finding unique vector elements in a list efficiently

Question

I have a list of numerical vectors, and I need to create a list containing only one copy of each vector. There isn\'t a list method for the identical function, so I wrote a func

Answer 1

You could hash each of the vectors and then use !duplicated() to identify unique elements of the resultant character vector:

library(digest)  

## Some example data
x <- 1:44
y <- 2:10
z <- rnorm(10)
ll <- list(x,y,x,x,x,z,y)

ll[!duplicated(sapply(ll, digest))]
# [[1]]
#  [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
# [26] 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
# 
# [[2]]
# [1]  2  3  4  5  6  7  8  9 10
# 
# [[3]]
#  [1]  1.24573610 -0.48894189 -0.18799758 -1.30696395 -0.05052373  0.94088670
#  [7] -0.20254574 -1.08275938 -0.32937153  0.49454570

To see at a glance why this works, here's what the hashes look like:

sapply(ll, digest)
[1] "efe1bc7b6eca82ad78ac732d6f1507e7" "fd61b0fff79f76586ad840c9c0f497d1"
[3] "efe1bc7b6eca82ad78ac732d6f1507e7" "efe1bc7b6eca82ad78ac732d6f1507e7"
[5] "efe1bc7b6eca82ad78ac732d6f1507e7" "592e2e533582b2bbaf0bb460e558d0a5"
[7] "fd61b0fff79f76586ad840c9c0f497d1"