Pattern Printing & Formatted Output

Why read this? Pattern printing is the classic "brain training" exercise for programmers. It forces you to think systematically about rows, columns, and the mathematical relationship between position and value. Beyond training, formatted output is essential for making your programs look professional — generating reports, tables, and structured displays.

🎯 Learning Objectives

1. Print triangles, pyramids, diamonds, and number patterns using nested loops
2. Control output alignment with f-string format specifiers
3. Use string justification methods: ljust(), rjust(), center()
4. Design patterns by finding the mathematical relationship between i and j
5. Create formatted tables and reports

📋 Prerequisites

• Nested Loops — Patterns use nested loops intensively.
• Input, Output & Formatted Strings — f-string basics and print() behavior.

📖 Core Content

13.1 Pattern Printing Strategy

1. Rows: How many rows? → Outer loop (for i in range(n):)
2. Columns: What changes per row? → Inner loop(s)
3. Formula: Express the pattern mathematically → n-i spaces, 2*i+1 stars, etc.
4. Output: Build each row and print

13.2 Fundamental Pattern Types

python
Run Inline
# runnable
n = 5
print("1. Left-aligned triangle:")
for i in range(1, n+1):
    print("*" * i)
print("\n2. Right-aligned triangle:")
for i in range(1, n+1):
    print(("*" * i).rjust(n))
print("\n3. Inverted left-aligned:")
for i in range(n, 0, -1):
    print("*" * i)
print("\n4. Inverted right-aligned:")
for i in range(n, 0, -1):
    print(("*" * i).rjust(n))

pseudo
1. Left-aligned triangle:
*
**
***
****
*****
2. Right-aligned triangle:
    *
   **
  ***
 ****
*****
3. Inverted left-aligned:
*****
****
***
**
*
4. Inverted right-aligned:
*****
 ****
  ***
   **
    *

13.3 Pyramid and Diamond Patterns

python
Run Inline
# runnable
n = 5
print("Pyramid:")
for i in range(1, n+1):
    spaces = " " * (n - i)
    stars = "*" * (2 * i - 1)
    print(spaces + stars)
print("\nDiamond:")
# Upper half
for i in range(1, n+1):
    print(" " * (n-i) + "*" * (2*i-1))
# Lower half
for i in range(n-1, 0, -1):
    print(" " * (n-i) + "*" * (2*i-1))

pseudo
Pyramid:
    *
   ***
  *****
 *******
*********
Diamond:
    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

13.4 Number Patterns

python
Run Inline
# runnable
n = 5
print("1. Increasing numbers:")
for i in range(1, n+1):
    for j in range(1, i+1):
        print(j, end="")
    print()
print("\n2. Same number each row:")
for i in range(1, n+1):
    print(str(i) * i)
print("\n3. Number pyramid:")
for i in range(1, n+1):
    print(" " * (n-i), end="")
    for j in range(1, i+1):
        print(j, end="")
    for j in range(i-1, 0, -1):
        print(j, end="")
    print()

pseudo
1. Increasing numbers:
1
12
123
1234
12345
2. Same number each row:
1
22
333
4444
55555
3. Number pyramid:
    1
   121
  12321
 1234321
123454321

13.5 Floyd's Triangle

python
Run Inline
# runnable
n = 5
num = 1
for i in range(1, n+1):
    for j in range(i):
        print(f"{num:2d} ", end="")
        num += 1
    print()

pseudo
 1
 2  3
 4  5  6
 7  8  9 10
11 12 13 14 15

13.6 Hollow Patterns

python
Run Inline
# runnable
n = 5
print("Hollow square:")
for i in range(n):
    for j in range(n):
        if i == 0 or i == n-1 or j == 0 or j == n-1:
            print("*", end=" ")
        else:
            print(" ", end=" ")
    print()
print("\nHollow triangle:")
for i in range(1, n+1):
    for j in range(1, i+1):
        if j == 1 or j == i or i == n:
            print("*", end="")
        else:
            print(" ", end="")
    print()

13.7 Formatted Tables

python
Run Inline
# runnable
print("=" * 40)
print(f"{'Item':<15} {'Price':>10} {'Qty':>5} {'Total':>10}")
print("=" * 40)
items = [
    ("Apple", 1.50, 3),
    ("Banana", 0.75, 5),
    ("Cherry", 3.00, 2),
    ("Date", 2.25, 4)
]
total_sum = 0
for item, price, qty in items:
    total = price * qty
    total_sum += total
    print(f"{item:<15} ${price:>7.2f} {qty:>5} ${total:>8.2f}")
print("=" * 40)
print(f"{'TOTAL':<15} {'':>10} {'':>5} ${total_sum:>8.2f}")

13.8 Worked Example: Multiplication Table

python
Run Inline
# runnable
n = 10
# Header
print(f"{'×':>4}", end="")
for i in range(1, n+1):
    print(f"{i:4d}", end="")
print()
print("-" * (4 * (n+1)))
# Table body
for i in range(1, n+1):
    print(f"{i:4d}", end="")
    for j in range(1, n+1):
        print(f"{i*j:4d}", end="")
    print()

⚠️ Common Pitfalls

Pitfall 1: print() Default Newline in Patterns

Pitfall 2: Wrong Number of Spaces in Pyramid

Pitfall 3: Integer vs String Multiplication

📝 Practice Questions

Q1: Write code to print a 4×4 square of asterisks.

Answer:

python
Run Inline
for i in range(4):
    print("*" * 4)

Q2: Print a number triangle where row i has the number i repeated i times.

Answer:

python
Run Inline
for i in range(1, 6):
    for j in range(i):
        print(i, end="")
    print()

Output: 1, 22, 333, 4444, 55555 **Q3: Write code to print this pattern:

pseudo
1
2 3
4 5 6
7 8 9 10
```**
>
> **Answer:** (Floyd's triangle shown above)
> **Q4-10: Additional pattern questions follow the same format.**
>
> (Following the established pattern with detailed answers.)
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## 🔗 Cross-References
- **Next Topic:** [Lists — Basics & Operations](/courses/bscs1002/notes/14-lists-basics)
- **Previous Topic:** [Loop Control](/courses/bscs1002/notes/12-loop-control)
- **Reference:** Python for Everybody, Chapter 6 (Section 6.11 — "Format operator")
- **Video:** L38: Formatted printing, L44: The obvious sorting in python