Append value to empty vector in R?

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Appending to an object in a for loop causes the entire object to be copied on every iteration, which causes a lot of people to say “R is slow”, or “R loops should be avoided”.

As BrodieG mentioned in the comments: it is much better to pre-allocate a vector of the desired length, then set the element values in the loop.

Here are several ways to append values to a vector. All of them are discouraged.

Appending to a vector in a loop

# one way
for (i in 1:length(values))
  vector[i] <- values[i]
# another way
for (i in 1:length(values))
  vector <- c(vector, values[i])
# yet another way?!?
for (v in values)
  vector <- c(vector, v)
# ... more ways

help("append") would have answered your question and saved the time it took you to write this question (but would have caused you to develop bad habits). 😉

Note that vector <- c() isn’t an empty vector; it’s NULL. If you want an empty character vector, use vector <- character().

Pre-allocate the vector before looping

If you absolutely must use a for loop, you should pre-allocate the entire vector before the loop. This will be much faster than appending for larger vectors.

set.seed(21)
values <- sample(letters, 1e4, TRUE)
vector <- character(0)
# slow
system.time( for (i in 1:length(values)) vector[i] <- values[i] )
#   user  system elapsed 
#  0.340   0.000   0.343 
vector <- character(length(values))
# fast(er)
system.time( for (i in 1:length(values)) vector[i] <- values[i] )
#   user  system elapsed 
#  0.024   0.000   0.023

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