Zip or enumerate in R?

Answer for python enumerate:

In R, a list is ordered (see this answer). Thus, all you need is to index either keys (using names()[i]) or values (using [[i]]).

Using seq_along (alternatively can do for(i in 1:length(mylist)){...}):

> mylist <- list('a'=10,'b'=20,'c'=30)
> for (i in seq_along(mylist)){
+   print(paste(i,names(mylist)[i],mylist[[i]]))
+ }
[1] "1 a 10"
[1] "2 b 20"
[1] "3 c 30"

Answer for python zip:

See one of the above answers to mimic the list of tuples. My preference is towards a data frame as shown in BondedDust's answer:

> x <- 1:3
> y <- 4:6
> data.frame(x=x, y=y)
  x y
1 1 4
2 2 5
3 3 6

There have been some discussions around list comprehension for R, e.g. here or there. The hash package even offers dictionary-like structure. However, as others said, it is difficult to try to map one language facilities onto another (even if this is what Comparison of programming languages actually offers) without a clear understanding of what it is supposed to be used to. For example, I can mimic Python zip() in R as follows:

Python

In [1]: x = [1,2,3]
In [2]: y = [4,5,6]
In [3]: zip(x, y)
Out[3]: [(1, 4), (2, 5), (3, 6)]

> x <- 1:3
> y <- 4:6
> list(x, y)                     # gives a simple list
> as.list(paste(x, y))           # three tuples, as a list of characters
> mapply(list, x, y, SIMPLIFY=F) # gives a list of 3 tuples
> rbind(x, y)                    # gives a 2x3 matrix

As can be seen, this really depends on what you want to do with the result afterwards.

zip and enumerate are not particularly difficult to implement in R:

#' zip(1:5,1:10)
zip <- function(...) {
  mapply(list, ..., SIMPLIFY = FALSE)
}

Enumerate is simple to define in terms of zip:

#' enumerate(l=LETTERS)
enumerate <- function(...) {
  zip(ix=seq_along(..1), ...)
}

Since these are proper functions, we can use ... to make them fairly flexible and terse, and take advantage of mapply's behavior, such as recycling inputs and naming output correctly.

Zip or enumerate in R?

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