ATM Machine

February 26, 2019

Today’s exercise asks you to simulate an ATM machine:

  1. Prompt for userid and password. If userid and password are not in the database, reprompt.
  2. Display balance.
  3. Present a menu asking to deposit, withdraw or exit. If user selects withdraw or deposit, perform the indicated transaction and goto Step 2. Deny withdrawals that would overdraw the account.
  4. Exit the program.

Your task is to write a program to simulate an ATM machine. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.


Pages: 1 2

4 Responses to “ATM Machine”

  1. matthew said

    Your suggested solution has a large chunk missing (comparing with the ideone version, looks like a section in angle brackets has disappeared).

  2. programmingpraxis said

    @matthew: Thanks. I think it’s fixed now. On the other hand, last night when I edited the exercise it looked right, too. WordPress has been playing with their editor lately, improving it. Things may change tomorrow.

  3. Daniel said

    Here’s a solution in Python.

    database = {
        'user1': ['password1', 20],
        'user2': ['password2', 100],
        'user3': ['password3', 0],
    while True:
        user = input('user> ')
        password = input('password> ')
        entry = database.get(user)
        if entry is None or entry[0] != password:
            print('login error')
        menu = (
            '[q] quit\n'
            '[d] deposit\n'
            '[w] withdraw'
        while True:
            balance = entry[1]
            print('balance: {}'.format(balance))
            option = input('choice> ')
            if option == 'q':
            elif option == 'd':
                amount = int(input('amount> '))
                entry[1] += amount
            elif option == 'w':
                amount = int(input('amount> '))
                if amount > balance:
                    print('insufficient funds')
                    entry[1] -= amount


    user> user4
    password> password4
    login error
    user> user2
    password> password2
    balance: 100
    [q] quit
    [d] deposit
    [w] withdraw
    choice> d
    amount> 5
    balance: 105
    [q] quit
    [d] deposit
    [w] withdraw
    choice> w
    amount> 200
    insufficient funds
    balance: 105
    [q] quit
    [d] deposit
    [w] withdraw
    choice> q
  4. Tim said

    Scheme version for completeness:

    (import (chicken io) simple-md5 srfi-69)
    (define account-db (make-hash-table))
    (define (prompt-for-line prompt)
      (print* prompt)
    (define (get-value prompt)
      (let ((v (string->number (prompt-for-line prompt))))
        (if (and v (>= v 0))
              (print "Invalid amount.")
              (get-value prompt)))))
    (define (make-account-dispatcher value)
      (define (dispatch)
        (print "\nYou currently have " value " CHF.\n\n"
               "- [D]eposit funds\n"
               "- [W]ithdraw funds\n"
               "- e[X]it")
        (case (string->symbol (prompt-for-line "> "))
          ((D d) (let ((v (get-value "Amount to deposit: ")))
                   (set! value (+ value v))
          ((W w) (let ((v (get-value "Amount to withdraw: ")))
                   (if (> v value)
                       (print "Insufficient funds!")
                       (set! value (- value v)))
          ((X x) (print "Don't forget your card."))
          (else (print "Invalid option.")
    (define (make-login username password)
      (cons username
            (string->md5sum (string-append username password))))
    (define (get-login)
       (prompt-for-line "Username --> ")
       (prompt-for-line "Password --> ")))
    (define (add-account login funds)
      (hash-table-set! account-db login (make-account-dispatcher funds)))
    (define (open-account)
      (print "Assign name and password for new account:")
      (add-account (get-login) 0)
      (print "Account created.  You may now log in."))
    (define (atm)
      (print "\nWelcome to E-Corp ATM\n"
      (let ((login (get-login)))
        (if (hash-table-exists? account-db login)
            ((hash-table-ref account-db login))
            (print "** Error: Invalid account."))
    (add-account (make-login "user1" "pass1") 10)
    (add-account (make-login "user2" "pass2") 20)
    (add-account (make-login "user3" "pass3") 30)

    Example output:

    $ csi atm.scm
    Welcome to E-Corp ATM
    Username --> user1
    Password --> pass1
    You currently have 10 CHF.
    - [D]eposit funds
    - [W]ithdraw funds
    - e[X]it
    > d
    Amount to deposit: 123
    You currently have 133 CHF.
    - [D]eposit funds
    - [W]ithdraw funds
    - e[X]it
    > w
    Amount to withdraw: 12
    You currently have 121 CHF.
    - [D]eposit funds
    - [W]ithdraw funds
    - e[X]it
    > x
    Don't forget your card.

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