What is a nested loop in Python?

A nested loop is a loop placed inside another loop—for example, a for loop inside a for loop. For each iteration of the outer loop, the inner loop runs from start to finish.

How many times does the inner loop body run?

If the outer loop runs m times and the inner loop runs n times per outer iteration, the inner body runs m × n times in total (assuming no break or early exit).

How do you print a grid on multiple lines with nested loops?

Use print(..., end=" ") inside the inner loop to keep values on one row, then call print() with no arguments after the inner loop finishes to move to the next line—one row per outer iteration.

Can nested loops use strings instead of range?

Yes. for works on any sequence. Nesting for i in "abc" and for j in "12" pairs every outer character with every inner character, similar to nested ranges.

Nested Loops - Beginner's Guide - Vimal Thapliyal

Nested loop = loop inside another loop (e.g. for inside for).
How it runs: outer runs once → inner runs fully → outer runs again → inner runs fully again ...
Total iterations: outer count × inner count (e.g. 5×5 = 25).
Examples: (i,j) grid, simple star patterns, nested loops over strings.

Let’s go! 🚀

Part 1: What is a Nested Loop?

📌 A nested loop is a loop written INSIDE another loop.

for i in range(5):        ← outer loop
    for j in range(5):    ← inner loop (nested)
        print(i, j)

You can mix: for inside while, while inside for, etc. Here we use for inside for.

Part 2: How Does It Run?

📌 Rule: For each iteration of the OUTER loop, the INNER loop runs completely.

Outer: i = 0 → inner runs j = 0, 1, 2, 3, 4 → then outer goes to next.
Outer: i = 1 → inner runs j = 0, 1, 2, 3, 4 again (starts fresh).
Outer: i = 2 → inner runs j = 0, 1, 2, 3, 4 again.
... same for i = 3, i = 4.

So the inner body runs 5 × 5 = 25 times. In general: outer m times, inner n times → m × n times.

Demo: (i, j) for i in range(3), j in range(3) → 9 pairs

for i in range(3):
    for j in range(3):
        print(f"   ({i},{j})", end=" ")
    print()

Part 3: Print on One Line (end=" ")

📌 print(i, j) by default goes to a new line each time. To print on the same line, use: print(i, j, end=" ") Then after the inner loop, use print() to go to the next line (one line per i).

So each "row" is one value of i; each "column" is j.

Demo: grid with end=' ', one row per i

for i in range(3):
    for j in range(3):
        print(i + j, end=" ")
    print()

Part 4: Simple Patterns (condition on i, j)

📌 You can print only when a condition is true. Examples:

Print "*" only when j <= i → triangle of stars (first row 1, second 2, ...).
Print "*" only when i <= j → other triangle.
No condition → full rectangle of stars.

Demo: 5×5 stars (full block)

for i in range(5):
    for j in range(5):
        print("*", end=" ")
    print()

Demo: triangle (print * when j <= i)

for i in range(5):
    for j in range(5):
        if j <= i:
            print("*", end=" ")
        else:
            print(" ", end=" ")
    print()

Part 5: Nested Loops over Strings

📌 for works on any sequence. So you can do: for i in "abc": for j in "xyz": print(i, j) That gives: a x, a y, a z, b x, b y, b z, c x, c y, c z.

Demo: for i in 'ab', j in '12'

for i in "ab":
    for j in "12":
        print(f"   {i}{j}", end=" ")
    print()

Part 6: Summary

✅ Nested loop = loop inside loop. Inner runs fully for each outer iteration.

✅ Total runs = (outer iterations) × (inner iterations).

✅ print(..., end=" ") keeps output on same line; print() starts new line.

✅ Use conditions (e.g. j <= i) to draw patterns. Nested loops work with range, strings, etc.

Congratulations! You now understand Nested Loops in Python! 🎉

Key Takeaways:

Inner loop runs completely for every outer iteration
Total executions = outer × inner
Use end=" " + print() for grids and patterns
Works with strings, ranges, lists, etc.

Next Steps:

Create more patterns (diamond, pyramid, multiplication table)
Try nested loops over two strings
Combine with previous lessons (prime check inside a range)