Lab 10 10 pts Week 4 Due Apr 22

Lab 10: Loop Drills

Three loop types, sentinel input, accumulators, and validation

while-loop for-loop do-while sentinel accumulator
Clone from GitHub
3 checkpoints 5 autograder 2 timeliness
Prerequisites: lesson-1-4 lesson-1-10

Learning Objectives

After completing this lab, you will be able to:

  • Choose the right loop type (while, for, do-while) for a given problem
  • Implement a sentinel loop that terminates on a special input value
  • Use an accumulator variable to compute a running total
  • Validate user input with a do-while loop

What You’re Building

Three independent methods, each demonstrating a different loop type. Together they form a mini toolkit: a sentinel-based sum calculator, a factorial computer, and an input validator. Each method is called from main so the autograder can test them independently.

Concepts and Common Misconceptions

Concept What students get wrong
while loop Forgetting to update the loop variable inside the body, causing an infinite loop.
for loop Off-by-one errors in the bounds. for (int i = 1; i <= n; i++) runs n times; i < n runs n - 1 times.
do-while Forgetting the semicolon after the closing parenthesis: } while (condition);
Sentinel value Including the sentinel in the sum. If -1 is the sentinel, do not add -1 to the total.
Accumulator init Initializing the accumulator to 1 for a sum (should be 0) or to 0 for a product (should be 1).

Checkpoint 1: Sentinel Sum (1 pt)

Write a method sentinelSum(Scanner scanner) that repeatedly reads integers until the user enters -1. Return the sum of all entered values, excluding the sentinel.

Enter numbers (-1 to stop): 10 20 30 -1
Sum: 60

Use a while loop. Read the first value before the loop, then check if it is the sentinel before entering the loop body.

Debugging tip: If your sum is off by -1, you are adding the sentinel to the total. Read the value, check it, then add it – in that order.

Checkpoint 2: Factorial (1 pt)

Write a method factorial(int n) that returns n! using a for loop. Return 1 for n == 0 (by definition, 0! = 1).

factorial(5) -> 120
factorial(0) -> 1

Use a long return type to avoid overflow for larger values.

Debugging tip: If factorial(0) returns 0, your accumulator is initialized to 0 instead of 1. For multiplication accumulators, always start at 1.

Checkpoint 3: Input Validation (1 pt)

Write a method getValidAge(Scanner scanner) that prompts the user for an age between 0 and 150 inclusive. Use a do-while loop to keep asking until the input is valid, then return the valid value.

Enter age (0-150): -5
Invalid. Enter age (0-150): 200
Invalid. Enter age (0-150): 25

The method returns 25.

Debugging tip: If the loop runs forever, print the value you read and the condition you are checking. Make sure you are reading a new value inside the loop, not outside it.

How to Debug

  1. Trace by hand. Before running, write down what each variable holds after each iteration for a small input (3 numbers for sentinel, factorial of 3).
  2. Test boundaries. Sentinel: what if the first input is -1? Factorial: what about 0 and 1? Validation: what about exactly 0 and 150?
  3. Print inside the loop. Add System.out.println("i=" + i + " product=" + product); to see each step.
  4. Run the autograder after each checkpoint. Fix issues early.

Scoring Breakdown

Component Points
Checkpoint 1: Sentinel sum with while loop 1
Checkpoint 2: Factorial with for loop 1
Checkpoint 3: Input validation with do-while 1
Autograder correctness 5
Timeliness (on-time submission) 2
Total 10