CS150 - Fall 2013 - Class 8

  • exercise problem 2
       - why write it the first way?

  • run add_circles function in conditional-turtle.py code
       - similar to the add_circles method that we wrote for assignment 2
          - randomly draws circles throughout the screen
       - how are the circles being colored?
          - upper left quadrant are all purple
          - lower left blue
          - lower right red
          - upper right yellow
       - how could we do this?
          - set the fill color depending on what the x and y value are that the circle will be drawn

  • look at 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

  • prime numbers
       - what is a prime number?
          - a number that is only divisible by 1 and itself
       - what are the first 10 prime numbers?
          - the first 100?
          - the first 1000?
       - How could we write a program that figured this out?
       - To start with, how can we tell if a number is prime?
          - try and divide it by all of the numbers between 1 and the number
          - if none of them divide evenly, then it's prime, otherwise it's not
       - A few questions:
          - do we need to check all of the numbers up to that number?
             - just need to check up to sqrt(number)
          - how can we check to see if a number divides evenly?
             - use the remainder/modulo operator and see if it equals 0 (i.e. no remainder)
          - how can we check all of the numbers?
             - use a for loop

  • look at isprime function in while.py code
       - for loop starting at 2 up to the sqrt of the number
          - there are multiple versions of the range function
             - range with a simple parameter starts counting at 0 up to but not including the specified number
             - range with 2 parameters starts counting at the first number up to, but not including, the second number

                for i in range(10, 20):
                   print i

                would print out the numbers from 10 - 19 (but not 20)

          - the if statement checks to see if the number is divisible by i
          - if we find this we can stop early!
             - the minute we find this, we know it's not prime so we can return False
          - what does "return True" do?
             - if we've checked all of the numbers and none of them were divisible (otherwise we would have exited the function with the return False), so return True
       - we can use this to see if a number is prime

          >>> isprime(5)
          >>> isprime(6)
          >>> isprime(100)
          >>> isprime(101)

  • how could we use this to print out the first 10 (100, 1000, etc) prime numbers?
       - like to do some sort of loop
       - will a for loop work?
          - we don't know when we're going to stop
          - we'd like to keep a count of how many we've seen and only stop when we've reached the number we want

  • while loop
       - another way to do repetition

       while <bool expression>:


       as long as the <bool expression> evaluates to True, it continues to repeat the statements, when it becomes False, it then continues on and executes statement3, etc.

       - specifically:
          evaluates the boolean expression
             - if it's False
                - it "exits" the loop and goes on to statement3 and continues there
             - if it's True
                - executes statement1, statement2, ... (all statements inside the "block" of the loop, just like a for loop)
             - go back to beginning and repeat
       - how could we use a while loop for our prime numbers problem?
          - keep a count of how many primes we've found (initially starts at 0)
          - start count from 1 and work our way up
          - check each number
          - if it's prime
             - print it out
             - increment the counter of how many primes we've found
          - keep repeating this as long as (while) the number of primes we've printed is less than the number we want

  • look at firstprimes function in while.py code
       - current += 1 every time through the loop we increment the number we're examining
       - if that current number happens to be prime, we increment count
       - the loop continues "while" count < num, that is as long as the number we've found is less than the number we're looking for

  • run number_guessing_game in while.py code
       - picks a random number between 1 and 20 and you try and guess it
          - keeps prompting you until you get it right
          - gives you hints as to whether you need to guess higher or lower
       - how could we implement this?
          - pick a random number
          - as long as (while) the user hasn't guessed the right answer
             - get the guess from the user
             - if it's the right answer
                - print out "Good job!"
                - somehow indicate that we're done looping
             - otherwise, if the guess it too low
                - print out higher
             - otherwise (i.e. the guess must be too high)
                - print out lower

  • bool variables
       - just like any other variables except it's of type bool
          - we've used variables to store ints, floats and strings
          - this works the same way
       - for example

          >>> x = True
          >>> x
          >>> x = 10 < 0
          >>> x

       - we need some way of keeping track whether or not the user has guessed correctly or not
       - we can us a bool variable and initially set it to some value
       - condition the while loop on this variable
          - change the value when we get it correct

  • look at number_guessing_game function in while.py code
       - use a variable called "finished" and initially set it to False
          - could have also use a variable like "stillguessing" and set it to True and then had "while stillguessing:"
       - when they get the number right we set finished = True and we will therefore exit the loop
       - notice there are other ways of writing this function, e.g. number_guessing_game2

  • \
       - 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

  • infinite loops
       - what would the following code do?

       while True:
          print "hello"

       - will never stop
       - in Wing it will just appear as if you're program has hung
       - you can stop this by selecting "reset shell"
       - be careful about these with your program. They're called an infinite loop.
       - if you think you might have an infinite loop
          - make sure that you can see the Debug I/O tab (if not, select it under the "Tools" menu)
          - run your program using the debugging button (two buttons over from the green arrow)