Lab 11 10 pts Week 4 Due Apr 24

Lab 11: Pattern Printer

Nested loops, print vs println, and ASCII art

nested-loops print-vs-println fencepost loop-bounds
Clone from GitHub
3 checkpoints 5 autograder 2 timeliness
Prerequisites: lesson-1-14

Learning Objectives

After completing this lab, you will be able to:

  • Use nested for loops where the inner loop bound depends on the outer loop variable
  • Distinguish between System.out.print() (stays on the line) and System.out.println() (moves to the next line)
  • Apply the fencepost pattern to handle leading/trailing spaces
  • Translate a visual pattern into loop bounds systematically

What You’re Building

Three static methods that each print an ASCII pattern of a given size n. You will build from simple (rectangle) to complex (centered diamond). Each method takes an int n parameter and prints directly to the console.

Concepts and Common Misconceptions

Concept What students get wrong
print vs println Using println("*") for every star, which puts each on its own line. Use print("*") inside the inner loop and println() after the inner loop to move to the next row.
Nested loop bounds Making the inner loop always run n times. For a triangle, the inner loop count must change with the outer loop variable.
Fencepost / spaces Forgetting leading spaces in centered patterns. The diamond requires n - 1 - row spaces before the first star on each row.
Off-by-one A size-5 diamond has 9 rows (5 top half including middle, 4 bottom half), not 10.

Checkpoint 1: Rectangle (1 pt)

Write printRectangle(int n) that prints an n x n grid of * characters.

For n = 4:

****
****
****
****

Outer loop: rows 0 to n - 1. Inner loop: columns 0 to n - 1. Print * with print, then println after each row.

Debugging tip: If all stars appear on one line, you forgot the println() after the inner loop. If each star is on its own line, you used println("*") instead of print("*").

Checkpoint 2: Right Triangle (1 pt)

Write printTriangle(int n) that prints a right triangle with n rows.

For n = 5:

*
**
***
****
*****

The inner loop runs from 0 to row (inclusive) on each row, printing row + 1 stars.

Debugging tip: If your triangle is upside down, your inner loop bound is n - row instead of row + 1. If the first row has no stars, check whether your outer loop starts at 0 or 1.

Checkpoint 3: Centered Diamond (1 pt)

Write printDiamond(int n) that prints a centered diamond. n is the half-height (the number of rows in the top half, including the middle row). The total height is 2 * n - 1.

For n = 4:

   *
  ***
 *****
*******
 *****
  ***
   *

Break this into two halves. Top half (rows 0 to n - 1): print n - 1 - row spaces, then 2 * row + 1 stars. Bottom half (rows 0 to n - 2): mirror the top half.

Debugging tip: If your diamond is lopsided, print the space count and star count for each row before printing the actual characters. Verify the numbers match the pattern above.

How to Debug

  1. Start with n = 3. Small inputs make it easy to count characters by hand.
  2. Print row and column indices. Replace * with row + "," + col temporarily to see exactly which iteration produces each character.
  3. Build a table. For each row, write down: number of spaces, number of stars. Then translate that table into loop bounds.
  4. Test the top half alone. Get the top half of the diamond working before adding the bottom half.

Scoring Breakdown

Component Points
Checkpoint 1: Rectangle 1
Checkpoint 2: Right triangle 1
Checkpoint 3: Centered diamond 1
Autograder correctness 5
Timeliness (on-time submission) 2
Total 10