What are elegant ways of in-order splicing things (vectors and dataframes) in R?

Question

Suppose I have three vectors a.1, a.2, a.3 -- they could hold arbitrary values (meaning the values would not necessarily be sorted)

> a.1 <- seq(1,15,3)
> a.2 <- seq(15,2,-3)
> a.3 <- seq(2,15,3)
> a.1
[1]  1  4  7 10 13
> a.2
[1] 15 12  9  6  3
> a.3
[1]  2  5  8 11 14

Now I want to produce a new vector C where I take all elements of a except elements at indices 2, 3, 5 and only elements at indices 2, 3 from vector a.2 and element at index 5 from a.3; combine them in order

One way I know how to do this is:

> C <- a.1
> C[c(2,3)] <- a.2[c(2,3)]
> C[5] <- a.3[5]
> C
[1]  1 12  9 10 14

I wonder if there are are better ways of doing this -- better may include:

More elegant
Extendable to say splices from multiple elements
More Terse (syntax wise)

Notes

One of the answer authors has suggested using replace -- I wonder if replace can be used in multiway splices
Also note the vales in the source or target vector need not be sorted
It would be great if the same solution can be used (barring some minor modifications) on both vectors and dataframes

@Psidom thanks ... ... good .... do you think we could extend this to multi-way splices — user1172468, May 30 '17 at 21:46
@user1172468 - but what do you mean by multiway splices? Joining three vectors? How do you see that working? — thelatemail, May 30 '17 at 22:04
@thelatemail -- just updated the question -- please take a look and see if it makes sense -- many thanks — user1172468, May 30 '17 at 22:13
@AidanGawronski -- they did -- however those were known to me -- I'm looking for something more elegant. — user1172468, Jun 01 '17 at 01:29
@user1172468 if they were known to you why did you use an inferior version in the first place ... not only that both answers are 2) Extendable & 3) More Terse — AidanGawronski, Jun 01 '17 at 02:34
@AidanGawronski -- those are answers I could have constructed via mapply or say a loop -- the replace was interesting (had forgotten that) — user1172468, Jun 01 '17 at 02:36

thelatemail · Answer 1 · 2017-05-31T01:17:08.033

In this case, I'd just use a boring old loop and overwrite the corresponding indexes from each object:

multchange <- function(x, repl, idx) {
  for (i in seq_along(repl)) {
    x[idx[[i]]] <- repl[[i]][idx[[i]]]
  }
  x
}

df_list <- list(a.2, a.3)
cool_list <- list(c(2,3), 5)
multchange(a.1, df_list, cool_list)
#[1]  1 12  9 10 14

And no, using a loop is not slower on a large dataset:

samp <- rep(1:2,each=1000000)
df_list <- list(a.2, a.3)[samp]
cool_list <- list(c(2,3), 5)[samp]

system.time({
  multchange(a.1, df_list, cool_list)
})
#   user  system elapsed 
#   4.24    0.00    4.25 

c <- a.1
system.time({
  mapply(function(X, Y) { c[Y] <<- X[Y] }, X=df_list, Y=cool_list)
})
#   user  system elapsed 
#   7.46    0.00    7.49

AidanGawronski · Answer 2 · 2017-05-30T23:59:07.350

2

df_list <- list(a.2, a.3)
cool_list <- list(c(2,3), 5)
c <- a.1
mapply(function(X, Y) { c[Y] <<- X[Y] }, X=df_list, Y=cool_list)

edited May 30 '17 at 23:59

answered May 30 '17 at 21:45

AidanGawronski

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Thanks for the answer -- but this assumes the vectors are sorted -- let me clarify the question – user1172468 May 30 '17 at 21:47
Hi, clarified the question a bit -- see if it makes sense – user1172468 May 30 '17 at 22:13
@user1172468 Your example is broken, and it is very unclear what your expected output is. – AidanGawronski May 30 '17 at 23:16
The above answer will extend to any number of vectors and "splices". – AidanGawronski May 31 '17 at 00:00
I'd avoid using `< – thelatemail May 31 '17 at 00:39
Everyone is so afraid of using the super assignment operator ... I like it! – AidanGawronski May 31 '17 at 00:40
@AidanGawronski - it's not about being afraid, it's that it can give unintended results if the function is called inside a different function. E.g. - https://stackoverflow.com/questions/9851655/why-is-using-frowned-upon-and-how-can-i-avoid-it/9851794 and https://stackoverflow.com/questions/14166207/avoiding-global-variables/14166812 for example. – thelatemail May 31 '17 at 00:42
Still doesn't seem like an unexpected result. Basically the fear is people use it without understanding it. – AidanGawronski May 31 '17 at 00:55
I didn't downvote your answer by the way if you're wondering. – thelatemail May 31 '17 at 01:24

What are elegant ways of in-order splicing things (vectors and dataframes) in R?

2 Answers2