Append value to empty vector in R? - r

Question

I'm trying to learn R and I can't figure out how to append to a list.
If this were Python I would . . .
#Python
vector = []
values = ['a','b','c','d','e','f','g']
for i in range(0,len(values)):
vector.append(values[i])
How do you do this in R?
#R Programming
> vector = c()
> values = c('a','b','c','d','e','f','g')
> for (i in 1:length(values))
+ #append value[i] to empty vector

Answer 1

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

Answer 2

FWIW: analogous to python's append():
b <- 1
b <- c(b, 2)

Answer 3

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)

Answer 4

Just for the sake of completeness, appending values to a vector in a for loop is not really the philosophy in R. R works better by operating on vectors as a whole, as #BrodieG pointed out. See if your code can't be rewritten as:
ouput <- sapply(values, function(v) return(2*v))
Output will be a vector of return values. You can also use lapply if values is a list instead of a vector.

Answer 5

Sometimes we have to use loops, for example, when we don't know how many iterations we need to get the result. Take while loops as an example. Below are methods you absolutely should avoid:
a=numeric(0)
b=1
system.time(
{
while(b<=1e5){
b=b+1
a<-c(a,pi)
}
}
)
# user system elapsed
# 13.2 0.0 13.2
a=numeric(0)
b=1
system.time(
{
while(b<=1e5){
b=b+1
a<-append(a,pi)
}
}
)
# user system elapsed
# 11.06 5.72 16.84
These are very inefficient because R copies the vector every time it appends.
The most efficient way to append is to use index. Note that this time I let it iterate 1e7 times, but it's still much faster than c.
a=numeric(0)
system.time(
{
while(length(a)<1e7){
a[length(a)+1]=pi
}
}
)
# user system elapsed
# 5.71 0.39 6.12
This is acceptable. And we can make it a bit faster by replacing [ with [[.
a=numeric(0)
system.time(
{
while(length(a)<1e7){
a[[length(a)+1]]=pi
}
}
)
# user system elapsed
# 5.29 0.38 5.69
Maybe you already noticed that length can be time consuming. If we replace length with a counter:
a=numeric(0)
b=1
system.time(
{
while(b<=1e7){
a[[b]]=pi
b=b+1
}
}
)
# user system elapsed
# 3.35 0.41 3.76
As other users mentioned, pre-allocating the vector is very helpful. But this is a trade-off between speed and memory usage if you don't know how many loops you need to get the result.
a=rep(NaN,2*1e7)
b=1
system.time(
{
while(b<=1e7){
a[[b]]=pi
b=b+1
}
a=a[!is.na(a)]
}
)
# user system elapsed
# 1.57 0.06 1.63
An intermediate method is to gradually add blocks of results.
a=numeric(0)
b=0
step_count=0
step=1e6
system.time(
{
repeat{
a_step=rep(NaN,step)
for(i in seq_len(step)){
b=b+1
a_step[[i]]=pi
if(b>=1e7){
a_step=a_step[1:i]
break
}
}
a[(step_count*step+1):b]=a_step
if(b>=1e7) break
step_count=step_count+1
}
}
)
#user system elapsed
#1.71 0.17 1.89

Answer 6

In R, you can try out this way:
X = NULL
X
# NULL
values = letters[1:10]
values
# [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"
X = append(X,values)
X
# [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"
X = append(X,letters[23:26])
X
# [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "w" "x" "y" "z"

Answer 7

> vec <- c(letters[1:3]) # vec <- c("a","b","c") ; or just empty vector: vec <- c()
> values<- c(1,2,3)
> for (i in 1:length(values)){
print(paste("length of vec", length(vec)));
vec[length(vec)+1] <- values[i] #Appends value at the end of vector
}
[1] "length of vec 3"
[1] "length of vec 4"
[1] "length of vec 5"
> vec
[1] "a" "b" "c" "1" "2" "3"

Answer 8

What you're using in the python code is called a list in python, and it's tottaly different from R vectors, if i get what you wanna do:
# you can do like this if you'll put them manually
v <- c("a", "b", "c")
# if your values are in a list
v <- as.vector(your_list)
# if you just need to append
v <- append(v, value, after=length(v))

Answer 9

in R you create a "list" doing this:
v <- numeric() (is a numeric vector, or int in Python)
v <- character() (is a character vector, or str in Python)
then, if you want yo append a single value you have to do this:
v[1] <- 10 (append to vector "v", in position "1" a value 10)
v[2] <- 11 (append to vector "v", in position "2" a value 11)
So, if yoy want yo append multiple values in a for loop, try this:
v <- numeric()
for (value in 1:10) {
v[value] <- value
}
v
[1] 1 2 3 4 5 6 7 8 9 10