• A for loop repeats a chunk of code many times, once for each element in a set of input.

for (value in that) {
  this
}

• The that object should be a set of objects (often a vector of numbers or character strings).
  • 1:10
  • 1:length(x)
  • 1:nrow(x)
  • c("hello", "this", "is", "a", "vector", "of", "strings")

• The value symbol in a for loop acts like an argument in a function.
• The for loop will
  • create an object named value
  • and assign it a new value on each run of the loop.
• The code in your loop can access this value by calling the value object.

for (value in c("My", "first", "for", "loop")) {
  print("one run")
}

## [1] "one run"
## [1] "one run"
## [1] "one run"
## [1] "one run"

for (value in c("My", "second", "for", "loop")) {
  print(value)
}

## [1] "My"
## [1] "second"
## [1] "for"
## [1] "loop"

• Example: What will be the value of a after the loop?

a <- 100

for (a in 1:5) {
  print(a)
}

a

• Task: Write a for loop to compute the sum of 5 numbers that are randomly selected from 1 to 100 with replacement.

• Task: Write a for loop to compute the sum of 5 numbers that are randomly selected from 1 to 100 with replacement.

• First, let’s generate the 5 random numbers!

• Second, write the for loop.

• To save output from a for loop, you must write the loop so that it saves its own output as it runs.
• What that means:
  • Create an empty vector or list before you run the loop.
  • Use the for loop to fill up the vector or list.
  • When the for loop is finished, you’ll be able to access the vector or list, which will now have all of your results.

• From a given list of fruits, select only the ones that have 6 or fewer letters.

fruits <- c("apple", "pineapple", "watermelon", "orange", "peach", "plum",
            "honeydew", "banana", "kiwi", "papaya", "grapes", "strawberry",
            "blueberry", "blackberry")

• A while loop reruns a chunk while a certain condition remains TRUE.

while (condition) {
  code
}

• while will rerun condition (a logical test) at the start of each loop:
  • If condition evaluates to TRUE, while will run the code between its braces.
  • If condition evaluates to FALSE, while will finish the loop.

• Why might condition change from TRUE to FALSE?
  • The code inside the loop changes the certain values which change the result of condition. (We will look at an example next)
• If the code has no relationship to the condition, a while loop will run until you stop it.
  • Called infinite loop.

cash <- 20
n <- 0

while (cash > 0) {
  cash <- cash - 1
  n <- n + 1
}

n

## [1] 20

• Remember the game King of Hearts from Lab02?
• It costs $1 to play this game each time.
• If we win a jackpot, we gain $10.
• Let’s say we start with $20!
• How long can we play until we run out of money?

• The repeat loops repeat a chunk of code until you tell them to stop (by hitting Escape) or until they encounter the command break, which will stop the loop.

• We can use a repeat loop to recreata a loop for the example above.

• Read Chapter 12: Speed of Hands-On Programming with R.
• Bring your laptop to lecture on Wednesday (02/12/2020) for Lab 03.
• Lab 03 will be posted either late evening on Tuesday or early Wednesday.

• Hands-On Programming with R, by Garrett Grolemund.