Control Structures and the while Loop in Python

Structure of Programs

Overview of Program Control Structures

In Python programming, control structures determine the flow of execution within a program. The three fundamental types are sequence, decision, and repetition structures.

• Sequence Structure: Executes statements one after another in a linear order.

• Decision Structure: Executes statements based on conditions (e.g., if, elif, else).

• Repetition Structure: Repeats a block of code as long as a condition is true (e.g., while loops).

Structure Type	Description	Example
Sequence	Linear execution of statements	input → processing → output
Decision	Conditional branching	if-else statements
Repetition	Looping based on condition	while loops

Decision Structures

Conditional Execution in Python

Decision structures allow a program to choose different paths of execution based on boolean conditions.

• if Statement: Executes a block if the condition is true.

• if-else Statement: Executes one block if the condition is true, another if false.

• if-elif-else Statement: Allows multiple conditions to be checked in sequence.

Syntax Example:

if condition: expression elif another_condition: expression else: expression

The condition must evaluate to True or False.

The while Loop

Introduction to the while Loop

The while loop is a fundamental repetition structure in Python. It repeatedly executes a block of code as long as a specified condition remains true.

• Syntax:

while condition: indented block of statements

• The condition is a boolean expression.

• The loop body is executed repeatedly until the condition becomes false.

Planning the while Loop

Effective use of the while loop requires careful planning of initialization, condition, and update steps.

• Initialize variables before the loop.

• Update variables within the loop to eventually make the condition false.

Example: Compound Interest Calculation

balance = 100 TARGET = 1000 RATE = 0.05 years = 0 while balance < TARGET: balance = balance * 1.05 years = years + 1 print("Your balance is ${0:,.2f} after {1:n} years.".format(balance, years))

Formula:

$\text{New Balance} = \text{Old Balance} \times (1 + \text{Rate})$

while Loop Syntax and Common Errors

Proper syntax and indentation are crucial for while loops in Python.

• Statements inside the loop must be indented to the same column position.

• Place a colon : after the while condition.

Common Errors:

• Incorrect Test Condition: The loop may not execute if the condition is wrong.

• Infinite Loops: Occur if the test variable is not updated, causing the condition to never become false.

• Off-by-One Errors: Mistakes in initializing or updating counters can lead to executing the loop one time too many or too few.

Types of while Loops

Event-Controlled Loops

An event-controlled loop continues until a specific event occurs, such as reaching a target value.

• Example: Continue looping until balance reaches TARGET.

Count-Controlled Loops

A count-controlled loop uses a counter variable to determine how many times the loop executes.

counter = 1 while counter <= 10: print(counter) counter = counter + 1

Input Validation and Sentinel Values

Input Validation Using while Loops

while loops are commonly used to validate user input, ensuring that only acceptable values are processed.

• Prompt the user until a valid input is received.

response_list = ['1', '2', '3'] response = '0' while response not in response_list: response = input("Enter 1, 2, or 3: ")

Sentinel Values

A sentinel value is a special value used to signal the end of input. It is not part of the data set.

• Commonly used sentinel: -1 for numeric input.

sum1 = 0 i = 0 number = eval(input("Enter a number: ")) while number != -1: i += 1 sum1 += number number = eval(input("Enter a number: ")) if i > 0: print("The average is {0:.2f}".format(sum1/i)) else: print("No values entered.")

Common Loop Algorithms

Summing and Averaging Values

• Initialize total and count to zero.

• Use a while loop with a sentinel value to process input.

• Divide total by count to compute the average.

Counting Matches

• Count occurrences of a specific condition (e.g., negative numbers).

negatives = 0 number = input("Enter value: ") while number != "": value = eval(number) if value < 0: negatives += 1 number = input("Enter value: ") print("There were", negatives, "negative values.")

Finding Maximum and Minimum Values

• Initialize largest or smallest with the first input value.

• Update the variable if a new input is larger or smaller.

inputStr = input("Enter a value: ") if inputStr != "": largest = float(inputStr) inputStr = input("Enter a value: ") while inputStr != "": value = float(inputStr) if value > largest: largest = value inputStr = input("Enter a value: ") print("The maximum is:", largest) else: print("No values entered.")

Comparing Adjacent Values

• Detect duplicate consecutive inputs.

value = input("Enter a value: ") if value == "": print("No values entered.") else: value = eval(value) inputStr = input("Enter a value: ") while inputStr != "": previous = value value = int(inputStr) if value == previous: print("Duplicate input") inputStr = input("Enter a value: ")

Loop Control Statements

The break Statement

The break statement immediately terminates the loop when executed, typically used within an if statement.

while True: num = eval(input("Enter a nonnegative number: ")) if num == -1: break # process num

The continue Statement

The continue statement skips the rest of the current loop iteration and returns to the loop header.

for x in my_list: if not isinstance(x, int): continue if x % 11 == 0: print(x, "is the first int divisible by 11.") break

Creating Menus with while Loops

Menu-Driven Programs

Menus allow users to make selections repeatedly until they choose to exit.

while True: print("1. Capital") print("2. National Bird") print("3. National Flower") print("4. Quit") num = int(input("Make a selection from the menu: ")) if num == 1: print("Washington, DC is the capital of the United States.") elif num == 2: print("The American Bald Eagle is the national bird.") elif num == 3: print("The Rose is the national flower.") elif num == 4: break

Summary Table: Loop Control Statements

Statement	Purpose	Typical Use
break	Exit loop immediately	Terminate on special input or event
continue	Skip to next iteration	Skip unwanted values

Additional info: These notes cover foundational control structures and loop algorithms in Python, suitable for introductory programming courses. Examples and tables have been expanded for clarity and completeness.