Iteration Chapter Notes | AP Computer Science Principles - Grade 9 PDF Download

Introduction

Iteration is a fundamental concept in AP Computer Science Principles, allowing programs to repeat tasks efficiently. This chapter explores iterative statements, also known as loops, which repeat a block of code until a specific condition is met. It covers two main types of loops: one that repeats a set number of times and another that repeats until a condition is true. The chapter also discusses how to avoid infinite loops and how to use logical operators in loops. Understanding iteration helps programmers automate repetitive tasks and make programs more efficient.

Iterative Statements

• Iterative statements, or loops, are programming tools that repeat a set of instructions multiple times.
• They continue running until a specific condition is met.
• Loops help automate repetitive tasks, making programs more efficient.

Types of Loops

There are two types of loops:

Repeat n Times

• This loop repeats a block of code a specific number of times, represented by 'n'.
• In pseudocode:
  • REPEAT n TIMES { block of statements }
  • The block of statements runs 'n' times, where 'n' can be a number or a variable.
• Example in pseudocode:
  • If n = 5, REPEAT n TIMES: print (n) will print the value of n five times.
• In Python, this is similar to a for...in range loop:
  • Example: for x in range(0, 6) runs 5 times (from 0 to 5, excluding 6).
  • Can also be written as: for x in range(6), which also runs 5 times starting from 0.
• For...in range loop:
  • A loop that repeats a block of code for a set number of times based on a range of numbers.

Repeat Until (condition)

• This loop repeats a block of code until a specific condition becomes true.
• In pseudocode:
  • REPEAT UNTIL (condition) { block of statements }
  • The block of statements runs until the Boolean condition evaluates to true.
• In Python, this is similar to a while loop, which runs while a condition is true and stops when it becomes false.
• Example in Python:
  • If tacos = 5, while tacos > 0: print("Nom Nom"); tacos -= 1 prints "Nom Nom" 5 times.
  • tacos -= 1 reduces the tacos variable by 1 each loop; can also be written as tacos = tacos - 1.
• While loop:
  • A loop that runs a block of code as long as a condition remains true.
• REPEAT UNTIL (condition) loop:
  • A loop that repeats until a condition becomes true, checking the condition after each iteration.

Iteration Practice Problem

• Example in Python to match REPEAT UNTIL format:
  • Code: while not tacos == 0: print("Nom Nom"); tacos -= 1
  • Using the NOT operator, the loop runs until tacos equals 0.
  • tacos -= 1 reduces the tacos variable by 1 each loop to prevent the loop from running forever.
• If the tacos variable isn’t reduced (e.g., tacos -= 1 is missing), the loop becomes an infinite loop, repeating forever until the program crashes or stops.
• Infinite loops:
  • Loops that run indefinitely because the condition is always true or no condition exists.
• If the condition is already true at the start (e.g., tacos = 0; while tacos > 0), the loop won’t run at all.

More Operators in Loops

• Loops can use AND and OR operators to combine conditions.
• AND operator:
  • Returns true only if both conditions are true; otherwise, returns false.
  • Example: while tacos > 0 and avocados > 0: reduces both tacos and avocados by 1 each loop until one reaches 0.
• OR operator:
  • Returns true if at least one condition is true; otherwise, returns false.
  • Example: while tacos > 0 or quesadillas > 0: prints "Nom Nom" and reduces both tacos and quesadillas by 1 until both reach 0.
  • In the example with tacos = 5 and quesadillas = 10, it prints "Nom Nom" 10 times because the loop continues until both conditions are false.

Key Terms

• AND Operator: A logical operator that evaluates to true only if both conditions are true; otherwise, it returns false.
• For...in range loop: A Python loop that iterates over a sequence of numbers for a set number of times, defined by the range function.
• Infinite Loops: Loops that run indefinitely due to a condition that never becomes false or the absence of a terminating condition.
• Iterative Statements: Also called loops, these allow repeated execution of code based on a condition, automating repetitive tasks.
• OR Operator: A logical operator that evaluates to true if at least one condition is true; otherwise, it returns false.
• REPEAT UNTIL (condition) loop: A loop that executes until a specified condition becomes true, checking the condition after each iteration.
• While loop: A Python loop that repeatedly executes as long as its condition remains true, checking the condition before each iteration.