Nested Loops

A nested loop is a loop inside another loop. The inner loop completes all its iterations for every single iteration of the outer loop.

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

Output:

i=1, j=1
i=1, j=2
i=1, j=3
i=2, j=1
i=2, j=2
i=2, j=3
i=3, j=1
i=3, j=2
i=3, j=3

Example: Multiplication table (1–5)

for i in range(1, 6):
    for j in range(1, 11):
        print(i * j, end="\t")
    print()   # newline after each row

Example: Star pattern (right triangle)

rows = 5
for i in range(1, rows + 1):
    for j in range(i):
        print("*", end="")
    print()

Output:

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

Example: Number pyramid

rows = 4
for i in range(1, rows + 1):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

Output:

1
1 2
1 2 3
1 2 3 4

while inside for (mixed nesting)

for i in range(3):
    j = 0
    while j < 2:
        print(f"outer={i} inner={j}")
        j += 1

Time complexity note

Nested loops running n times each cost O(n²) — important to remember when analyzing performance of pattern-printing or matrix programs.