Purpose and Overview

Learning Objectives

1. 1Explain what a value and a variable are, and describe the role of variables in programs.
2. 2Trace assignment statements step by step, including reassignment and accumulator patterns.
3. 3Identify the type of any basic value: integer, float, string, or Boolean.
4. 4Perform and interpret integer arithmetic, including integer division and the remainder operator.
5. 5Describe what strings are and apply string operations including concatenation, repetition, and indexing.
6. 6Construct and evaluate Boolean expressions using comparison and Boolean operators.
7. 7Apply operator precedence rules and use parentheses to control evaluation order.
8. 8Use input() and print() correctly, including type conversion from input strings.
9. 9Perform explicit type conversions and explain the difference between int, float, str, and bool conversions.
10. 10Describe a program's state and trace it through a sequence of assignments using a trace table.
11. 11Read simple programs and predict their output correctly.

Sections at a Glance

• 7.1Values and VariablesWhat values are; variables as named storage; variable naming rules; why variables matter
• 7.2AssignmentThe assignment operator; reading vs. writing; reassignment; right side evaluates first; trace tables
• 7.3NumbersIntegers and floats; arithmetic operators; integer division and remainder; division by zero; rounding errors
• 7.4StringsText data; concatenation; repetition; length; zero-based indexing; escape characters; f-strings
• 7.5BooleansTrue and False; comparison operators; and, or, not; truth tables; = vs == distinction
• 7.6Expressions and OperatorsWhat an expression is; operator precedence; mixing types; compound expressions
• 7.7Input and Outputprint() for output; input() for user data; converting input to numbers; type mismatch errors
• 7.8Type Conversionint(), float(), str(), bool(); implicit vs. explicit; truncation vs. rounding; falsy values
• 7.9State and Changing ValuesWhat state means; accumulator variables; augmented assignment; swapping variables
• 7.10Reading Simple ProgramsReading habits; tracing execution; three complete program examples; the code-reading habit

Values and Variables

What a Value Is

What a Variable Is

Variable Naming Rules

1. In your own words, explain the difference between a value and a variable. Give one example of each.
2. Why is it useful to give a variable a descriptive name rather than a single letter?
3. Which of the following are valid Python variable names? For those that are invalid, explain why.
   
   first_name  ·  2ndScore  ·  total_cost  ·  my variable  ·  _count  ·  MAX

Assignment

The Assignment Statement

age = 17
score = 100
name = "Jordan"
temperature = 98.6
is_active = True

Reassignment

score = 100
score = 95
score = 87
print(score)   # Output: 87

The Right Side Evaluates First

x = 5
x = x + 1    # look up x (5), add 1, get 6, store 6 in x
print(x)      # Output: 6

Trace Tables

Build a trace table for each program and write the output.

1. x = 8  /  y = 3  /  x = x * y  /  print(x)
2. a = 20  /  b = a // 3  /  c = a % 3  /  print(b)  /  print(c)
3. name = "Alex"  /  greeting = "Hello, " + name  /  print(greeting)

Numbers

Integers

students = 30
temperature = -5
items = 0
year = 2025

Floats

price = 4.99
height = 1.75
pi = 3.14159

Arithmetic Operators

1. 15 + 4 * 3
2. 20 // 6
3. 20 % 6
4. 3 ** 3
5. 100 / 8
6. 100 // 8
7. 17 % 5
8. 2 ** 10

Also: A program receives 245 seconds. Using // and %, compute the full minutes and remaining seconds. Show your work.

Strings

String Operations

Zero-Based Indexing

word = "algorithm"
print(word[0])    # a
print(word[4])    # r
print(word[8])    # m
print(len(word)) # 9

f-Strings

name = "Jordan"
score = 94
print(f"Student: {name}, Score: {score}")
# Output: Student: Jordan, Score: 94

1. len("computer")
2. "ab" * 4
3. "Hello" + " " + "there"
4. "science"[3]
5. "science"[0]
6. len("")

Also: Write an f-string that displays a person's full name and age on one line. Define the variables first, then write the print statement.

