Arithmetic

Announcements

• By MONDAY 11:59 PM: "Problem Set 1: Karel" assignment.
  • One problem (probably) using "def" and "for"
  • One problem using at least "while" and maybe the other three.
• Sections reminder: Wed & Thurs PM.
  • Problems: email jjrembold@, you may cc ckdeutschbein@
  
• By NEXT 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.

Today

• Do we need homework screencapture slides?
  • Doing homework: It's fun.
• I can live-code any homework questions.
  • Let's crush this homework... together.
• Introducing Python, cont.
  
  if / elif / else
+ - * // / % ** - # operators
= == # assignment vs comparison
return

if / elif / else

• We know 'if', but 'else' is very useful on Checkerboard
• E.g., place beeper after moving ONLY if moving from an unbeeped square. def check_move(): if no_beepers_present(): move() put_beeper() else: move()
• What happens if we: import karel def main(): while front_is_clear(): check_move()

if / elif / else

• 'elif', short for 'else if', allows longer, more complicated checks. def sign_check(x): if x < 0: print("negative") elif x > 0: print("positive") else: print("zero")

Today

• Introducing Python, cont.
  
  ✓: if / elif / else
+ - * // / % ** - # operators
= == # assignment vs comparison
return

Operators

• Most arithmetic in Python is "as expected" with a few differences.
• Easy ones:

  Addition	a + b
  Subtraction	a - b
  Multiplication	a * b
  Division	a / b

• Weirder ones:

  Integer Division	a // b	Leaves a "Remainder"
  Modulo	a % b	calculates the "Remainder"
  Exponentiation	a ** b	Note: The caret ^ is something else

Operators

Note: Division takes whole numbers (integers) and returns numbers with a decimal ('floats').

Operators

It may seem weird, but % ("modulo" or "remainder") is extremely useful, and I use it more than anything except maybe + ("plus" or "addition")

Two divisions

• Integer division // is nice when we might want to calculate a value then loop that many times. >>> for i in range(5//2): ... print(i) ... 0 1 >>> for i in range(5/2): ... print(i) ... Traceback (most recent call last): File "<stdin>", line 1, in <module>> TypeError: 'float' object cannot be interpreted as an integer

type()

• If you ever aren't sure what something will be, ask Python with "type()" >>> type(5/2) <class 'float'> >>> type(5//2) <class 'int'> >>> type(5) <class 'int'> >>> type(5.0) <class 'float'>

'for' loops need "<class 'int'>"

Today

• Introducing Python, cont.
  
  ✓: if / elif / else
✓: + - * // / % ** - # operators
= == # assignment vs comparison
return

=

• We use '=', which I term 'single equals' or 'assignment', to 'assign' a value (like 7) to a variable (like x). >>> x = 7
• Then x does all the things seven does. >>> 7 7 >>> x 7 >>> x + 6 13

  It means "make this equal to that"

Weird thing

• We are allowed to set a variable, like x, equal to some arithmetic over x, like x = 1. >>> x = 7 >>> x = x + 1 >>> x 8 >>>
• This is useful to e.g. count things.
• Imagine if we could count how many beepers Karel placed, or how many moves. def ten_even_numbers(): x = 0 for i in range(10): x = x + 2 print(x)

==

• We use '==', which I term 'doubles equals', 'equality test', or 'comparison', to 'check' if two things, either values or variables, are equal.
• '==' is kind like '+': given two things, it gives back a value - but that value is either true or false. >>> x = 7 >>> 7 == x True >>> x == 7 True >>> 6 == x False >>> 1 == 2 False >>> x + 6 13

Comparison Operators

Checking Numbers

• We check if numbers are even... def is_even(x): # check if x is even if x % 2 == 0: # no remainder when divided by two print("even") elif x % 2 == 1: print("odd") else: print("not a whole number")
• Test it: >>> is_even(2) even >>> is_even(1) odd >>> is_even(1.5) not a whole number

Today

• Introducing Python, cont.
  
  ✓: if / elif / else
✓: + - * // / % ** - # operators
✓: = == # assignment vs comparison
return

Return

• Print is how the computer tells us things.
• Sometimes we want it to tell itself things, like:
  • Whether some number is divisible by another number...
  • The greater of two numbers..
• Print has a limitation here.
  def is_even(x): print(is_divisible_by(x,2)) def bigger(x,y): if x > y: print(x) else: print(y)
• What if I want to assign some variable 'c' to be the greater of 'a' and 'b'?

Return

• If we write "is_divisible_by" like this: def is_divisible_by(value,divisor): print(value % divisor == 0) # has no remainder }
• This code won't work: def is_even(x): print(is_divisible_by(x,2))

Return

• Why? 'is_divisible_by' prints True or False, but doesn't tell 'is_even' what it prints.
• The way to 'tell' a computer what happened in a def/function is with 'return': def is_divisible_by(value,divisor): return (value % divisor == 0) # has no remainder
• We use return on its own line, and it doesn't need parenthesis or colons. def is_even(x): return is_divisible_by(x,2)
• Something like 'front_is_blocked()" returned either True or False.

Return

• We can compare the two: >>> is_divisible_by(10,2) True >>> print(is_divisible_by(10,2)) True >>> x = is_divisible_by(10,2) >>> x True >>> if is_divisible_by(10,2): ... print('10 is even') ... 10 is even >>>

Return

• Until you write code with zero errors on the first time...
• You should never write a function (def) with no return.
• Most programers expect Python to return when it does not. def is_divisible_by(value,divisor): # checks if value is evenly divided by divisor check = value % divisor == 0 # compare remainder to zero if check: # print what we find print("Value is evenly divided by divisor") else: print("Value is not evenly divided by divisor") # do not forget to return return check
• Print is for you to read, return is for the computer or for you to use.

Exercise

• Do "Problem Set 1: Karel"
  • Use comments
  • The first problem "newspaper" benefits from functions, and perhaps for loops
    • It is possible, but annoying, to do without either
  • The second problem, "painting" benefits from 'while' loops (and could use 'if')
    • I used four (4) 'while' loops, and no 'if' statements or 'for' loops

Announcements

• By MONDAY 11:59 PM: "Problem Set 1: Karel" assignment.
  • One problem (probably) using "def" and "for"
  • One problem using at least "while" and maybe the other three.
• Sections reminder: Wed & Thurs PM.
  • Problems: email jjrembold@, you may cc ckdeutschbein@
  
• By NEXT 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.