Oct 7, 2020 Notes

IT 212 -- Oct 7, 2020

Review Exercises

Answer these three Zoom Poll questions. The correct answer is marked with *:
1. What are the meanings of these Python operators?
  = ==
  a. numeric comparison; string comparison
  b. bitwise comparison; assignment
  c. object comparison; numeric comparison
  *d. assignment; comparison
2. What are the meanings of these Python operators?
  and &
  a. logical or; bitwise exclusive or
  *b. logical and; bitwise and
  c. logical bitwise or; concatenationbitwise
  d. and; concatenation
3. What are the meanings of this Python operator?
  *
  a. multiplication and exponentiation
  *b. multiplication and repetition
  c. multiplication only
  d. exponentiation only

How many stars do these for loops print?

# a. Sequential for loops.
for i in range(0, 3):
    print('*')
for i in range(0, 2):
    print('*')
# Ans: Loops are sequential, so add: 
#      3 + 2 = 5.

# b. Nested for loops.
for i in range(0, 3):
    for j in range(0, 2):
        print('*')
# Ans: Loops are nested so multiply: 
#      3 * 2 = 6

# c. Watch out for the indentation.        
for i in range(0, 9):
    print('*')
    print('*')
print('*')
# Ans: 9 * 2 + 1 = 19

Answer these five Zoom Poll questions about for loops.
1. ```
for i in range(0, 200):
    print("*")
for j in range(0, 300):
    print("*")
for k in range(0, 500):
    print("*")
```
  a. 200 b. 300 c. 500 *d. 1,000
2. ```
for i in range(0, 200):
    for j in range(0, 300):
        for k in range(0, 500):
            print("*")
```
  a. 500 b. 1,000 c. 300,000 *d. 60,000,000
3. ```
for i in range(0, 200):
    for j in range(0, 300):
        print("*")
for i in range(0, 500):
    print("*")
    print("*")
print("*")
```
  a. 1,000 b. 1,003 *c. 61,001 d. 30,000,000
4. The class A has both __str__ and __repr__ dunder methods defined. For the statements
```
a = A( )
print(a)
```
  which dunder method is used to represent a as a string?
  *a. __str__ b. __repr__
5. The class A has both __str__ and __repr__ dunder methods defined. For the statements
```
a = A( )
print([a])
```
  which dunder method is used to represent a as a string within the list?
  a. __str__ *b. __repr__

BankAccount Class

Here are the instance members obtained for the BankAccount class by the breakout groups in class yesterday (Oct 5):
Instance Variables: acct_num acct_type name email balance
Instance Methods: __init__ deposit withdraw __str__

UML Diagram:

+-----------------------------------------------------+
|                     BankAccount                     |
+-----------------------------------------------------+
| + acct_number : int                                 |
| + acct_type : str                                   |
| + name : str                                        |
| + email : str                                       |
| + balance : float                                   |
+-----------------------------------------------------+
| - __init__(self : BankAccount, acct_number : int,   |
|         acct_type : str, name : str, email : str)   |
| + deposit(self : BankAccount, amt : double) : bool  |
| + withdraw(self : BankAccount, amt : double) : bool |
| - __str__(self : BankAccount) : str                 |
+-----------------------------------------------------+

Source Code with Errors:
BankAccount Class Test1 Module
Each source code module has about 10 errors. Fix the errors.

Practice Problem

Using the Person class (see the Person Example), create and print list of Person objects from the data in the input file persons.txt. Once you get your script (named test3.py) working, try it on this larger input file: kids.txt. This second file uses space delimiters, not comma delimiters.
Ans:

# Module: person.py
class Person:

    def __init__(self, the_name, the_gender, the_age):
        self.name = the_name
        self.gender = the_gender
        self.age = the_age

    def have_birthday(self):
        self.age += 1

    def __str__(self):
        return f"{self.name};{self.gender};{self.age}"

    def __repr__(self):
        return str(self)

#----------------------------------------------------
# Module: test3.py
from person import Person

# Initialize list of Person objects
persons = [ ]

fin = open("persons.txt", "r")
print(fin)

# Throw away first line
fin.readline( )

for line in fin:
    fields = line.split(",")
    # strip eliminates leading or trailing white
    # space
    name = fields[0].strip( )
    gender = fields[1].strip( )
    age = int(fields[2].strip( ))
    p = Person(name, gender, age)
    persons.append(p)

fin.close( )
print(persons)
#------------------------------------------------------

To read the kids.txt file, replace the line

fin = open("persons.txt", "r")

fin = open("persons.txt", "r")

Also replace the line

fields = line.split(",")

fields = line.split(" ")

Sorting

We will finish talking about sorting a list of Person objects next week.
To sort a list of items, use the sort instance method.

Sort these lists:

a = ['dog', 'gerbil', 'cat', 'rabbit', 'mouse']
b = [28, 15, 93, 31, 11, 10]
Ans:
a.sort( )
print(a)
b.sort( )
print(b)

What happens when you try to sort an array of Person objects?
Ans: You get this error message:
```
TypeError: '<' not supported between 
    instances of 'Person' and 'Person'
```
To sort an array of user defined objects, define a dunder method __lt__ that defines a less than ( < ) operator for the class.
For example, to sort Person objects by age, use this __lt__ method:
```
def __lt__(self, other):
    return self.age < other.age
```

Here are traditional and unit tests for the Person < operator:

# Source code file: test1.py
from person import Person
p1 = Person("Alice", "F", 11)
p2 = Person("Bob", "M", 13)
print(p1 < p2, p2 < p1)

# Source code file: test2.py
from person import Person
import unittest

class MyUnitTestClass(unittest.TestCase):
    def test_1(self):
        self.assertEqual(p1 < p2, True)
        self.assertEqual(p2 < p1, False)
        
if __name__ == '__main__':
    unittest.main( )