11.3 While Loops and Repeat

In addition to for loop in Section 11.2 where you have to specify the range of values to iterate, it is sometimes more convenient to use another type of loop: the while loop.

11.3.1 while loop

Let’s first review the syntax of for loops.

for (val in val_seq) {
    statement1
    statement2
}

Here, we have to specify the val_seq along which we will assign each element to val and execute the statements, unless break is called. Sometimes, it is not clear what the val_seq would be. For example, if we want to find the Fibonacci sequence up to 100, it is not clear how many elements we will have in the val_seq. In this case, the while loop comes to rescue. Let’s take a look of the syntax of while loop.

while (cond_expr) {
    statement1
    statement2
}

In the while loop, we put a logical statement in the cond_expr. The loop will continue as long as cond_expr take the value of TRUE. Let’s see the actual code in action.

fib_seq <- c(0, 1)
fib_last_value <- fib_seq[1]
fib_cur_value <- fib_seq[2]
fib_next_value <- fib_last_value + fib_cur_value
while (fib_next_value < 100) {
    fib_seq <- c(fib_seq, fib_next_value)
    fib_cur_value <- fib_next_value
    fib_last_value <- fib_cur_value
    fib_next_value <- fib_last_value + fib_cur_value
}
fib_seq  #print the sequence
#> [1]  0  1  1  2  4  8 16 32 64

avoid infinite loops: it is very important to ensure that cond_expr will eventually be TRUE at some iterations. Otherwise, the loop will be infinite. The following is an infinite loop.

counter <- 1
while (counter > 0) {
    print(counter)
    counter <- counter + 1
}

11.3.2 repeat Loop

The last type of loops is the repeat loop, which doesn’t have neither the range of values, nor the logical expression. The syntax is as below.

repeat {
    expr1
    expr2
    if (cond_expr) {
        break
    }
    expr3
}

As you can see in the syntax of the repeat loop, you always need to put a conditional break statement to avoid infinite loops.

Let’s rewrite the previous example using repeat loop.

fib_seq <- c(0, 1)
fib_last_value <- fib_seq[1]
fib_cur_value <- fib_seq[2]
fib_next_value <- fib_last_value + fib_cur_value
repeat {
    fib_seq <- c(fib_seq, fib_next_value)
    fib_cur_value <- fib_next_value
    fib_last_value <- fib_cur_value
    fib_next_value <- fib_last_value + fib_cur_value
    if (fib_next_value >= 100) {
        break
    }
}
fib_seq  #print the sequence
#> [1]  0  1  1  2  4  8 16 32 64

You can see that the same sequence was generated using the repeat loop.

11.3.3 Comparison of for, while, and repeat Loops

1. for Loops

   • Use when the number of iterations is known before starting the loop.

   • Typically iterates over elements in a vector, list, or sequence.

   • Example:

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

2. while Loops

   • Use when the loop termination depends on a condition that may change dynamically during execution.

   • The condition is checked at the beginning of each iteration.

   • Example:

     counter <- 1
     while (counter <= 5) {
       print(counter)
       counter <- counter + 1
     }

3. repeat Loops

   • Use for indefinite loops where the termination condition is not evaluated at the start.

   • Typically terminated manually using a break statement.

   • Example:

     counter <- 1
     repeat {
       print(counter)
       counter <- counter + 1
       if (counter > 5) break
     }

4. Key Differences
   • for loops are ideal for iterating over a known sequence.
   • while loops are better when the number of iterations depends on a condition that might change dynamically.
   • repeat loops are useful for loops that require manual control over termination.
5. When to Use
   • Prefer for loops for most R operations due to simplicity and readability.
   • Use while for scenarios where the termination condition is dynamic or unknown.
   • Use repeat sparingly, typically for indefinite or complex loops.

11.3.4 Exercises

1. Countdown Timer

Write a while loop that counts down from a given number n to 1 and prints each number. When the countdown is finished, print "Blast off!".

Example Input:

n <- 5

Expected Output:

5
4
3
2
1
Blast off!

2. Sum Until a Threshold

Write a while loop that keeps adding numbers (starting from 1) until their sum exceeds a given threshold threshold. Print the final sum and the last number added.

Example Input:

threshold <- 15

Expected Output:

"The sum is 21. The last number added is 6."

3. Repeat Until Correct

Use a repeat loop to simulate a simple guessing game. The loop should keep running until the user guesses the correct number, which is predefined (e.g., target <- 7). Use break to exit the loop when the guess is correct. Hint: guess <- readline("Guess a number: ") will prompt the user to enter a number and assigned it to the object guess.

Example Input:

target <- 7

Expected Output:

"Guess a number: "
7
"Congratulations! You've guessed the correct number."

4. Fibonacci Sequence

Write a while loop that generates the Fibonacci sequence until the value exceeds a given limit limit. Print all the Fibonacci numbers generated.

Example Input:

limit <- 50

Expected Output:

1, 1, 2, 3, 5, 8, 13, 21, 34