Announcements

• First HW and Project gets released at the start of next week
• Technical Office Hours (cs61a.org/office-hours/)
  • For help with additional setup from Lab 00
  • 1:00 PM - 4:00 PM in Cory 521
  • 4:00 PM - 6:00 PM online

• I talk very fast please scream at me if I'm talking too fast

Questions/comments from last section

• What do lab rooms have?
  • They all come with computers running Linux (I think Ubuntu)
  • You all should have after-hours access to the lab rooms if you need to use those computers!
• How fast are you at cubing?
  • Competition Times:
    • One-Handed: 6.85 Single, 10.48 Average
    • 3x3 Blindfolded: 18.38 Single, 22.06 Average
• What sort of involvements do you have with music?
  • Composing/Arranging Music
  • Piano/Voice + other instruments, but I suck at them

Temperature Check

• Expressions (e.g. 1 + 2, 10 ** 3, 7 // 2)
• Call Expressions (call expression order, what they look like)
• Values/Types (Integers, Strings, Floats, Booleans, etc.)
• Environment Diagrams
• Functions
• Control (not covered in lecture yet)
  • 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

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

Question 1 (2 Minutes)

What's the result of evaluating the following code?

• Answer: 12

Question 1 (2 Minutes)

What's the result of evaluating the following code?

• Answer: 19

Question 1 (2 Minutes)

What's the result of evaluating the following code?

• Answer: 12 (Return will prematurely exit a function )

Question 1

Notice how the first version and the third version of our function did the same thing?

• Question: When do you think using a series of if statements has the same effect as using if and elif blocks?
• Answer: A series of if statements has the same effect as using both if and elif cases if each if block ends in a return statement of some sort (that functionally acts the same)

Question 2 (5 Minutes)

Alfonso will only wear a jacket outside if it's below 60 degrees or if it's raining .

Write a function that takes in the current temperature temp (integer), and a boolean signifying whether it is raining or not.

Try to write this function both with an if statement and in a single line! Hints on how to do this are on the worksheet.

Question 2

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:

Question 4 (5 Minutes)

Write a function that returns True if a positive integer n is a prime number and False otherwise.

Hint: Use the % operator: x % y returns the remainder of x when divided by y.

Question 4

What should I put in the while loop? Where should my return values be?

Question 4

Question 5 (5 minutes)

Implement the fizzbuzz sequence, which prints out a single statement for each number from 1 to n.

• If i is divisible by 3 (only), print "fizz"
• If i is divisible by 5 (only), print "buzz"
• If i is divisible by 15, print "fizzbuzz"
• Else print the number i itself
• No return value

Things to note:

• If a number is divisible by 15, it is also divisible by 3 and 5 - how do you take account for that?

Question 5

What's wrong with the code above?

Question 5

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

Question 7 (5 minutes)

Draw out an environment diagram for the following statements:

• For future reference, you can use Python Tutor to verify your solutions!

Question 7

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

Example

• Keep track of the name, parameters, and parent!
• Uses pointers (unlike for primitive values)

Example

• Keep track of the name, parameters, and parent!
• Uses pointers (unlike for primitive values)

Question 8 (5 minutes)

Draw the environment diagram for the following:

• Start off with defining double and triple, then figure out what to do from there

Question 8

Mental Health Resources

• CAPS:
  • If you need to talk to a professional, please call CAPS at 510-642-9494.
• After Hours Assistance
  • For any assistance after hours, details on what to do can be found at this link