Welcome to the Bank of CS

Where we work hard not to lose any of your money!

(At least...not on purpose.)

All money stored as integers (Ex: 123 cents)
All input/output shown as floats (Ex: "$1.23")

Starter Code

Download: jmu_bank.py

Functions defined so far:

• money()
• open_account()
• deposit()
• withdraw()

Exercise 1

Fix the bug in money()

• This function is used to format money for output
• Ex: money(1234500) should return "$12,345.00"
• Hint #1: Assign the variables dollars and cents
• Hint #2: Format with :,d and :02d (in an f-string)

Global Variables

import random

accounts = []  # account numbers
balances = []  # money, in cents

numbers = list(range(10000, 100000))
random.shuffle(numbers)

• range is a built-in sequence type
• numbers is a list of all 5-digit positive integers
• Can you guess what random.shuffle() does?

Using the Bank

>>> open_account()
  Opening new account
  Opened account #81017
81017

>>> deposit(81017, 123.4)
  Depositing $123.40 into account #81017
  New balance is $123.40
123.4

>>> withdraw(81017, 23.4)
  Withdrawing $23.40 from account #81017
  New balance is $100.00
100.0

Exercise 2

Define a test_bank() function:

• Open an account
• Deposit into account
• Withdraw from account

Add these lines at the very end:

if __name__ == "__main__":
    test_bank()

Exercise 3

Run with Thonny's debugger:

• Set a breakpoint on the line that calls test_bank()
• Step through the entire program, one line at a time
• Pay attention to variables when functions are called

If you haven't already, enable View > Variables (for globals)

Local Variables

• When a function is called, a new "frame" is created
• New variables are created/visible within that frame
• When a function returns, the variables are deleted

def deposit(acct, amount):
    amount = int(round(amount * 100))
    index = accounts.index(acct)
    balances[index] += amount
    return balances[index] / 100

• Parameters: acct and amount
• Local variable: index

Modifying Global list

Global variables in jmu_bank.py

• numbers (list): available account numbers
• accounts (list): numbers of open accounts
• balances (list): balances of open accounts

def open_account():
    acct = numbers.pop(0)
    accounts.append(acct)
    balances.append(0)
    return acct

Exercise 4

Write a close_account(acct) function

Ex: Account 12345 has a balance of $6.78

• At the start, print "Closing account #12345"
• Get the index for the account (and balance)
• Pop the index from accounts and balances
• Print "Closed account #12345 with $6.78"
• Return the account's balance (as a float)

Hint: Look at the use of index in deposit()