Append value to empty vector in R?

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 wayfor (i in 1:length(values))  vector[i] <- values[i]# another wayfor (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)# slowsystem.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 

FWIW: analogous to python's append():

b <- 1b <- c(b, 2)

You have a few options:

• c(vector, values)

• append(vector, values)

• vector[(length(vector) + 1):(length(vector) + length(values))] <- values

The first one is the standard approach. The second one gives you the option to append someplace other than the end. The last one is a bit contorted but has the advantage of modifying vector (though really, you could just as easily do vector <- c(vector, values).

Notice that in R you don't need to cycle through vectors. You can just operate on them in whole.

Also, this is fairly basic stuff, so you should go through some of the references.

Some more options based on OP feedback:

for(i in values) vector <- c(vector, i)