Range

Calvin (Deutschbein)

W3Wed: 11 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)) range(0, 10)
Um... what?
Okay fun fact - range just starts at zero by default, but we can change that... >>> print(range(5,10)) range(5, 10)
Variety of uses for this - usually reading a file and cutting off headings for me.

`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) ... 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) ... 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) ... 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) ... 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) ... 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) <class 'int'> >>> type(1.0) <class 'float'> >>> type(1==1.0) <class 'bool'> >>> type(False) <class 'bool'>
'True' and 'False' are booleans and nothing else is a boolean.

Booleans

The is such thing as boolean arithmetic
Easy ones:

a and b True if 'a' and 'b' are True

a or b True if 'a' or 'b' be True

not a True if 'a' is not True ('a' is False)

>>> a = True
>>> b = False
>>> a and b
False
>>> a or b
True
>>> not a
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 (???)

>>> if 0:
...     print('hi')
...
>>> if "":
...     print('hi')
...
>>> if print():
...     print('hi')
...

>>> if .1:
...     print('hi')
...
hi
>>> bool(0)
False

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

Truthiness

While goofy, it allows for some shortcuts.

Let's find some multiples of 3.

`val % div`	Is non-zero if 'val' divided 'div' has some remainder
`val % div == 0`	Is True if 'val' divided 'div' has no remainder
`bool(val % div)`	Is True if 'val' divided 'div' has some remainder
`not val % div`	Is True if 'val' divided 'div' has no remainder

>>> for i in range(10,20):
...     if i % 3 == 0:
...             print(i)
...     if not i % 3:
...             print(i)
...
12
12
15
15
18
18
>>>

All together

>>> for i in range(0,50,5):
...     if not i % 3:
...             print(i)
...
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

Type hints

You basically know what types are now:
- int
- float
- bool
- Whatever "hello world" is - we'll get there
>>> type('hi') # to be continued...
The ONLY way I wrapped my head around functions was by remembering this: def add(x,y): return x + y
Was a function that takes two ints and returns one int
Call this a "function type"

Type hints

PS2 Problem 2 asks for a function that takes two ints and returns one int.
And it ALSO prints a whole bunch of stuff (ints, more or less - we'll get there)
In Python we can state what the function type is with "type hints":
- This are basically a special kind of comment - they don't affect code.
def divide(x, y): # no hints return x / y def intdiv(x:int, y:int) -> int: # yes hints return x // y
You are not required to use type hints in this class...
But my brain requires me to use function types to teach this class.

Type hints

We term the things that go into function "parameters" or "arguments"
- Sometimes we say "inputs" but that can get confusing - stay tuned.
We term the things that come out of functions "return values"
A function with no return value, like 'print', returns "None" - which is a special value and a special type. >>> type(None) <class 'NoneType'>>>> type(print('hi')) hi <class 'NoneType'>
Why do I bring this up?

Type hints

You should only ever omit return values *on purpose* (realistically: never until you are an expert) >>> def print_sum_of(x:int,y:int) -> None: ... print(x + y) ... >>> type(print_sum_of(3,4)) 7 <class 'NoneType'> >>>
Code like this will be easier to fix if you try to, say: >>> x = print_sum_of(3,4) 7 >>> x >>>
Wait - I said print_sum should return None! Why am I saving that in x!

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! 🚀