Lecture-17_4 Iteration Python - nested while 24-25

Introduction to Iteration in Python

Focus: Understanding nested while loops
Topic: Iteration and control flow in Python programming using nested loops.

Nested Loops Overview

Nested loops allow loops to contain other loops, providing a way to iterate through collections or repeat tasks driven by another iteration.
Types of nested loops:
- For loop with nested for loop
- For loop with nested while loop
- While loop with nested while loop
- While loop with nested for loop
Outer Loop: The primary loop that contains another loop.
Inner Loop: The nested loop contained within the outer loop.
An outer loop can contain multiple inner loops.

Control Structures in Nested Loops

Outer Loop Control: The outer loop dictates the number of times the inner loop will execute.
Inner Loop Execution: For each iteration of the outer loop, the inner loop executes all its code, possibly multiple times.
Calculation of Total Iterations: Total iterations can be calculated as:
- Total iterations = (outer loop iterations) * (inner loop iterations)
- Example: Outer loop iters = 3, inner loop iters = 7 → Total iterations = 3 * 7 = 21 iterations.

Example of Nested While Loops

Use case: Grading an unknown number of student submissions.

Code Snippet:

student = input(‘Students y/n: ‘)
while student == 'y':
    work = input ('Work to grade y/n: ')
    while work == 'y':
        grade = int(input('Enter grade: '))
        work = input ('More work y/n: ')
    student = input (‘Another student y/n: ')

Iterating Through Data Sets

Task: Find integer 8 in a matrix of 9 rows and 38 columns.
Given data set: A matrix containing digits which needs iteration to find specified values.
- Example Data (simplified):
  - 62524262625363363309774646435353635355
  - ...

Handling Control Flow with Break Statement

Example of using break in a nested while loop to terminate execution at a certain condition.
```
nums = 10
while nums > 0:
    nums -= 1
    if nums == 5:
        break
    print(nums)
```
Caution: break only exits the current loop but can cause issues if overused in nested structures.

Using Continue Statement

continue skips the current iteration but continues the loop.

nums = 10
while nums > 0:
    nums -= 1
    if nums == 5:
        continue
    print(nums)

While-Else Construct

The else block after a while loop executes only when the loop ends due to a false condition, not via break.

Example:

number = 1
while number < 6:
    print(number)
    number += 1
else:
    print(‘number is now 6’)

Practical use of while-else is limited; clearer structure is preferred without it.

Level-2 Nesting: Combining Constructs

Example of combining while-else with if statements for user input attempts.

attempt = 3
while attempt > 0:
    password = input("Enter your password:")
    if password == 'COMP101':
        print("Password Accepted.")
        break
    else:
        attempt -= 1
else:
    print("Maximum attempts reached - access failed")

The loop handles input attempts and employs break efficiently.

Python-Specific Syntax

While-else Structure: Syntax overview and notes about practical implications.
Caution against overuse due to possible logic confusion and implementation complexity.

Use of Pass Statement

pass serves as a placeholder for future code, effectively doing nothing but maintaining structure.
```
nums = 10
while nums > 0:
    nums -= 1
    if nums == 5:
        pass
```

Flow Control Statements Categorization

Control statements in Python are categorized as:
1. Conditional Statements (evaluate True or False)
2. Transfer Statements (alter execution flow)
3. Iterative Statements (repeat code blocks indefinitely based on condition)

Limitations and Best Practices

Python supports a maximum of 20 levels of nested structures.
Check design choices when using significant nesting for better readability and maintenance.