Append Value to Empty Vector in R

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 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 

Cannot create an empty vector and append new elements in R

append does something that is somewhat different from what you are thinking. See ?append.

In particular, note that append does not modify its argument. It returns the result.

You want the function c:

> a <- numeric()
> a <- c(a, 1)
> a
[1] 1

Do something then append that value do an empty vector

There are a few ways of doing this. If you want to use a loop, the trick is to define an empty vector before initiating the loop, and store values in that. Here's an example using rnorm to generate values from a standard normal, since I don't have the code for your rnormout function:

M <- 1000
means <- vector()
for (i in 1:M){
  means[i] = mean(rnorm(100, 0, 1))
}

Alternatively, it's better not to use a for loop at all. You can use map_dbl to replace the loop in one line:

library(tidyverse)
M <- 1000
means <- map_dbl(1:M, ~ mean(rnorm(100, 0, 1)))

How to append an empty array in R

We need to assign (<-) to 'values' to update the original object

for (n1 in 1:50) {

    n2 = n1+1
    temp_val = (n1+n2)
    values <-  append(values,temp_val)
  }

-output

values
#[1]   3   5   7   9  11  13  15  17  19  21  23  25  27  29  31  33  35  37  39  41  43  45  47  49  51  53  55  57  59  61  63  65  67
#[34]  69  71  73  75  77  79  81  83  85  87  89  91  93  95  97  99 101

How do I append to a vector using a `while` loop?

Try this:

vec_teamCodes <- c()
x <- 0
while (x < 10) {
  vec_teamCodes <- c(vec_teamCodes,"Hello")
  # OR
  # vec_teamCodes <- append(vec_teamCodes,"Hello")
  x <- x+1
}


[1] "Hello" "Hello" "Hello" "Hello" "Hello" "Hello" "Hello" "Hello" "Hello" "Hello"

How do I create an empty vector in r?

There are several ways to create empty vector in r. For example,

v2<-NULL

or

v2<-c()

Try this:

v2 <- NULL

for (i in 1:length(df)){ 
if (df$Texture[i] == 'Silt Loam'){ 
  new_val <- df$`pH 1:1`[i]}
v2[i] <- new_val
}

how to append an element to a list without keeping track of the index?

There is a function called append:

ans <- list()
for (i in 1992:1994){
n <- 1 #whatever the function is
ans <- append(ans, n)
}

  ans
## [[1]]
## [1] 1
## 
## [[2]]
## [1] 1
## 
## [[3]]
## [1] 1
## 

Note: Using apply functions instead of a for loop is better (not necessarily faster) but it depends on the actual purpose of your loop.

Answering OP's comment: About using ggplot2 and saving plots to a list, something like this would be more efficient:

plotlist <- lapply(seq(2,4), function(i) {
            require(ggplot2)
            dat <- mtcars[mtcars$cyl == 2 * i,]
            ggplot() + geom_point(data = dat ,aes(x=cyl,y=mpg))
})

Thanks to @Wen for sharing Comparison of c() and append() functions:

Concatenation (c) is pretty fast, but append is even faster and therefor preferable when concatenating just two vectors.

Appending a vector in R

You need to use <<- instead of <-.

The operator <<- is normally only used in functions, and cause a search to be made through parent environments for an existing definition of the variable being assigned.

fun <- function() {
  s <- sample(a, 1)
  b <<- c(b,s)
  print(paste("Sample:", s)) 
  print(paste("Selected so far:",b))
}
fun()
#[1] "Sample: 7"
#[1] "Selected so far: 7"
fun()
#[1] "Sample: 3"
#[1] "Selected so far: 7" "Selected so far: 3"

Append value to vector if condition met in other column

You can do :

#Remove last 2 rows
tmp <- head(df, -2)
#Get the corresponding value of HOD where TEST = 1
v <- as.numeric(tmp$HOD[tmp$TEST == 1])
v
#[1] 1.30 8.00 1.36

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