Variables and Assignments Chapter Notes | Information and Communications Technology (ICT) for Grade 8 PDF Download

Introduction

This chapter introduces variables and assignments, fundamental concepts in the AP Computer Science Principles course. It covers how variables store and manage data, assignment operations in pseudocode and Python, and different data types like integers, floats, strings, lists, and booleans. Understanding variables and their proper use is crucial for writing effective programs and succeeding in the AP CSP exam.

Variables

A variable acts as a container in your program, holding a value that can change, similar to variables in algebra. They are typically named using letters or descriptive words to represent data.

Assigning and Modifying Variables

In programming, you assign values to variables using an assignment operator, and you can update these values as needed.

• College Board Pseudocode: Uses the ← symbol for assignment.
• Python: Uses the  symbol to assign values, as in a = expression .
• ​The value a variable holds is always the most recently assigned one.

Example: Consider this Python example:

• number = 7
• changed_number = number
• number = 17
• print(changed_number)
• Output: 7, because changed _number was assigned the value of number when it was still 7.

Tracking variable values can be tricky, especially with frequent changes. Techniques like adding extra print   statements or hand-tracing (from Big Idea 1) can help clarify what’s happening in your code.

Data Types

Data types categorize the kinds of data a program can handle, such as integers, strings, lists, and booleans. Each type determines what operations can be performed on the data.

Numerical Data Types

Python supports several numerical types, primarily int and float for AP CSP:

• Integer (int): Whole numbers, positive or negative, e.g., int_example = 5.
• Float (float): Numbers with decimal points, e.g., float_example = 5.52.

Strings

Strings are sequences of characters enclosed in quotation marks. They can represent words, letters, or even numbers as text:

• "Hello, World!"
• "Fiveable"
• "123456789"
• "y + x"

Strings cannot be used for mathematical operations, even if they contain numbers.

Lists

Lists, also called arrays, are ordered collections of items, allowing multiple values to be stored in a single variable. They can hold numbers, strings, or a mix:

• list_example = ["Fish", "fish", "fish"]
• num_list_example = [2, 4, 6, 8]

Booleans

Booleans represent two values: True or False , often used for decision-making:

• Boolean_value = True

Data Type Limitations

The data type of a variable dictates what operations are valid. For example, mathematical operations work on numbers but not on strings:

• y = 25
• x = 5
• print(y / x) # Outputs: 5
• y = "25"
• x = "5"
• print(y / x) # Error: cannot divide strings

Choosing the appropriate data type is crucial for your program’s functionality.

A variable can hold only one value at a time, but that value can be a collection, like a list containing multiple items.

Variable Naming

Use clear, descriptive names for variables to make your code self-documenting and easier to understand, especially in longer programs. Short, single-letter names are fine for small scripts but less ideal for complex ones.

Variable names are case-sensitive and must be spelled correctly:

• number = 7 is distinct from Number = 7.

Key Terms