Day 2: Conditionals

CMSC 201: Conditionals P1

HW0

Went pretty OK I think :)

Reminder: Do not submit HW through blackboard for this class, it will not be looked at. (You can submit something if it makes you feel good! But it will still not be looked at). TAs will grade sometime between soon and soonish.

HW1 is much more in depth and worth more points! Some of you have already started, that's great!

Looking at HW1: input, printing, and variables

Open source software

We know the difference between compiled vs interpreted programs: compiled are very hard to read (without special tools).

If you wrote a program and distributed just the binary, it would be very difficult for people to understand or change!

You could easily:

• Collect and sell user data (e.g., Meta/facebook)
• Hide malicious code in it (e.g., Sony rootkit scandal)
• Write very bad code and sell it, with no ways for users to change it (e.g., crowdstrike)

Open source software or free and open source software (FOSS) is code that is licensed to assure the following freedoms:

(from gnu.org): A program is free software if the program's users have the four essential freedoms: [1]

• The freedom to run the program as you wish, for any purpose (freedom 0).
• The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
• The freedom to redistribute copies so you can help others (freedom 2).
• The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.

Common memes:

• Free as in freedom (or libre, freedom, vs gratis, free of charge). Facebook is gratis, Linux is libre
• Thankless open source maintainer whose software runs the whole world (probably like, bind9 developers)

  Obligatory XKCD 2347:

  
• Uhm, what you're referring to as Linux is actually GNU/Linux...

Open source software is very cool and much of the world runs on it. It's written almost entirely by volunteers. I'm running almost all free software!

Who cares?

Let's continue with the game theme :) Let's make a dialogue tree! We'll use input, variables, and a bit of conditionals. Ours will be text only, naturally :)

Conditionals

Basic idea: IF this, then THAT. And sometimes, IF this, then THAT, otherwise THIS.

if temperature > 70:
    print("summer clothes!")

if temperature > 70:
    print("summer clothes!")
else:
    print("maybe wear a jacket")

The if statement

if condition:
    print("if body")
print("rest of program")

condition can be anything that evaluates to a boolean (or Truthy/Falsy...) value. Such as:

• True
• False
• 3 > 4
• input() == "hi"
• 60 + 100 < 1000
• int(input()) >= 60

Now you see the power of conditionals. We can make decisions on ANYTHING!

print("welcome!")
if input() == "hello":
    print("hello!")
print("how can I help you?")

The else statement

if condition:
    print("if body")
else:
    print("else body")
print("rest of program")

The elif statement

if condition:
    print("if body")
elif condition:
    print("elif body")
else:
    print("else body")
print("rest of program")

if condition:
    print("if body")
elif condition:
    print("elif body")
elif condition:
    print("elif body")
elif condition:
    print("elif body")
else:
    print("else body")
print("rest of program")

else and elif are used for mutually exclusive conditions. Use them when you only want ONE path to execute.

These two programs are different. How?

power_level = int(input())
if power_level < 10:
    print("your power level is too low; I am impervious to attack!")
if power_level % 2 == 0:
    print("your power level is even! I am too odd; I am impervious to attack!")
else:
    print("oh no!")

power_level = int(input())
if power_level < 10:
    print("your power level is too low; I am impervious to attack!")
elif power_level % 2 == 0:
    print("your power level is even! I am too odd; I am impervious to attack!")
else:
    print("oh no!")

The latter one uses elif, making each if "body" mutually exclusive (meaning only one of them will run).

Truthy/Falsey values

Python tries to be helpful by having things that are not booleans still evaluate to true or false.

• ints and floats: anything except 0 is true
• string: any non-empty string

The None type

Python has a special "type" or object: None. It's the closest thing Python has to a NULL or empty value. It represents a value of nothingness. It is always considered false (False).

Preview of next class's material, to prime the mind, and help demonstrate what we learned!

Combining conditions

Python keywords and and or let us combine multiple conditionals into one statement.

if temperature > 70 and ac_broken == True:
    print("summer clothes!")

We can group these with parentheses. Normally they are evaluated left to right.

True and False

True and (False or True)

Short circuiting

When the outcome of a condition with multiple parts is obvious, Python won't bother evaulating unnecessary parts.

Why are these two code blocks different?

temperature = 60
if temperature > 70 and ac_broken == True:
    print("summer clothes!")

temperature = 80
if temperature > 70 and ac_broken == True:
    print("summer clothes!")

Cursed example: Combining our type lore knowledge, what would this print?

if print("condition one") and print("condition two"):
    print("wow!")

Review: Operators

Let's write some code! Dialogue tree time :)

Here's what I'm imagining:

• Shopkeeper interaction: quality of interaction determines price
• Buy an item

# shopkeeper.py --> cool shopkeeper interaction
item = ""
discount = 1
currency = 15

## interact : determines discount
print("Shopkeep: Welcome to walmart!")
user_greeting = input("User: ")

if user_greeting == "beanz":
    discount = 0.5
elif user_greeting == "Soupz":
    discount = 0.7

## buy item
print("Shopkeep: What do you want?")
print("TP - $5")
print("Milk - $3")
user_item = input("User: ")

if user_item == "TP":
    price_after_discount = 5 * discount
    currency -= price_after_discount
    item = "TP"
    print("Cool!")
elif user_item == "Milk":
    price_after_discount = 3 * discount
    currency -= price_after_discount
    item = "Milk"
    print("Milky!")
else:
    print("Why'd you come to walmart?")

## print summary
print("Item:", item)
print("Discount:", discount)
print("Currency:", currency)