rep()

rep(x, times = 1, length.out = NA, each = 1)

The rep() function replicates the elements of a vector a specified number of times. It is fundamental for creating repeated patterns, expanding datasets, and constructing test data in R.

Syntax

rep(x, times = 1, length.out = NA, each = 1)

Parameters

Parameter	Type	Default	Description
x	vector or list	—	The object to replicate
times	integer	1	Number of times to repeat the entire vector
length.out	integer	NA	Desired length of the output vector
each	integer	1	Number of times to repeat each element individually

Examples

Basic usage

# Repeat the entire vector 3 times
rep(c("a", "b"), times = 3)
# [1] "a" "b" "a" "b" "a" "b"

# Repeat each element 2 times
rep(c("a", "b"), each = 2)
# [1] "a" "a" "b" "b"

Using length.out

# Specify exact output length (truncates or recycles)
rep(1:3, length.out = 7)
# [1] 1 2 3 1 2 3 1

# Useful for creating equal-length vectors
rep(c("train", "test"), each = 50, length.out = 100)
# Creates 100 labels: 50 train, 50 test

Combining times and each

# Both parameters together
rep(1:2, times = 2, each = 3)
# [1] 1 1 1 2 2 2 1 1 1 2 2 2

Common Patterns

Creating indicator variables

groups <- rep(c("control", "treatment"), each = 10)
# Creates 20-group assignment vector

Expanding data for simulation

set.seed(42)
values <- rnorm(5)
repeated <- rep(values, each = 100)
# Each original value repeated 100 times for bootstrap-style analysis

Cycling with length.out

# Alternate pattern for stratified sampling
idx <- rep(1:3, length.out = 90)
table(idx)
# idx
#  1  2  3 
# 30 30 30

See Also

• c()
• seq()
• sample()