Cmu Cs Academy Answers Key Unit 3

CMU CS Academy Unit 3 Answers: A Comprehensive Guide

This guide provides comprehensive answers and explanations for the exercises within Carnegie Mellon University's (CMU) CS Academy Unit 3. Unit 3 typically covers fundamental programming concepts, building upon the introductory material of previous units. Remember, understanding the why behind the code is crucial, not just memorizing solutions. This guide aims to help you achieve that deeper understanding. We'll break down each section, providing detailed explanations and alternative approaches where applicable. Let's dive in!

Section 1: Basic Control Flow (if, else if, else)

This section reinforces the core concepts of conditional statements. You'll likely encounter problems requiring you to implement logic based on various conditions.

Exercise 1.1: Simple Conditional Statements

Problem: Write a program that checks if a number is positive, negative, or zero.

Solution:

num = float(input())

if num > 0:
    print("Positive")
elif num < 0:
    print("Negative")
else:
    print("Zero")

Explanation: The float(input()) line allows the user to input a number (including decimals). The if, elif, and else statements check the conditions sequentially. Only the code block associated with the first true condition is executed.

Exercise 1.2: Nested Conditional Statements

Problem: Write a program that determines the grade based on a numerical score. (e.g., 90+ is A, 80-89 is B, etc.)

Solution:

score = float(input())

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(grade)

Explanation: This uses nested conditional logic, effectively creating a range-based grading system. The order of the if and elif statements is crucial here, ensuring the correct grade is assigned.

Exercise 1.3: Conditional Expressions (Ternary Operator)

Problem: Rewrite Exercise 1.1 using a conditional expression.

Solution:

num = float(input())
result = "Positive" if num > 0 else ("Negative" if num < 0 else "Zero")
print(result)

Explanation: This demonstrates the ternary operator, a concise way to write conditional logic in a single line. It's particularly useful for simple conditional assignments. While elegant, avoid overly complex nested ternary expressions for readability.

Section 2: Loops (for and while)

This section introduces iterative structures, allowing you to repeat code blocks efficiently.

Exercise 2.1: The for Loop

Problem: Write a program that prints numbers from 1 to 10 using a for loop.

Solution:

for i in range(1, 11):
    print(i)

Explanation: range(1, 11) generates a sequence of numbers from 1 (inclusive) to 11 (exclusive), effectively producing numbers 1 through 10.

Exercise 2.2: Nested for Loops

Problem: Print a multiplication table (e.g., up to 10x10).

Solution:

for i in range(1, 11):
    for j in range(1, 11):
        print(i * j, end="\t")  # \t creates a tab for better formatting
    print()  # Newline after each row

Explanation: The nested for loops create a table. The outer loop iterates through rows, and the inner loop iterates through columns, calculating and printing the product of i and j.

Exercise 2.3: The while Loop

Problem: Write a program that counts down from 10 to 1 using a while loop.

Solution:

count = 10
while count > 0:
    print(count)
    count -= 1

Explanation: The while loop continues as long as the condition (count > 0) is true. count -= 1 decrements the counter in each iteration. Crucially, ensure your while loop condition will eventually become false to prevent infinite loops.

Exercise 2.4: break and continue Statements

Problem: Write a program that sums numbers from 1 to 100, but stops if the sum exceeds 500.

Solution:

total = 0
i = 1
while i <= 100:
    total += i
    if total > 500:
        break  # Exit the loop
    i += 1
print(total)

Explanation: The break statement immediately terminates the loop when the condition is met. The continue statement (not used in this example) would skip the rest of the current iteration and proceed to the next.

Section 3: Functions

Functions are fundamental to modular and reusable code.

Exercise 3.1: Defining and Calling Functions

Problem: Write a function that calculates the area of a rectangle.

Solution:

def rectangle_area(length, width):
    area = length * width
    return area

length = float(input("Enter length: "))
width = float(input("Enter width: "))
area = rectangle_area(length, width)
print("Area:", area)

Explanation: The def keyword defines a function. Parameters (length, width) are input values. The return statement sends the calculated area back to the caller.

Exercise 3.2: Functions with Multiple Return Values

Problem: Write a function that calculates both the area and perimeter of a rectangle.

Solution:

def rectangle_calculations(length, width):
    area = length * width
    perimeter = 2 * (length + width)
    return area, perimeter

length = float(input("Enter length: "))
width = float(input("Enter width: "))
area, perimeter = rectangle_calculations(length, width)
print("Area:", area)
print("Perimeter:", perimeter)

Explanation: Multiple values can be returned using a comma-separated list. The caller must unpack these values using a similar comma-separated assignment.

Exercise 3.3: Function Parameters (Default Arguments)

Problem: Write a function to calculate the volume of a rectangular prism, with a default height of 1.

Solution:

def prism_volume(length, width, height=1):
    volume = length * width * height
    return volume

length = float(input("Enter length: "))
width = float(input("Enter width: "))
volume = prism_volume(length, width) #height defaults to 1
print("Volume:", volume)

volume2 = prism_volume(length,width,2) #height is explicitly set to 2
print("Volume with height 2:", volume2)

Explanation: Default arguments provide flexibility. If the caller omits the height argument, it defaults to 1.

Exercise 3.4: Recursive Functions (Optional, but highly recommended)

Problem: Write a recursive function to calculate the factorial of a number.

Solution:

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n - 1)

num = int(input("Enter a non-negative integer: "))
if num>=0:
    result = factorial(num)
    print("Factorial:", result)
else:
    print("Please enter a non-negative integer.")

Explanation: Recursion involves a function calling itself. The base case (n == 0) stops the recursion; otherwise, it recursively calls itself with a smaller input.

Section 4: Strings and String Manipulation

This section delves into working with textual data.

Exercise 4.1: Basic String Operations

Problem: Concatenate two strings.

Solution:

string1 = input("Enter the first string: ")
string2 = input("Enter the second string: ")
result = string1 + string2
print("Concatenated string:", result)

Explanation: The + operator concatenates strings.

Exercise 4.2: String Slicing

Problem: Extract a substring from a larger string.

Solution:

string = "Hello, world!"
substring = string[7:12]  # Extracts "world"
print("Substring:", substring)

Explanation: String slicing uses [start:end] notation, extracting characters from the start index (inclusive) to the end index (exclusive).

Exercise 4.3: String Methods (e.g., upper(), lower(), find())

Problem: Convert a string to uppercase and find the index of a specific character.

Solution:

string = "Hello, World!"
uppercase_string = string.upper()
index = string.find("o")
print("Uppercase:", uppercase_string)
print("Index of 'o':", index)

Explanation: Python provides many built-in string methods for various manipulations.

Exercise 4.4: String Formatting (f-strings)

Problem: Format a string to include variable values.

Solution:

name = "Alice"
age = 30
formatted_string = f"My name is {name} and I am {age} years old."
print(formatted_string)

Explanation: f-strings offer a clean and efficient way to embed variables within strings.

Conclusion

This comprehensive guide provides detailed solutions and explanations for CMU CS Academy Unit 3 exercises. Remember, the key to mastering programming is not just finding the correct answers but understanding the underlying logic and principles. Use this guide as a resource to deepen your comprehension and build a solid foundation in programming concepts. Practice consistently, explore different approaches, and don't hesitate to experiment! Good luck on your programming journey!