R + combine a list of vectors into a single vector

Question

I have a single list of numeric vector and I want to combine them into one vector. But I am unable to do that. This list can have one element common across the list element. Final vector should not add them twice. Here is an example:

>lst
`1`
[1] 1 2
`2`
[2] 2 4 5
`3`
[3] 5 9 1

I want final result as this

>result
[1] 1 2 4 5 9 1

I tried doing following things, without worrying about the repition:

>vec<-vector()
>sapply(lst, append,vec)

and

>vec<-vector()
>sapply(lst, c, vec)

None of them worked. Can someone help me on this?

Thanks.

Answer 1

A solution that is faster than the one proposed above:

vec<-unlist(lst)
vec[which(c(1,diff(vec)) != 0)]

Answer 2

Another answer using Reduce().

Create the list of vectors:

lst <- list(c(1,2),c(2,4,5),c(5,9,1))

Combine them into one vector

vec <- Reduce(c,lst)
vec
# [1] 1 2 2 4 5 5 9 1

Keep the repeated ones only once:

unique(Reduce(c,lst))
#[1] 1 2 4 5 9

If you want to keep that repeated one at the end, You might want to use vec[which(c(1,diff(vec)) != 0)] as in @Rachid's answer

Answer 3

You want rle:

rle(unlist(lst))$values

> lst <- list(`1`=1:2, `2`=c(2,4,5), `3`=c(5,9,1))
> rle(unlist(lst))$values
## 11 21 22 31 32 33 
##  1  2  4  5  9  1 

Answer 4

stack will do this nicely too, and looks more concise:

stack(lst)$values

Answer 5

Doing it the tidyverse way:

library(tidyverse)
lst %>% reduce(c) %>% unique

This uses the (uncapitalized) reduce version from purrr in combination with pipes. Also note that if the list contains named vectors, the final naming will be different depending on whether unlist or reduce methods are used.

Answer 6

Benchmarking the two answers by Rachit and Martijn

rbenchmark::benchmark(
  "unlist" = {
    vec<-unlist(a)
    vec[which(diff(vec) != 0)]
  },
  "reduce" = {
    a %>% reduce(c) %>% unique
  }
)

Output:

    test replications elapsed relative user.self sys.self user.child sys.child
2 reduce          100   0.036        3     0.036    0.000          0         0
1 unlist          100   0.012        1     0.000    0.004          0         0

This one clearly beat the other one.