Booleans

Comparison Operators

Boolean Operators and Truth Tables

1. 10 > 5
2. 3 == 3.0
3. 7 != 7
4. True and False
5. True or False
6. not True
7. 5 > 3 and 10 < 4
8. 5 > 3 or 10 < 4
9. not (5 == 5)
10. not False or False

Expressions and Operators

Operator Precedence

1. 5 + 3 * 2
2. (5 + 3) * 2
3. 2 ** 3 + 1
4. 10 - 4 / 2
5. 15 % 4 + 1
6. 3 > 1 and 4 < 2
7. not 5 == 5
8. 3 > 1 or 4 < 2

Input and Output

Output: print()

print("Hello!")
print(42)
print(3.14)
print("Name:", name, "Age:", age)

Input: input()

name = input("What is your name? ")
print("Hello,", name)

Converting Input to Numbers

age = int(input("Enter your age: "))
next_year = age + 1
print(f"Next year you will be {next_year}")

1. Write a program that asks the user for their name and prints a personalized greeting using an f-string.
2. Write a program that asks for a temperature in Celsius and prints the converted Fahrenheit value. Formula: F = (C * 9/5) + 32. What conversion function do you need to apply to the input?
3. A student writes age = input("Age: ") and then print(age + 1). What error will this produce and why?

Type Conversion

Part A: Identify the type of each value (int, float, str, or bool).

1. 42
2. "42"
3. 3.14
4. True
5. "True"
6. 0
7. ""
8. -7.5

Part B: What does each conversion produce?

1. int(3.9)
2. int(-3.9)
3. float("7")
4. str(100)
5. bool(0)
6. bool(-5)
7. int("hello")
8. bool("")

State and Changing Values

Accumulator Variables

score = 0
score += 10    # score is now 10
score += 25    # score is now 35
score -= 5     # score is now 30
print(score)   # 30

Swapping Two Variables

Build a trace table and write the final output for each program.

1. score = 0 / score += 10 / score += 25 / score -= 5 / score *= 2 / print(score)
2. x = 8 / y = 3 / x, y = y, x / print(x) / print(y)
3. A student runs this code to swap two values, but the result is wrong. Explain why it fails and write the corrected version:
   
   p = 5 / q = 9 / p = q / q = p / print(p, q)

Reading Simple Programs

print("Simple Calculator")
x = float(input("Enter first number: "))
y = float(input("Enter second number: "))

total      = x + y
difference = x - y
product    = x * y

print(f"Sum:        {total}")
print(f"Difference: {difference}")
print(f"Product:    {product}")

first = input("First name: ")
last  = input("Last name: ")
full_name = last + ", " + first
print(f"Hello, {full_name}!")

celsius = float(input("Enter temperature in Celsius: "))
fahrenheit = (celsius * 9 / 5) + 32
print(f"{celsius}°C = {fahrenheit}°F")

Predict the output of each program without running it. Then verify.

Program A:

a = 10
b = 4
c = a + b
a = c - a
print(a)
print(b)
print(c)

Program B:

score = 0
score += 10
score += 25
score -= 5
score *= 2
print(score)

Program C:

word = "science"
print(word[0])
print(len(word))
print(word[3])
print("cs " * 3)

Program D:

x = True
y = False
print(x and y)
print(x or y)
print(not x)
print(x and not y)

Chapter Summary

• Sec 7.1A value is a piece of data. A variable is a named storage location that holds a value. Variables should have clear, descriptive names. Python names start with a letter or underscore and are case-sensitive.
• Sec 7.2The assignment operator = stores a value into a variable. The right side evaluates first. Reassignment replaces the previous value. Trace tables track variable state step by step.
• Sec 7.3Integers are whole numbers; floats have decimal points. Arithmetic operators include +, -, *, **, /, //, and %. Integer division and remainder are especially useful for time, indexing, and parity checks.
• Sec 7.4Strings are sequences of characters. Concatenation uses +; repetition uses *. Strings are indexed from zero. f-strings embed variable values into output cleanly. Strings and numbers are different types and cannot be mixed with + directly.
• Sec 7.5Booleans are True or False. Comparison operators produce Boolean values. Boolean operators and, or, and not combine or modify them. = is assignment; == is comparison — never confuse them.
• Sec 7.6Expressions combine values, variables, and operators to produce one value. Precedence rules determine evaluation order; parentheses override them. Mixing integers and floats produces floats. Mixing strings with numbers requires explicit conversion.
• Sec 7.7print() displays output; input() receives a string from the user. Input must always be converted to the correct type before arithmetic. Type mismatches cause TypeErrors.
• Sec 7.8Type conversion functions: int(), float(), str(), bool(). int() on a float truncates — it does not round. Falsy values include 0, 0.0, "", and None.
• Sec 7.9A program's state is all variables and their current values at any moment. Accumulator variables build up values using += and similar operators. Swapping two variables requires a temporary variable to preserve the original value.
• Sec 7.10Reading a program means tracing execution top to bottom, tracking state at each step. Predict output before running code. The code-reading habit is as important as code-writing and improves debugging, comprehension, and design.

