Range

Calvin (Deutschbein)

W3Wed: 10 Sep

Announcements

• By MONDAY 11:59 PM: "Problem Set 2: Checkers" assignment.
  • One Karel problem (probably) using "if" and "while"
  • One Python problem using arithmetic and (probably) "for"
  • One writing problem.
• Sections reminder: Wed & Thurs PM.
  • Problems: email jjrembold@, you may cc ckdeutschbein@
  
• PS1: Crushed. Mission: All A's is a go! 🚀

Today

range()
and / or / not
def / return

range()

The Anapurna range of the Himalayas (that mountain is ~5 miles tall)

range()

• Remember this: def turn_right(): for i in range(3): turn_left()
• Like the Anapurna range (look it was just a cool picture okay) of the Himalayas...
• A range of some integer n is part of the number line (from neg to pos infinity)
• In Karel we didn't worry about the numbers, but we are Arithmetic Appreciators™ now

range()

• The easiest way to examine range() is with print() print(range(10))
• We see: range(0, 10)
• Um... what?
• Okay fun fact - range just starts at zero by default, but we can change that... print(range(5,10))
• We see: range(5, 10)

range()

• The easiest way to examine range() is with print()...
• But we shouldn't just print the range itself!
• We often use 'range()' with 'for' for i in range(3): # smaller so its easy to work with print(i)
• We see: 0 1 2
• Recall: we say "range(3)" but we get "range(0,3)"
• So ranges include the first number and stop right before the second.
• Why include zero?
  • What is the first hour of the day? (12hr or 24hr clock?)
  • What is the fewest number of apples you can have?

range()

• The easiest way to examine range() is with print()... for i in range(4,6): print(i)
• We see: 4 5
• We can also do non-zero starts.

range()

• Often we use for and range when we want to check for something, like multiples of 5. for i in range(25,50): if i % 5 == 0: print(i)
• We see: 25 30 35 40 45
• One can imagine how this may be useful
  • Check every 50th entry to see if it seems valid.
  • Divide up some tasks among n people.

range()

• There is a forbidden (it's not forbidden) third value that can be supplied to a range, that we usually call the step size: for i in range(25,50,10): print(i)
• We see: 25 35 45
• I use this sometimes, but usually inner if is fine, unless...

range()

• By the way you can make any of these values negative. for i in range(15,10,-2): print(i)
• We see:
15 13 11
• For whatever reason, I'm always reversing orders:
  • Review the homeworks that were turned in last, first
  • Use reverse alphabetical order to offset years of alphabetization
  • In a big file: start reading from the end (latest entries)

range()

• I like range... for i in range(15,10,-2): print(i)
• ... but also like while: i = 15 while i > 10: print(i) i = i - 2
• These are identical. Use what you like.
• You will need* to use on PS2 Problem 2.
  • There is a very challenging way to use only 'if'
  • I don't have it working yet but it looks possible. Office hours.

Today

✓: range()
and / or / not
def / return

Booleans

• There's these things called booleans.

  type(1) type(1.0) type(1==1.0)

  <class 'int'> <class 'float'> <class 'bool'>

• This is new:
<class 'bool'>
• 'True' and 'False' are booleans and nothing else is a boolean.

Booleans

• There is such thing as boolean arithmetic a = True b = False
• Easy ones:

  a and b	True if 'a' and 'b' are True	False
  a or b	True if 'a' or 'b' be True	True
  not a	True if 'a' is not True ('a' is False)	False

  In general: if you aren't sure, just check.

Truthiness

• If Python expects a boolean, like in an 'if', it turns to turn whatever it gets into a boolean.
• You can test by writing 'if' statements, or by turning things into booleans with 'bool()'
• This is sometimes called "truthiness" - I don't love it (do you?)
• Here's some "falsy" values

  0	Non-zero values are treated like True
  0.0	Non-zero values are treated like True
  ""	Imagine if we want to print nothing.
  None	None is the return value of a function with no return (???)

Checking Truthiness

• We try:
if 0: print('zero') if "": print('empty') if print("anything"): print('printing') if .1: print('.1') print(bool(0))
• We see
anything .1 False

Truthiness

All together

for i in range(0,50,5): if not i % 3: print(i)
• We see:
0 15 30 45
• Same as: for i in range(50): if i % 3 == 0 and i % 5 == 0: print(i)

I took a whole class in undergrad on "counting two ways" (it was fun!)

Disambiguate

• If you get confused looking at something: for i in range(50): if i % 3 == 0 and i % 5 == 0: print(i)
• Add parenthesis: for i in range(50): if ((i % 3) == 0) and ((i % 5) == 0): print(i)
• Or use variables. for i in range(50): rem3 = (i % 3) rem5 = (i % 5) if not rem3 and not rem5: # 'not' any remainder, print(i)

Today

✓: range()
✓: and / or / not
def / return

Exercise

• Do "Problem Set 2: Checkers"
  • One Karel problem (probably) using "if" and "while" def check_move(): if no_beepers_present(): move() put_beeper() else: move()
  • One Python problem using arithmetic and (probably) "for" def divisible_by_six_or_seven(x:int, y:int) -> int: # there must be the possibility of something printing in here return # must return an int
  • One writing problem

Announcements

• By MONDAY 11:59 PM: "Problem Set 2: Checkers" assignment.
  • One Karel problem (probably) using "if" and "while"
  • One Python problem using arithmetic and (probably) "for"
  • One writing problem.
• Sections reminder: Wed & Thurs PM.
  • Problems: email jjrembold@, you may cc ckdeutschbein@
  
• PS1: Crushed. Mission: All A's is a go! 🚀