Practice Problems: Variable Scope (Pyret & Python)

Solutions are found at the bottom of the page!

Problem 1: Basic Scope Understanding

Problem Statement: Understand the difference between global and local variables by predicting the output of code snippets.

Part A: Pyret Scope For each Pyret code snippet, predict what will be printed or if there will be an error.

# Snippet 1
global-value = 10

fun test-function(x :: Number) -> Number:
  local-value = 5
  x + local-value + global-value
end

result = test-function(3)
# What is the value of `result`?

# Snippet 2
name = "Alice"

fun greet() -> String:
  name + " says hello!"
end

greeting = greet()
# What is the value of `greeting`?

# Snippet 3
var counter = 0

fun increment() -> Number block:
  counter := counter + 1
  counter
end

first-call = increment()
second-call = increment()
# What are the values of `first-call` and `second-call`?

Part B: Python Scope For each Python code snippet, predict what will be printed or if there will be an error.

# Snippet 1
global_value = 10

def test_function(x):
    local_value = 5
    return x + local_value + global_value

result = test_function(3)
# What is the value of `result`?

# Snippet 2
name = "Alice"

def greet():
    return name + " says hello!"

greeting = greet()
# What is the value of `greeting`?

# Snippet 3
counter = 0

def increment():
    counter = counter + 1  # Will this work?
    return counter

# What happens when we call `increment()`?

Problem 2: Pyret vs Python Comparison

Problem Statement: Compare how the same functionality is implemented in Pyret and Python.

Task: Bank Account Simulation Implement the same bank account functionality in both languages:

Pyret Version:

# PYRET
var account-balance = 1000.0
var transaction-count = 0

fun deposit(amount :: Number) -> Number:
  # Add amount to balance, increment transaction count
  # Return new balance
end

fun withdraw(amount :: Number) -> Number:
  # Subtract amount if sufficient funds
  # Increment transaction count
  # Return new balance or raise error
end

fun get-balance() -> Number:
  # Return current balance without modifying it
end

fun get-transaction-count() -> Number:
  # Return number of transactions
end

fun reset-account() -> String:
  # Reset balance to 1000 and transaction count to 0
  # Return confirmation message
end

Python Version:

# PYTHON
account_balance = 1000.0
transaction_count = 0

def deposit(amount):
    # Add amount to balance, increment transaction count
    # Return new balance
    pass

def withdraw(amount):
    # Subtract amount if sufficient funds
    # Increment transaction count
    # Return new balance or raise error
    pass

def get_balance():
    # Return current balance without modifying it
    pass

def get_transaction_count():
    # Return number of transactions
    pass

def reset_account():
    # Reset balance to 1000 and transaction count to 0
    # Return confirmation message
    pass

Solutions

Problem 1:

Part A: Pyret Scope

# result in 18 (3 + 5 + 10)
# greeting is "Alice says hello!"
# first-call is 1, second-call is 2

Part B: Python Scope

# result is 18 (3 + 5 + 10)
# greeting is "Alice says hello!"

def increment():
    counter = counter + 1  # UnboundLocalError
    return counter

# The above raises UnboundLocalError because Python sees the assignment and treats counter as local, but tries to read it before assignment

Problem 2:

Pyret Version:

var account-balance = 1000.0
var transaction-count = 0

fun deposit(amount :: Number) -> Number:
  doc: "deposits amount into account"
  if amount > 0 block:
    account-balance := account-balance + amount
    transaction-count := transaction-count + 1
    account-balance
  else:
    raise("Deposit amount must be positive")
  end
where:
  account-balance := 1000.0
  transaction-count := 0
  
  deposit(100) is 1100.0
  deposit(50) is 1150.0
  transaction-count is 2
end

fun withdraw(amount :: Number) -> Number:
  doc: "withdraws amount from account if sufficient funds"
  if amount > 0:
    if account-balance >= amount block:
      account-balance := account-balance - amount
      transaction-count := transaction-count + 1
      account-balance
    else:
      raise("Insufficient funds")
    end
  else:
    raise("Withdrawal amount must be positive")
  end
where:
  account-balance := 1000.0
  transaction-count := 0
  
  withdraw(100) is 900.0
  withdraw(50) is 850.0
  transaction-count is 2
end

fun get-balance() -> Number:
  doc: "returns current balance without modifying it"
  account-balance
where:
  account-balance := 500.0
  get-balance() is 500.0
end

fun get-transaction-count() -> Number:
  doc: "returns number of transactions"
  transaction-count
end

fun reset-account() -> String block:
  doc: "resets balance to 1000 and transaction count to 0"
  account-balance := 1000.0
  transaction-count := 0
  "Account reset successfully"
where:
  reset-account() is "Account reset successfully"
  get-balance() is 1000.0
  get-transaction-count() is 0
end

Python Version:

account_balance = 1000.0
transaction_count = 0

def deposit(amount):
    """deposits amount into account"""
    global account_balance, transaction_count
    if amount > 0:
        account_balance += amount
        transaction_count += 1
        return account_balance
    else:
        raise ValueError("Deposit amount must be positive")

def withdraw(amount):
    """withdraws amount from account if sufficient funds"""
    global account_balance, transaction_count
    if amount > 0:
        if account_balance >= amount:
            account_balance -= amount
            transaction_count += 1
            return account_balance
        else:
            raise ValueError("Insufficient funds")
    else:
        raise ValueError("Withdrawal amount must be positive")

def get_balance():
    """returns current balance without modifying it"""
    return account_balance 

def get_transaction_count():
    """returns number of transactions"""
    return transaction_count

def reset_account():
    """resets balance to 1000 and transaction count to 0"""
    global account_balance, transaction_count
    account_balance = 1000.0
    transaction_count = 0
    return "Account reset successfully"

Problem 1: Basic Scope Understanding​

Problem 2: Pyret vs Python Comparison​

Solutions

Problem 1:​

Problem 2:​

Problem 1: Basic Scope Understanding

Problem 2: Pyret vs Python Comparison

Problem 1:

Problem 2: