13 Loops

13.1 Loops

Loops are a fundamental component of (procedural) programming.

There are two main types of loops:

• conditional loops are executed as long as a defined condition holds true
  • construct while
  • construct repeat
• deterministic loops are executed a pre-determined number of times
  • construct for

13.2 While

The while construct can be defined using the while reserved word, followed by the conditional statement between simple brackets, and a code block. The instructions in the code block are re-executed as long as the result of the evaluation of the conditional statement is TRUE.

current_value <- 0

while (current_value < 3) {
  cat("Current value is", current_value, "\n")
  current_value <- current_value + 1
}

## Current value is 0 
## Current value is 1 
## Current value is 2

13.3 For

The for construct can be defined using the for reserved word, followed by the definition of an iterator. The iterator is a variable which is temporarily assigned with the current element of a vector, as the construct iterates through all elements of the list. This definition is followed by a code block, whose instructions are re-executed once for each element of the vector.

cities <- c("Derby", "Leicester", "Lincoln", "Nottingham")
for (city in cities) {
  cat("Do you live in", city, "?\n")
}

## Do you live in Derby ?
## Do you live in Leicester ?
## Do you live in Lincoln ?
## Do you live in Nottingham ?

13.4 For

It is common practice to create a vector of integers on the spot in order to execute a certain sequence of steps a pre-defined number of times.

for (i in 1:3) {
  cat("This is exectuion number", i, ":\n")
  cat("    See you later!\n")
}

## This is exectuion number 1 :
##     See you later!
## This is exectuion number 2 :
##     See you later!
## This is exectuion number 3 :
##     See you later!

13.5 Loops with conditional statements

3:0

## [1] 3 2 1 0

#Example: countdown!
for (i in 3:0) {
  if (i == 0) {
    cat("Go!\n")
  } else {
    cat(i, "\n")
  }
}

## 3 
## 2 
## 1 
## Go!