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R For Loop


For Loops

A for loop is used for iterating over a sequence:

Example

for (x in 1:10) {
  print(x)
}
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This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a vector, array, list, etc..

You will learn about lists and vectors, etc in a later chapter.

Example

Print every item in a list:

fruits <- list("apple", "banana", "cherry")

for (x in fruits) {
  print(x)
}
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Example

Print the number of dices:

dice <- c(1, 2, 3, 4, 5, 6)

for (x in dice) {
  print(x)
}
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The for loop does not require an indexing variable to set beforehand, like with while loops.


Break

With the break statement, we can stop the loop before it has looped through all the items:

Example

Stop the loop at "cherry":

fruits <- list("apple", "banana", "cherry")

for (x in fruits) {
  if (x == "cherry") {
    break
  }
  print(x)
}
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The loop will stop at "cherry" because we have chosen to finish the loop by using the break statement when x is equal to "cherry" (x == "cherry").


Next

With the next statement, we can skip an iteration without terminating the loop:

Example

Skip "banana":

fruits <- list("apple", "banana", "cherry")

for (x in fruits) {
  if (x == "banana") {
    next
  }
  print(x)
}
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When the loop passes "banana", it will skip it and continue to loop.


Yahtzee!

If .. Else Combined with a For Loop

To demonstrate a practical example, let us say we play a game of Yahtzee!

Example

Print "Yahtzee!" If the dice number is 6:

dice <- 1:6

for(x in dice) {
  if (x == 6) {
    print(paste("The dice number is", x, "Yahtzee!"))
  } else {
    print(paste("The dice number is", x, "Not Yahtzee"))
  }
}
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If the loop reaches the values ranging from 1 to 5, it prints "No Yahtzee" and its number. When it reaches the value 6, it prints "Yahtzee!" and its number.


Nested Loops

A nested loop is a loop inside a loop.

The "inner loop" will be executed one time for each iteration of the "outer loop":

Example

Print the adjective of each fruit in a list:

adj <- list("red", "big", "tasty")

fruits <- list("apple", "banana", "cherry")
  for (x in adj) {
    for (y in fruits) {
      print(paste(x, y))
  }
}
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