What is rep() Function in R and How to Use It

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The rep() is a built-in R function that repeats elements of an object a specified number of times. The syntax is rep(x, times, each), where x is the object to be repeated, times is the number of times to repeat the object, and each is an optional argument that specifies the number of times to repeat each element of x (by default, each is set to 1, which means that each element of x is repeated once).

The rep() function in R has two faster-simplified versions.

  1. rep.int()
  2. rep_len()

Syntax

rep(rv, …)

Parameters

The rep() function takes a maximum of two arguments.

The rv is a vector (of any mode, including a list) or a factor.

The (…) are further arguments for or from other methods. For example, it can be one of the following.

  1. times: It is an integer-valued vector giving the (non-negative) number of times to repeat each item if of length. For example, length(x), or to repeat the whole vector if of length 1. 
  2. length.out: It is a non-negative integer—the desired length of the output vector. 
  3. each: It is a non-negative integer. Each item of x is repeated each time.

Example 1: How to use the rep() function in R

rep(11, 4)

Output

[1] 11 11 11 11

In this example, we are repeating vector 11 four times.

To repeat NA values more than once, use the rep() function.

rep(NA, 5)

Output

[1] NA NA NA NA NA

Example 2: Repeat counting from number to number in R

Repeat the counting numbers from 1 to 4 three times.

rep(1:4, 3)

Output

 [1] 1 2 3 4 1 2 3 4 1 2 3 4

You can see that 1 through 4 are repeated four times.

Example 3: Repeat vector with an incomplete cycle in R

To repeat a vector with an incomplete cycle, use the length.out argument in the rep() function.

rep(1:4, 3, length.out=9)

Output

[1] 1 2 3 4 1 2 3 4 1

By providing length.out argument, you can restrict the length of the output.

So, for example, you can see that the cycle is incomplete because the output should consist of 12 integers, but instead, it contains 9 integers.

Example 4: Passing each argument to the rep() function

Each parameter is a non-negative integer. Thus, each element of x is repeated each time.

rep(1:4, each=2)

Output

[1] 1 1 2 2 3 3 4 4

You can see that each element from 1 to 4 is repeated 2 times.

Example 5: Automated length repetition

A vector can expand to a biased panel by replacing the length parameter with a vector that defines the number of times each item in the vector will repeat.

rep(1:4, 1:4)

Output

 [1] 1 2 2 3 3 3 4 4 4 4

Here, you can see that 1 appears 1 time, 2 two times, 3 three times, and 4 four times.

Replicating elements

Example 1: Replicating a single element

Use the rep() function with a single value as the first argument and the number of times you want to replicate the value as the second to replicate a single element in R.

rep(c(19), times = 3)

Output

[1] 19 19 19

Example 2: Replicating a vector of elements

Use the rep() function with the vector of elements as the first argument and the number of times you want to replicate the vector as the second to replicate the vector elements in R.

data <- c(11, 19, 21)

rep(data, times = 2)

Output

[1] 11 19 21 11 19 21

Example 3: Replicating elements of a matrix or array

To replicate elements of a matrix or array in R, you can use the rep() function with the matrix or array as the first argument and the number of times you need to replicate the matrix or array as the second.

mat <- matrix(1:9, 3, 3)
matrep <- rep(mat, times = 2)
print(matrep)

Output

 [1] 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Example 4: Replicating elements of a list

Use the rep() function to replicate a list in R.

data <- list(netflix = 1:4)
rep(data, 4)

Output

$netflix
[1] 1 2 3 4

$netflix
[1] 1 2 3 4

$netflix
[1] 1 2 3 4

$netflix
[1] 1 2 3 4

In the above example, the Netflix list of 1 to 4 has been replicated four times.

Example 5: Replicating elements of a factor

The rep() function in R can replicate the factor.

result <- factor(LETTERS[1:4])
rep(result, 4)

Output

[1] A B C D A B C D A B C D A B C D
Levels: A B C D

Other rep() functions

rep.int() function in R

The rep.int() function returns no attributes (except the class returns a factor). The rep.int() is a simple case provided as a separate function, partly for compatibility and speed. The syntax is rep.int(x, times).

rep.int(1:4, 2)

Output

[1] 1 2 3 4 1 2 3 4

rep_len() function in R

The rep_len() function in R is similar to the rep() function but is used to repeat elements of a vector, matrix, or array to achieve a specific output length. 

The rep_len() function takes two arguments: the first is the vector, matrix, or array of elements to be repeated, and the second is the expected length of the output.

The syntax is rep_len(x, length.out).

rep_len(1:3, 10)

Output

 [1] 1 2 3 1 2 3 1 2 3 1

In this example, 1 to 3 is repeated until the length.out is reached.

FAQ

Can you use the rep() function to repeat elements in a non-uniform manner?

Yes, the rep() function helps repeat elements in a non-uniform manner by determining a vector of the number of times to repeat each element in the ‘times’ argument.

What happens if the ‘times’ argument is negative?

If the “times” argument is negative, the rep() function will return an empty vector.

Can you use the rep() function to repeat elements in a cyclic manner?

Yes, the rep() function can repeat elements in a cyclic manner by specifying the ‘each’ argument.

Conclusion

The rep() function in R is excellent for replicating the values of a list or vector, and it is also time and memory-effective.

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