Nested Loops — How They Work

A nested loop is a loop inside another loop. The inner loop runs completely for each iteration of the outer loop.

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Basic Example

for i in range(3):        # Outer loop: runs 3 times
    for j in range(2):    # Inner loop: runs 2 times PER outer iteration
        print(f"i={i}, j={j}")

Output:

i=0, j=0
i=0, j=1
i=1, j=0
i=1, j=1
i=2, j=0
i=2, j=1

Think of it like clock hands — the inner loop is the minute hand (spins fast) and the outer loop is the hour hand (moves after the minute hand completes a full cycle).

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Counting Total Iterations

The total number of times the inner code runs = outer × inner.

for i in range(4):       # 4 times
    for j in range(3):   # 3 times each
        print("*")       # Runs 4 × 3 = 12 times

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Common Pattern: Grid / Rectangle

rows = 3
cols = 5

for r in range(rows):
    for c in range(cols):
        print("*", end=" ")
    print()  # New line after each row

Output:

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

The end=" " makes print stay on the same line. The print() after the inner loop moves to the next line.

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Common Pattern: Triangle

for i in range(1, 6):
    for j in range(i):
        print("*", end=" ")
    print()

Output:

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

The inner loop runs i times — so row 1 gets 1 star, row 2 gets 2 stars, etc.

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Common Pattern: Multiplication Table

for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i * j:4}", end="")
    print()

Output:

   1   2   3   4   5
   2   4   6   8  10
   3   6   9  12  15
   4   8  12  16  20
   5  10  15  20  25

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Nested Loop with a List

teams = ["Red", "Blue"]
players = ["Alex", "Sam", "Jordan"]

for team in teams:
    for player in players:
        print(f"{player} joins {team} team")

Output:

Alex joins Red team
Sam joins Red team
Jordan joins Red team
Alex joins Blue team
Sam joins Blue team
Jordan joins Blue team

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Using break in Nested Loops

break only exits the inner loop, not the outer one:

for i in range(3):
    for j in range(5):
        if j == 2:
            break       # Only breaks the inner loop
        print(f"i={i}, j={j}")
    print(f"--- Done with i={i} ---")

Output:

i=0, j=0
i=0, j=1
--- Done with i=0 ---
i=1, j=0
i=1, j=1
--- Done with i=1 ---
i=2, j=0
i=2, j=1
--- Done with i=2 ---

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Key Takeaways

• Nested loops = a loop inside a loop
• Inner loop runs completely for each outer iteration
• Total iterations = outer count × inner count
• break only exits the innermost loop
• Common uses: grids, tables, coordinate pairs, comparing all pairs