Lab 04 10 pts Week 2 Due Apr 8

Lab 04: Unit Converter

Integer division, modulus, type casting, and formatted output

integer-division modulus casting printf
Clone from GitHub
3 checkpoints 5 autograder 2 timeliness
Prerequisites: lesson-1-6

After this lab, you will be able to:

  • Use integer division (/) and modulus (%) to decompose values into larger units
  • Cast between int and double to control division behavior
  • Format output with System.out.printf and format specifiers
  • Distinguish when integer division is useful vs. when it causes bugs

What You’re Building

You will write a unit converter that takes a total number of minutes and breaks it into hours and minutes, computes a percentage, and formats everything with printf. This lab drills the difference between integer division (which truncates) and floating-point division (which preserves decimals) — a distinction that does not exist in Python 3.

Concepts and Misconceptions

Concept Common Mistake What the Test Catches
Integer division Expecting 7 / 2 to produce 3.5 (it produces 3) Incorrect hours or minutes value
Modulus Confusing % with percentage; writing minutes % 100 instead of minutes % 60 Remainder exceeds 59
Casting Casting the result instead of the operand: (double)(a / b) vs (double) a / b Percentage is 0.0 instead of correct value
printf format Missing %n or \n at end of format string Output lines merge together

Checkpoints

Checkpoint 1: Convert Minutes to Hours and Minutes

What to do: Given int totalMinutes, compute hours = totalMinutes / 60 and remaining = totalMinutes % 60. Print in the format: "3 hours and 27 minutes".

What the test checks: For input 207, output is exactly "3 hours and 27 minutes".

Debugging tip: If hours is correct but minutes is wrong, double-check your modulus divisor. % 60 gives minutes remaining after full hours. % 100 is a common mistake that gives a meaningless value.

Checkpoint 2: Compute a Percentage with Casting

What to do: Compute what percentage of a full day (1440 minutes) the input represents. Cast to double before dividing: (double) totalMinutes / 1440 * 100.

What the test checks: The percentage is a double with the correct value (e.g., 14.375 for 207 minutes). Integer division would produce 0.0.

Debugging tip: If your result is 0.0, check where the cast is. (double)(207 / 1440) casts after integer division — the damage is already done. The cast must go on one of the operands before the / operator.

Checkpoint 3: Format Output with printf

What to do: Rewrite your output using System.out.printf. Display hours and minutes as integers and the percentage to two decimal places: printf("%d hours, %d minutes (%.2f%%)%n", hours, remaining, pct).

What the test checks: Output matches the exact format, including two decimal places and the % sign.

Debugging tip: To print a literal % with printf, use %%. A single % is a format specifier prefix, so printf("85%") will crash with java.util.MissingFormatArgumentException. Write printf("85%%") instead.

How to Debug

  1. Test with edge cases. Try totalMinutes = 0, 60, 61, and 1440. Zero and exact multiples of 60 reveal off-by-one errors. 1440 should produce 100.00%.

  2. Print the raw division. Before formatting, print totalMinutes / 1440 and (double) totalMinutes / 1440 on separate lines. If both produce the same integer result, your cast is missing or misplaced.

  3. Check printf argument count. printf crashes at runtime (not compile time) if the number of % specifiers does not match the number of arguments. Count them: each %d, %f, or %s needs exactly one corresponding argument, in order.

Scoring

Component Points Criteria
Checkpoints 3 1 pt each. Binary: the checkpoint test passes or it does not.
Autograder 5 Correctness across all test cases. Partial credit by proportion of tests passed.
Timeliness 2 Full credit if submitted by the due date. 0 if late.
Total 10