Using loops is generally discouraged in R when it is possible to avoid them using vectorized alternatives. Vectorized solution are be both faster to write, read and execute – except sometimes they aren’t and the definition of vectorization isn’t always straightforward.

In any event, solutions using loops can be:

• The fastest to prototype
• The easiest to understand for people coming from other programming language
• Impossible to avoid – the only way to solve a specific problem.

Furthermore, loops are a great tool to play around with in order to gain a deeper understanding of R.

Solutions are available here.

Exercise 1

Write a for loop that iterates over the numbers 1 to 7 and prints the cube of each number using print().

Exercise 2

Write a for loop that iterates over the column names of the inbuilt iris dataset and print each together with the number of characters in the column name in parenthesis. Example output: Sepal.Length (12). Use the following functions print(), paste0() and nchar().

Exercise 3

Write a while loop that prints out standard random normal numbers (use rnorm()) but stops (breaks) if you get a number bigger than 1.

Exercise 4

Using next adapt the loop from last exercise so that doesn’t print negative numbers.

Exercise 5

Using a for loop simulate the flip a coin twenty times, keeping track of the individual outcomes (1 = heads, 0 = tails) in a vector that you preallocte.

Exercise 6

Use a nested for loop (a for loop inside a for loop) that produces the following matrix, preallocate the matrix with NA values.

    0     1     2     3     4
    1     0     1     2     3
    2     1     0     1     2
    3     2     1     0     1
    4     3     2     1     0

Exercise 7

Use a while loop to investigate the number of terms required before the product

1⋅2⋅3⋅4⋅…

reaches above 10 million.

Exercise 8

Use a while loop to simulate one stock price path starting at 100 and random normally distributed percentage jumps with mean 0 and standard deviation of 0.01 each period. How long does it take to reach above 150 or below 50?

Exercise 9

Implement a simple version of Guess the number game using a while loop. The user should guess a number between 1 and 10, you can use scan() to get user input. The loop should break if the user guesses 5.

Exercise 10

Implement a multiplication game. A while loop that gives the user two random numbers from 2 to 12 and asks the user to multiply them. Only exit the loop after five correct answers. Try using as.integer(readline()) instead of scan() this time.

If you enjoyed this exercise set you might also enjoy Writing custom functions or Answer probability questions with simulation.

(Image by Nicolas Raymond)