CS51A - Spring 2019 - Class 5

Example code in this lecture


Lecture notes

  • admin
       - lab this afternoon

  • run add_circles function from conditional-turtle.py using setcolor_xy
       - picks random x and y coordinates to draw a circle
          - uses randint
       - How are the colors chosen?
          - each quadrant of the xy-axes is a different color
          - how can we do this?
             - want to ask a question about the x and y

  • booleans
       - We've seen three types so far: int, float and str (string)
       - Python has another type called bool (short for boolean)
          - bool can only take the value True or False
       - bool's generally result from asking T/F question
       - What questions might we want to ask on data we've seen so far (e.g. numbers)?
          - comparison operators
             - == (equal)
                - notice that '=' is the assignment operator while '==' asks whether two things are equal
             - != (not equal)
             - < (less than)
             - > (greater than)
             - <= (less than or equal to)
             - >= (greater than or equal to)
       - Using the comparison operators
          >>> 10 < 0
          >>> 11 >= 11
          >>> 11 > 11.0
          >>> 11 >= 10.9
          >>> 10 == 10.1
          >>> "test" == "test"
          >>> "test" == "TEST"
          >>> 10 != 10
          >>> 10 != 11
          >>> "banana" < "apple"
          >>> type(True)
          <type 'bool'>
          >>> type(0 < 10)
          <type 'bool'>

  • combining booleans
       - we can also combine boolean expressions to make more complicated expressions
       - what kind of connectors might we want?
          - and
             <bool expression> and <bool expression>

             - only returns True if both expressions are True
             - otherwise, it returns false

             >>> x = 5
             >>> x < 10 and x > 0
             >>> x = -1
             >>> x < 10 and x > 0

             - Truth table
                - a truth table gives you a mapping from input to output for boolean functions

                A B | A and B
                T T | T
                T F | F
                F T | F
                F F | F

          - or
             <bool expression> or <bool expression>

             - returns True if either expression is True
             - False only if they are both False

             >>> x = 5
             >>> x < 10 or x > 0
             >>> x = -1
             >>> x < 10 or x > 0
             - Truth table
                A B | A or B
                T T | T
                T F | T
                F T | T
                F F | F
          - not
             not <bool expression>

             - negates the expression
                - if it's True returns False
                - if it's False returns True

             >>> not 5 == 5

             - Truth table

                A | not B
                T | F
                F | T
  • if statement
       - the key use of bool's is to make decisions based on the answers
       - the "if" statement allows us to control the flow of the program based on the result of a boolean expression

          if bool_expression:
             # do these statements if the bool is True

       - the if statement is called a "control" statement in that it changes how the program flows
          - as the program runs, it evaluates the boolean expression
          - if it is true, it evaluates all of the statements under the "if" block and then continue on
             - it will execute statement1, statement2 and then statement3
          - otherwise (i.e. the boolean was false), it will skip these statements and continue on
             - it will just execute statement3

       - look at simple_if function in conditionals.py code

  • run stupid_name function from conditionals.py code
       - input
          - takes a string as a parameter
          - it displays the string to the user
          - then waits for the user to enter some text. The program doesn't continue until the user hits enter/return
          - whatever the user typed will be returned by the raw_input function as a string
             - if you want a number you need to use int(...) or float(...)

       - first prompts the user for their name
       - depending on the input, the output of the program partially differs
          - if statements allow us to control the flow of the program
       - if-else: sometimes we'd also like to do something if the bool is False, in this case, we can include an "else"

          if <bool expression>:
             # do these statements if the bool is True
             # do these statements if the bool is False


          - if the boolean expression is true
             - execute statement1, statement2 then statement 5
          - if the boolean expression is false
             - execute statement3, statement4 then statement 5

  • look at stupid_name function from conditionals.py code
       - we first use the raw_input function to get the user's name
          - raw_input returns the text the user entered
          - in the variable "name" will be whatever the users entered
       - name == "Dave" checks whether the entered name is "Dave"
       - if statement directs the program's behavior depending on the answer
       - finally, regardless of the name, we print out "Nice to meet you..."

  • run forecast function in conditionals.py code
       - takes two parameters
          - temperature
          - amount of rain
       - depending on what these values are, gives us different answers
       - to start with, let's just look at the temperature. We want to define 4 temperature bands:
          - > 80 => "hot"
          - 71 - 80 => "warm"
          - 51 - 70 => "cool"
          - <= 50 => "cold"

          - say we have a variable called temperature with the temperature, how would we write the if statements for this?
             if temperature > 80:
                temp = "hot"
             if 70 < temperature <= 80:
                temp = "warm"
             if 51 < temperature <= 70:
                temp = "cool"
             if temperature <= 50:
                temp = "cold"
          - if we know the temperature is above 80, do we need to check any of the other ones?
             - no!
          - Python has another statement that allows us to represent this type of expression: elif

             if <bool expression>:
             elif <bool expression>:
             ... # we can have as many elif blocks as we want

             - the program starts with the first if statement. If it is true, it executes the statements in the if block then goes to the end (after else) and continues
             - if it is false, it goes to the first elif and see if it is true. If it is true, it executes the statements in the elif block then goes to the end (after the else) and continues
             - the program will keep going down the list of elif statements as long as none of them are true
             - if they are all false, then it will execute the statements under else
          - why is the elif useful?
             - avoids redundant calculations: if we know things are mutually exclusive, then once we find one that is true, don't need to check the others
             - can simplify the statements, when we get to an elif statement, we know that the above boolean checks are false and don't need to check those again

  • look at the temperature_report and precipitation_report functions in conditionals.py code
       - both use if-elif-else statements to calculate an answer

  • look at forecast function in conditionals.py code
       - uses the other functions to generate the answer

  • \
       - Python assumes one statement per line
       - We've seen multi-line strings. Python also allows you to put a statement over multiple lines
       - if you put a \ (backslash) at the end of a line, Python will continue reading on the next line

  • look at the add_circles function in conditional-turtle.py code
       - setcolor_xy function takes the x and y as a parameter and sets the fill color
       - what will this function look like?

  • look at setcolor_xy function in conditional-turtle.py code
       - uses the if-elif-else statement to select between the four options

  • run add_circles function in conditional-turtle.py code with setcolor_random function instead of setcolor_xy
       - what does this do?
          - randomly picks between blue, purple, red and yellow (instead of based on x, y)
       - how could we get this behavior?
          - use random.randint to select a number between 1 and 4
          - save this number and use it in an if-elif-else statement
             - you MUST save this number to a variable and not try and do your if/else statement based on new calls to random.randint
       - look at setcolor_random function