Nested Loops — Tips & Common Mistakes

Common Mistake 1: Forgetting print()

If you forget the print() after the inner loop, everything stays on one line:

# ❌ Wrong — no newline
for r in range(3):
    for c in range(4):
        print("*", end=" ")

# Output: * * * * * * * * * * * *  (all on one line!)

# ✅ Correct — add print() after inner loop
for r in range(3):
    for c in range(4):
        print("*", end=" ")
    print()  # ← this moves to the next line

---

Common Mistake 2: Wrong Variable Names

Using the same variable for both loops causes bugs:

# ❌ Wrong — both loops use 'i'
for i in range(3):
    for i in range(4):    # This overwrites the outer 'i'!
        print(i, end=" ")
    print()

# ✅ Correct — use different names
for i in range(3):
    for j in range(4):
        print(j, end=" ")
    print()

---

Common Mistake 3: Off-by-One in Range

Remember: range(n) goes from 0 to n-1, and range(a, b) goes from a to b-1.

# Want to print 1 to 5?
range(5)      # gives 0, 1, 2, 3, 4  ← starts at 0!
range(1, 5)   # gives 1, 2, 3, 4     ← misses 5!
range(1, 6)   # gives 1, 2, 3, 4, 5  ← correct!

---

Common Mistake 4: break Only Breaks the Inner Loop

for i in range(5):
    for j in range(5):
        if j == 2:
            break    # Only exits the inner loop!
    # Outer loop keeps going...

If you need to break both loops, use a flag:

done = False
for i in range(5):
    for j in range(5):
        if some_condition:
            done = True
            break
    if done:
        break

---

Debugging Tip: Add Print Statements

When your nested loop isn't working, add prints to see what's happening:

for i in range(3):
    print(f"--- Outer loop: i = {i} ---")
    for j in range(3):
        print(f"  Inner loop: j = {j}")

This shows you exactly how the loops are running.

---

Quick Reference

Pattern	Outer	Inner	Total
3×4 grid	range(3)	range(4)	12 runs
Triangle	range(1, n+1)	range(i)	varies
All pairs	range(len(list))	range(i+1, len(list))	n×(n-1)/2
Times table	range(1, 13)	(just multiply)	12 runs

---

When to Use Nested Loops

• Grids and tables — rows × columns
• Comparing all pairs — matching, finding duplicates
• Pattern printing — stars, numbers, shapes
• 2D data — game boards, spreadsheets, images