Python Nested For Loop

Nested For Loop in Python

In Python, a nested for loop is a loop inside another loop. The inner loop executes completely for each iteration of the outer loop, making it useful for working with multi-dimensional data structures such as matrices, grids, and nested lists. In this tutorial, we will explore how nested for loops work in Python with examples.

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Examples

1. Printing a Pattern using Nested For Loops

Nested loops can be used to generate patterns, such as a right-angled triangle made of stars.

# Number of rows
rows = 5

# Outer loop for rows
for i in range(1, rows + 1):
    # Inner loop for columns
    for j in range(i):
        print("*", end=" ")
    print()  # Move to the next line

Explanation:

1. The outer loop (for i in range(1, rows + 1)) iterates over the rows.
2. The inner loop (for j in range(i)) runs i times in each iteration of the outer loop, printing *.
3. The print() at the end moves the cursor to a new line after each row.

Output:

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

2. Iterating Over a 2D List (Matrix)

We can use nested loops to iterate through a two-dimensional list (a matrix).

# Defining a 2D list (Matrix)
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

# Iterating over the rows
for row in matrix:
    # Iterating over the elements in each row
    for element in row:
        print(element, end=" ")
    print()  # Move to the next line

Explanation:

1. The outer loop (for row in matrix) iterates over each list (row) in the matrix.
2. The inner loop (for element in row) iterates over elements in each row.
3. print(element, end=" ") prints elements in a single row, and print() moves to the next line.

Output:

1 2 3
4 5 6
7 8 9

3. Generating Multiplication Table

Nested loops can be used to generate a multiplication table.

# Multiplication table up to 5
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i * j:2}", end=" ")
    print()  # Move to the next line

Explanation:

1. The outer loop (for i in range(1, 6)) controls the row numbers (multiplicands).
2. The inner loop (for j in range(1, 6)) iterates through multipliers.
3. i * j calculates the product and prints it in a formatted way.

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

4. Finding Prime Numbers in a Range

We can use nested loops to check if numbers in a range are prime.

# Find prime numbers from 2 to 20
for num in range(2, 21):
    is_prime = True
    for div in range(2, int(num ** 0.5) + 1):
        if num % div == 0:
            is_prime = False
            break
    if is_prime:
        print(num, end=" ")

Explanation:

1. The outer loop (for num in range(2, 21)) iterates through numbers from 2 to 20.
2. The inner loop (for div in range(2, int(num ** 0.5) + 1)) checks if num is divisible by any number between 2 and its square root.
3. If a divisor is found, is_prime is set to False, and we exit the loop.
4. If no divisor is found, the number is prime and printed.

Output:

2 3 5 7 11 13 17 19

Conclusion

Nested for loops in Python are useful for iterating over multi-dimensional data and performing operations like pattern printing, working with matrices, generating multiplication tables, and more. Here are the key takeaways:

1. The outer loop runs first, and the inner loop completes its execution for each iteration of the outer loop.
2. They are useful in handling 2D lists, matrices, and structured data.
3. Breaking out of the inner loop does not affect the outer loop.