R For Loop - Learn By Example

The for statement in R is a bit different from what you usually use in other programming languages.

Rather than iterating over a numeric progression, R’s for statement iterates over the items of a vector or a list. The items are iterated in the order that they appear in the vector.

Syntax

Here’s the syntax of the for statement:

vector is a collection of objects that can be iterated over

statement(s) are executed once for each item in a vector

var takes items from a vector one by one

Basic Examples

# Iterate through a vector
colors <- c("red","green","blue","yellow")
for (x in colors) {
  print(x)
}
[1] "red"
[1] "green"
[1] "blue"
[1] "yellow"

# Iterate through a list
l <- list(3.14, "Hi", c(1,2,3))
for (x in l) {
  print(x)
}
[1] 3.14
[1] "Hi"
[1] 1 2 3

If you need to execute a group of statements for a specified number of times, use sequence operator : or built-in function seq()

# Print 'Hello!' 3 times
for (x in 1:3) {
  print("Hello!")
}
[1] "Hello!"
[1] "Hello!"
[1] "Hello!"

# Iterate a sequence and square each element
for (x in seq(from=2,to=8,by=2)) {
  print(x^2)
}
[1] 4
[1] 16
[1] 36
[1] 64

for Loop Without Curly Braces

If you have only one statement to execute, you can skip curly braces.

# Print the numbers 0 to 4
for (x in 0:4) print(x)
[1] 0
[1] 1
[1] 2
[1] 3
[1] 4

Nested for loop

A nested for loop is a loop within a loop. They are useful for when you want to repeat something several times for several things.

for(x in 1:3) {
  for(y in 1:2) {
    print(paste(x, y))
  }
}
[1] "1 1"
[1] "1 2"
[1] "2 1"
[1] "2 2"
[1] "3 1"
[1] "3 2"

Break in for Loop

In R, break statement is used to exit the loop immediately. It simply jumps out of the loop altogether, and the program continues after the loop.

# Break the loop at 'blue'
colors <- c("red","green","blue","yellow")
for (x in colors) {
  if (x == "blue")
    break
  print(x)
}
[1] "red"
[1] "green"

Next (continue) in for Loop

The next statement skips the current iteration of a loop and continues with the next iteration.

# Skip 'blue' using continue statement
colors <- c("red","green","blue","yellow")
for (x in colors) {
  if (x == "blue")
    next
  print(x)
}
[1] "red"
[1] "green"
[1] "yellow"