10  Python conditional loops with while

TipLearning Objectives

• Learn about while conditionals and how they are used

10.1 while Loops in Python

A while loop in Python is a control flow statement that allows code to be executed repeatedly based on a boolean condition. The loop continues to execute as long as the condition remains True. Here’s a breakdown of how while loops work:

Key Components

1. Condition: The loop starts with a condition that is evaluated before each iteration. If the condition is True, the code block within the loop is executed.

2. Code Block: The statements inside the loop are indented, and these will run repeatedly as long as the condition remains True.

3. Increment/Decrement: It’s crucial to modify the variable used in the condition within the loop; otherwise, you may create an infinite loop.

4. Exit: Once the condition evaluates to False, the loop stops, and the program continues with the next line of code following the loop.

Syntax

while condition:
    # Code block to execute
    # Update condition variable

Example

Here’s a simple example to illustrate how a while loop works:

count = 0

while count < 5:
    print("Count is:", count)
    count += 1  # Increment count

Explanation of the Example

• Initialization: The variable count is initialized to 0.

• Condition: The while loop checks if count is less than 5.

• Code Execution: If the condition is True, it prints the current value of count.

• Increment: The line count += 1 increments the value of count by 1 after each iteration.

• Termination: Once count reaches 5, the condition becomes False, and the loop exits.

This can be incredibly useful in simulations. Or when interacting in the environment. Here the number of iterations is not known beforehand and depend on a certain condition being met. They provide a way to repeat actions and process data dynamically within a program.

However

• Infinite Loops: Ensure the condition will eventually evaluate to False to avoid infinite loops.

• Break Statement: You can use the break statement to exit a loop prematurely if needed.

• Continue Statement: The continue statement can skip the current iteration and proceed to the next one based on a condition.

An example is:

import random

population = 1000
infected = 1
days = 0
infection_rate = 1.5
max_days = 30

while infected < population:
    days += 1

    if random.random() > 0.9:
        print(f"Day {days}: Lockdown in effect, no new infections today.")
        continue
    
    new_infections = int(infected * infection_rate)

    if new_infections + infected > population:
        new_infections = population - infected
    
    infected += new_infections

    print(f"Day {days}: {infected} infected.")

    if infected >= population:
        print(f"Day {days}: The entire population is infected.")
        break

    if random.random() > 0.8:
        print(f"Day {days}: Health measures implemented, slowing infection.")
        infection_rate -= 0.3
    
    if infection_rate <= 0.1:
        print(f"Day {days}: The infection has nearly stopped spreading.")
        break
    
    if days >= max_days:
        print("The simulation has reached its time limit.")
        break

ExerciseExercise 1 - Code Legibility

Level:

Discussion: What does the above code do? Is it legibly written? What can be done to improve this?

AnswerAnswer

# Commenting the code appropriately is important!
import random

# Parameters of the simulation
population = 1000       # Total population
infected = 1            # Initially 1 person is infected
days = 0                # Start at day 0
infection_rate = 1.5    # Rate of infection: how many people one infected person can infect per day
max_days = 30           # Maximum number of days to simulate

while infected < population:
    days += 1

    # Simulate random events like lockdown or vaccines
    if random.random() > 0.9:  # 10% chance of stopping the spread for the day
        print(f"Day {days}: Lockdown in effect, no new infections today.")
        continue  # Skip the infection calculation for this day and move to the next

    # Simulate daily infections
    new_infections = int(infected * infection_rate)

    # If new infections exceed the remaining healthy population, adjust them
    if new_infections + infected > population:
        new_infections = population - infected

    infected += new_infections

    # Display day-by-day status
    print(f"Day {days}: {infected} infected.")

    # Continue if there are still people to infect
    if infected >= population:
        print(f"Day {days}: The entire population is infected.")
        break

    # Random chance to reduce the infection rate due to interventions
    if random.random() > 0.8:  # 20% chance to reduce infection rate
        print(f"Day {days}: Health measures implemented, slowing infection.")
        infection_rate -= 0.3  # Decrease the infection rate

    # If the infection rate gets too low, break out of the loop (end of epidemic)
    if infection_rate <= 0.1:
        print(f"Day {days}: The infection has nearly stopped spreading.")
        break

    # Stop the simulation after a max number of days to avoid infinite loops
    if days >= max_days:
        print("The simulation has reached its time limit.")
        break

ExerciseExercise 2 - Code Legibility

Level:

What does the while statement in the expense_calculator.py code do? What are its limitations? Improve the code, by adressing these limitations

AnswerAnswer

Add code introducing a maximum number of iterations or a global valuse cap to force exit and stop an infinite loop You can also add an extra break statement to stop if a user has exceeded a limit

10.2 Summary

TipKey Points

• while loops can be very useful for tasks that do not have a set number of loops allowing simulation and user input
• It is important you do not get infinite loops in your code