How to improve this hash function

Question

Is there anyway to improve the speed of the initalization of this hash? Currently this takes around 20 minutes on my machine.

#prepare hash()
hash <- list();

mappedV <- # matrix with more than 200,000 elements
for( i in 1:nrow(mappedV) ) {
  hash[[paste(mappedV[i,], collapse = '.')]] <- 0;
}

Before this piece of code, I used a matrix, but this took me more than 3 hours. So I wont complain about the 20 minutes. I am just curious if there are better alternatives. I use the hash function to count each of the 200,000 possible combination.

PS: To concurrency is maybe one option. But this doesn't improve the hashing.

Answer 1

You'll often save significant time by pre-allocating a list of the desired length, rather than growing it at each iteration.

Behold:

X <- vector(mode="list", 1e5)
Y <- list()

system.time(for(i in 1:1e5) X[[i]] <- 0)
#    user  system elapsed 
#     0.3     0.0     0.3 
system.time(for(i in 1:1e5) Y[[i]] <- 0)
#    user  system elapsed 
#   48.84    0.05   49.34 
identical(X,Y)
# [1] TRUE

Because the entire list Y gets copied each time it's added to, appending additional elements only gets slower and slower as it grows in size.