Key Terms Review

Concept Check

Answer these questions to verify understanding before attempting practice problems.

1. What is the difference between a value and a variable?
2. What does the assignment operator = do? How is it different from the mathematical equals sign?
3. What are the four basic data types introduced in this chapter?
4. What does 7 // 3 produce? What does 7 % 3 produce?
5. Why can you not add a string and an integer using + without conversion?
6. What is the difference between = and ==?
7. What does input() always return, regardless of what the user types?
8. What is a trace table, and why is it useful?
9. What does int(3.9) return? Does it round?
10. What is the state of a program?

Skill Practice

Evaluate each expression and write the result.

1. 5 + 3 * 2
2. (5 + 3) * 2
3. 10 // 3
4. 10 % 3
5. 2 ** 8
6. 15 % 4
7. 7 / 2
8. 7 // 2
9. True and False
10. True or False
11. not True
12. 3 > 2 and 10 < 5
13. 3 > 2 or 10 < 5
14. not (5 == 5)
15. len("computer")

Construct a trace table for each program and write the output.

Program B1:

x = 4
y = 7
z = x + y
x = z - x
print(x)
print(y)
print(z)

Program B2:

a = 10
b = 3
a = a * b
b = a // b
print(a)
print(b)

Program B3:

count = 0
total = 0
count += 1
total += 50
count += 1
total += 30
count += 1
total += 70
print(count)
print(total)

Program B4:

word = "science"
first_letter = word[0]
length = len(word)
print(first_letter)
print(length)
print(word[3])

Design and Application Questions

For each question, define your variables clearly, write the program, and state what types you used and why.

1. Write a program that asks the user for two numbers and prints their sum, difference, product, and integer quotient.
2. Write a program that asks for a temperature in Fahrenheit and converts it to Celsius. Formula: C = (F - 32) * 5 / 9.
3. A store charges a base price for an item and adds 8.5% sales tax. Write a program that asks for the base price and prints the tax amount and total price, both rounded to two decimal places.
4. Write a program that asks for a person's first name, last name, and birth year. Print their full name (last, first format) and calculate approximately how old they are (use 2025 as the current year).
5. Write a program that asks for the length and width of a rectangle and prints the area and perimeter.

Debugging and Error Analysis

Each program below contains a bug. Identify the error type (TypeError, ValueError, NameError, logic error), explain what is wrong, and write the corrected version.

score = 85
bonus = "10"
final_score = score + bonus
print(final_score)

greeting = input("Name: ")
message = "Hello, " + greeting
length = len(message)
print("Length: " + length)

radius = float(input("Enter radius: "))
pi = 3.14159
area = pi * radius ^ 2
print(area)

p = 5
q = 9
p = q
q = p
print(p, q)

word = "hello"
print(word[5])

Chapter 7 Checkpoint

Real-World Applications

1. Banking: A bank account program stores the current balance as a float. Every deposit adds to the balance; every withdrawal subtracts. Why should the balance be a float rather than an integer? What would happen if a programmer accidentally stored it as a string?
2. Games: In a video game, a player has a score (int), a health level (float), a username (str), and an active status (bool). For each attribute, explain why that type is appropriate and give one example operation the game might perform on it.
3. Temperature sensors: A weather station records temperatures as floats. Why is float more appropriate than integer for this data? What information would be lost if integers were used?
4. Forms and databases: An online form collects a user's name, age, and email. The name is stored as a string, the age as an integer. Why is it wrong to store the age as a string even though the user types it as text?
5. Calendars: A program receives a total number of days and needs to express it as weeks and remaining days. Write the expression using // and %. Test with 100 days.

Mixed Review

These questions draw on Chapters 1–7.

1. What is binary representation, and why do computers use binary rather than decimal? (Chapter 4)
2. A program needs to find the largest number in a list of 1,000 values. Before writing code, describe the algorithm in pseudocode. (Chapter 5)
3. What is the difference between a syntax error and a logic error? Give an example of each in the context of Chapter 7 material. (Chapter 6)
4. In Chapter 4, you learned that all digital information is stored as bits. Explain how a string like "hello" is ultimately represented in binary in a computer's memory. What role does text encoding (such as ASCII or Unicode) play?
5. Why is abstraction (Chapter 3) relevant to the concept of a variable? What hardware detail does a variable hide from the programmer?