Announcements

• HW2 Releases on Friday
• Lab Checkoffs
  • You must get checked off to receive credit
  • You must also submit something to Gradescope for the points to count
    • This submission does not need to be finished!
• Pre-lab Checkoffs
  • If you want to get lab checked off without attending lab, you must finish both the required and the optional questions of the lab, and then send it to me!
    • For now, use email, but by next discussion, I'll have something for submitting using a Google Form which I'll be a lot better at keeping track of.

Questions/comments from last section

• Please fill out the attendance form with stuff you want to show up here!
• (Put some fun stuff - I like answering fun questions)
  • Have had questions about games/music/etc. if that's the stuff you want to fill
• Will also ask you to fill something for Lab so I have more material for this slide next time.

Temperature Check

• Call Expressions (call expression order, what they look like)
• Values/Types (Integers, Strings, Floats, Booleans, etc.)
• Environment Diagrams
• Functions
• Control
  • if, while

Boolean Operators

• not
  • Returns the opposite truthy value of the expression.
    • For example, if you type not 0, Python will return True
• and
  • Short circuits if it reaches a falsey value, and returns that value
    • This is not necessarily False
  • If all the values are truthy, the last value is returned
• or
  • Short circuits if it reaches a truthy value, and returns that value
    • This is not necessarily True
  • If all the values are falsey, the last value is returned.

Short Circuiting

• In call expressions, everything is evaluated from left to right, but this is not the case for when things short circuit.
• In and and or statements, all statements are not necessarily evaluated.
• and will keep on evaluating from left to right until it finds the first falsey value. If it finds a falsey value, it simply returns that value

Short Circuiting

• The last value will always be returned as is if you reach it:
  • For or, it will short circuit when it reaches the first truthy value, so if it sees only falsey values until the end, or will simply just return the last value as is.
  • Let's take a look at a smaller example:
    • False, True ⇒ True
    • False, False ⇒ False
    • Notice how the last elements are the same
    • This is why the last element is returned as is (it's very similar for and, except everything before is True)

Short Circuiting

Examples

• 1 and True and 1/0
  • This will error
• 1 and True and 0 and 1/0
  • Returns 0
• 1 or True or 1/0
  • Returns 1
• 0 or 1 or True or 1/0
  • Returns 1

If Statements

• Don't forget the colons!
• else does not need a conditional
• You can chain together as many elif blocks as you want
• Evaluate all ifs unless there's a return statement that ends the function
  • If you have a whole block of if/elif/else, you only evaluate at maximum 1 of the blocks.

Example (If Statements)

In this case, the console will output

return in Functions

• return will prematurely exit a function, and will make Python evaluate that function call to whatever gets returned

While Loops

• A while loop allows for a repeated execution of a certain block of code, allowing you to write just one thing that will end up being executed multiple times.
• The condition is checked before the execution of each iteration.
• To avoid an infinite loop, you must make sure your while loop changes the variable in the condition

While Loops Examples

Example 1

Output:

While Loops Examples

Example 2

Output:

Environment Diagrams

• Environment diagrams are a great way to learn how coding languages work under the hood
• Keeps track of all the variables that have been defined, and the values that they hold
  • Done with the use of frames
• Expressions evaluate to values:
  • 1 + 1 → 2
• Statements do not evaluate to values:
  • def statements, assignments, etc.
• Statements change our environment

Frames

• The Global Frame exists by default
• Frames list bindings between variables and their values
• Frames also tell us how to look up values

Assignment

• Assignment statements bind a value to a name
  • The right side is evaluated before being bounded to the name on the left
  • = is not the same in Python and mathematics
• These are then put in the correct frame in the environment diagram

Assignment

def statements

• Creates function (objects), and binds them to a variable name
• The function is not executed until called!
• Name of the variable is the name of the function
• Parent of the function is the frame where the function is defined
• Keep track of:
  • Name
  • Parameters
  • Parent

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