Python Practice

Most of you know Java but are new to Python. Given this, we’re going to do a translation activity, so that you can think about how the languages are similar and different. You will have to solve simple problems using both Python and Java.

1. Python Basics vs Java

1.1. Hello World

Write the equivalent in Python:

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }
}

Answer: In Python, the whole program is just:

print('Hello world!')

No class, no main, no semicolons – just one line.

1.2. Variables & Types

Write the equivalent in Python:

int x = 5;
double y = 3.14;
String name = "Alice";
boolean isHappy = true;

1.3. Exercise

Write a program that takes a string and counts the number of vowels in it.

2. Functions & Control Flow

2.1. Factorial

Write the equivalent in Python:

public static int factorial(int n) {
    if (n <= 1) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

2.2. FizzBuzz

Write the equivalent in Python:

for (int i = 1; i <= 20; i++) {
    if (i % 15 == 0) {
        System.out.println("FizzBuzz");
    } else if (i % 3 == 0) {
        System.out.println("Fizz");
    } else if (i % 5 == 0) {
        System.out.println("Buzz");
    } else {
        System.out.println(i);
    }
}

2.3. For Each Loop

Translate this into Python using a for … in loop:

String[] fruits = {"apple", "banana", "cherry"};

for (String fruit : fruits) {
    System.out.println(fruit.toUpperCase());
}

2.4. Classic For Loop

Translate this into Python using a for loop with the range() function:

for (int i = 0; i < 10; i++) {
    System.out.println("Number: " + i);
}

2.5. Exercise

Write a function that takes a list of numbers and returns a list of their squares. (Do it once with a loop, once with a comprehension.)

3. Objects & Classes

3.1. Simple Class

Translate this into a Python class.

public class Car {
    private String make;
    private String model;

    public Car(String make, String model) {
        this.make = make;
        this.model = model;
    }

    public void drive() {
        System.out.println("Driving " + make + " " + model);
    }
}

3.2. Inheritance

Translate into Python with inheritance:

public class ElectricCar extends Car {
    private int batteryLife;

    public ElectricCar(String make, String model, int batteryLife) {
        super(make, model);
        this.batteryLife = batteryLife;
    }

    public void charge() {
        System.out.println("Charging the car...");
    }
}

3.3. Exercise

Create a BankAccount class with methods:

deposit(amount)
withdraw(amount)
getBalance()

Midterm Exam Study Questions

Each challenge can be solved with nested loops, but a dictionary-based solution will be much more efficient. You should be able to implement the solutions to these practice problems using both the nested loop and the dictionary solution (but the latter is more efficient):

1. Word Frequency Counter:
   Given a paragraph of text, count how many times each word appears.
   • Naive way: For each word, loop through the list to count occurrences.
   • Efficient way: Use a dictionary with words as keys and counts as values.
2. First Duplicate Finder:
   Given a list of integers, find the first number that repeats.
   • Naive way: Nested loops checking every pair.
   • Efficient way: Keep a dictionary of numbers seen so far.
3. Anagram Checker:
   Given two strings, check if they are anagrams of each other. An anagram is a word or phrase formed by rearranging the letters of another word or phrase, using all the original letters exactly once. Examples:
   • “listen” → “silent”
   • “evil” → “vile”
   • “rail safety” → “fairy tales”
4. Student Grades:
   You are given a list of (student, grade) pairs. Compute the average grade for each student.
   • Naive way: For each student, scan the entire list for all their grades.
   • Efficient way: Use a dictionary mapping student → list (or sum/count).
5. Two-Sum Problem:
   Given a list of numbers and a target, find two numbers that add up to the target.
   • Naive way: Check all pairs with nested loops.
   • Efficient way: Use a dictionary to store numbers seen and check complements.
6. Inventory Management:
   You have two lists:
   • items = ["apple", "banana", "apple", "orange", "banana", "apple"]
   • prices = {"apple": 2, "banana": 1, "orange": 3}

   Compute the total cost of the shopping list.

7. Character Frequency Histogram:
   Given a string, produce a dictionary showing how many times each character appears.

8. Course Enrollment:
   You have a list of (student, course) pairs. Build a dictionary mapping each course → list of students enrolled.

9. Common Elements in Two Lists:
   Find the elements that appear in both lists.

10. Most Frequent Number:
    Given a list of integers, find the number that occurs most frequently.