1 Chapter 2: Loops

1.1 While Loop

1.1.1 Write a while loop

Let’s get you started with building a while loop from the ground up. Have another look at its recipe:

while (condition) {
  expr
}

Remember that the condition part of this recipe should become FALSE at some point during the execution. Otherwise, the while loop will go on indefinitely.

If your session expires when you run your code, check the body of your while loop carefully.

Have a look at the sample code provided; it initializes the speed variables and already provides a while loop template to get you started.

# Initialize the speed variable
speed <- 64

# Code the while loop
while (___) {
  ___
  ___ <- ___
}

# Print out the speed variable
speed

Instructions

Code a while loop with the following characteristics:

  • The condition of the while loop should check if speed is higher than 30.

  • Inside the body of the while loop, print out "Slow down!".

  • Inside the body of the while loop, decrease the speed by 7 units and assign this new value to speed again. This step is crucial; otherwise your while loop will never stop and your session will expire.

If your session expires when you run your code, check the body of your while loop carefully: it’s likely that you made a mistake.

Solution

# Initialize the speed variable
speed <- 64

# Code the while loop
while (speed > 30) {
  print("Slow down!")
  speed <- speed - 7
}

# Print out the speed variable
speed

1.1.2 Throw in more conditionals

In the previous exercise, you simulated the interaction between a driver and a driver’s assistant: When the speed was too high, “Slow down!” got printed out to the console, resulting in a decrease of your speed by 7 units.

There are several ways in which you could make your driver’s assistant more advanced. For example, the assistant could give you different messages based on your speed or provide you with a current speed at a given moment.

A while loop similar to the one you’ve coded in the previous exercise is already available for you to use. It prints out your current speed, but there’s no code that decreases the speed variable yet, which is pretty dangerous. Can you make the appropriate changes?

# Initialize the speed variable
speed <- 64

# Extend/adapt the while loop
while (speed > 30) {
  print(paste("Your speed is",speed))
  if ( ) {
    ___
    ___
  } else {
    ___
    ___
  }
}

Instructions

  • If the speed is greater than 48, have R print out “Slow down big time!”, and decrease the speed by 11. Otherwise, have R simply print out “Slow down!”, and decrease the speed by 6.

  • If the session keeps timing out and throwing an error, you are probably stuck in an infinite loop! Check the body of your while loop and make sure you are assigning new values to speed.

Solution

# Initialize the speed variable
speed <- 64

# Extend/adapt the while loop
while (speed > 30) {
  print(paste("Your speed is", speed))
  if (speed > 48) {
    print("Slow down big time!")
    speed <- speed - 11
  } else {
    print("Slow down!")
    speed <- speed - 6
  }
}

1.1.3 Stop the while loop: break

There are some very rare situations in which severe speeding is necessary: what if a hurricane is approaching and you have to get away as quickly as possible? You don’t want the driver’s assistant sending you speeding notifications in that scenario, right?

This seems like a great opportunity to include the break statement in the while loop you’ve been working on. Remember that the break statement is a control statement. When R encounters it, the while loop is abandoned completely.

# Initialize the speed variable
speed <- 88

while (speed > 30) {
  print(paste("Your speed is", speed))
  
  # Break the while loop when speed exceeds 80
  if ( ) {
    
  }
  
  if (speed > 48) {
    print("Slow down big time!")
    speed <- speed - 11
  } else {
    print("Slow down!")
    speed <- speed - 6
  }
}

Instructions

Adapt the while loop such that it is abandoned when the speed of the vehicle is greater than 80. This time, the speed variable has been initialized to 88; keep it that way.

Solution

# Initialize the speed variable
speed <- 88

while (speed > 30) {
  print(paste("Your speed is", speed))
  
  # Break the while loop when speed exceeds 80
  if (speed > 80) {
    break
  }
  
  if (speed > 48) {
    print("Slow down big time!")
    speed <- speed - 11
  } else {
    print("Slow down!")
    speed <- speed - 6
  }
}

1.1.4 Build a while loop from scratch

The previous exercises guided you through developing a pretty advanced while loop, containing a break statement and different messages and updates as determined by control flow constructs. If you manage to solve this comprehensive exercise using a while loop, you’re totally ready for the next topic: the for loop.

# Initialize i as 1 
i <- 1

# Code the while loop
while (i <= 10) {
  print(___)
  if (___) {
    ___
  }
  i <- i + 1
}

Instructions

Finish the while loop so that it:

prints out the triple of i, so 3 * i, at each run. is abandoned with a break if the triple of i is divisible by 8, but still prints out this triple before breaking.

Solution

# Initialize i as 1 
i <- 1

# Code the while loop
while (i <= 10) {
  print(3 * i)
  if ( (3 * i) %% 8 == 0) {
    break
  }
  i <- i + 1
}