vec")

# some demonstraton
A["a"]
# a1 a2 a3
#  1  2  3

A["b"]
# b1 b2 b3
#  4  5  6

A["b"] <- 7
A
# a1 a2 a3 b1 b2 b3
#  1  2  3  7  7  7

If the vector used for indexing is not of type character, the class "pmatch_vec" behaves as if it is an ordinary atomic vector:

A[1:2] <- 8
A[1:4]
# a1 a2 a3 b1
#  8  8  3  7