Lecture 3: For Loops, Booleans

• Back to turtle, how can we use a for loop to draw a square?
  • look at the iterative_square function in turtle-examples.py
• What does the simple_star function in turtle-examples.py do?
  • draws a 36-sided star (or asterisk)
• What if we wanted to have a star/asterisk with a different number of spokes?
  • look at asterisk_star function
    • figure out how we have to space the spokes
    • do a for loop over the number of spokes
      • each iteration in the loop draw a spoke
      • need to go backwards into the middle for the next spoke
      • rotate right based on the angle we calculated
• what will simple_spiral, spiral and rotating_circles in turtle-examples.py do?
  • simple_spiral
    • i
      • i is just the name of a variable
      • for small loops (i.e. just a couple of statements), it's common to just us the variable name i (short for index)
    • each time, the length of the edge drawn will be longer by a factor of 5
      • 0, 5, 10, 15, 20, …
    • and will be at a 50 degree angle
      • if it is < 90 it will spiral out
      • > 90, spiral in
  • spiral
    • similar to simple_spiral, however now we've parameterized the length of the sides and the angle
  • rotating_circles
    • draws num circles
    • each one rotated "angle" degrees from the previous

• run the walk function from turtle-examples.py
  • how is this being accomplished?
    • turns a random angle between -90 and 90
    • steps forward some step size
• if you only want to import a single function, you can do that like:

from random import randint

• before the * indicated all function and variables. Here, we just said the randint function
• You can print out a bunch of random numbers if you're curious:

for i in range(100):
    print(randint(0,10))

• look at walk function in turtle-examples.py
  • went through the loop num_steps times/iterations
  • uses the uniform function to get an angle between -90 and 90
• what will the pretty_picture function do?
  • draws a line
    • at an angle between -90 and 90
    • of length between 10 and 60
  • then draws a star
    • of length between 10 and 60
    • with int(spokes) spokes
      • remember, if we want an integer, for example to be used in range, you need to turn it into an int

• 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
      True
      >>> x = -1
      >>> x < 10 and x > 0
      False
      

    • Truth table

      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
      True
      >>> x = -1
      >>> x < 10 or x > 0
      True
      

    • 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
      False
      

    • Truth table

      A	not A
      T	F
      F	T

• the key use of bool 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
    statement1
    statement2

statement3

• the if statement is called a "control" statement in that it changes how the program flows
  • for is also a control statement
  • 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

• run silly_name function from conditionals.py
• 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 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
    statement1
    statement2
else:
    # do these statements if the bool is False
    statement3
    statement4

• look at the add_circles function in conditional_turtle.py
  • setcolor_random function picks a random color
  • uses the if-elif-else statement to select between options
  • note that it saves the random number generated to use later in if-elif-else statements
    • you MUST save this number to a variable and not try and do your if/else statement based on new calls to random.randint
• 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

Try copying and pasting each of these five strings into the Python console. What happens?

"hi\nthere"

"hi\
there"

"""hello"""

"""hello
    there"""

"""What happens
to
    whitespace?"""

• How does it know?
• Docstrings!
  • A nice long comment at the top of each function
• help(input)

• We'll be using docstrings on every function from here on out
• Always use triple quotes (even if it's just one line of documentation)
• Simple function, simple docstring
• Longer function: talk about what it does, what the parameters are for, and why it might be useful

• Use good variable/function names
• Use whitespace (vertical and horizontal) to make code more readable
• Comment code — docstrings but also tricky parts of your functions
• Try to write code as simply as possible
  • E.g. avoid repetition, prefer less error-prone approaches