COMP1701-004

fall 2023

lec-16

Is November Over Yet?

PREDICT what this displays
if the user enters bar at the prompt.

def is_vowel(s: str) -> bool:
    return s in "aeiou"


def blorp_index(s: str) -> int:
    i = len(s) - 1
    blorp_idx = -1

    while i >= 0 and blorp_idx == -1:
        c = s[i]
        if is_vowel(c):
            blorp_idx = i

        i -= 1

    return blorp_idx


def fun(s: str) -> str:
    i = blorp_index(s)
    return s[i]


def main() -> None:
    response = input("> ")
    c = fun(response)
    print("That", "was", c, sep="-")


main()

let's talk about these  things today:

⦾ lists: the concept

⦾ lists: making them

⦾ lists: referring to items in them

⦾ lists: iterating through them

⦾ lists: lists of lists

lists : the concept

lists : the concept

s t r i n g
0 1 2 3 4 5

The string type can be viewed as a sequence of characters in consecutive indexed boxes.

lists : the concept

112 -4 9
0 1 2

The list type can be viewed as a sequence of whatever-you-want in consecutive indexed boxes.

list of ints

0.2 1.0 -100.03 -4.81
0 1 2 3

list of floats

True True False True False
0 1 2 3 4

list of bools

"foo" "hi there"
0 1

list of strings

lists : the concept

You might be wondering "can I put different types of things in a list?"

True "cheese" 42 3.14 "toast"
0 1 2 3 4

list of whatever-the-heck-you-want

Yeah. But don't.

🙋🏻‍♂️❓🙋🏻‍♀️Why do you suppose this is a bad idea?

lists : making them

lists : making them

Let's talk about
2 ways
of creating a list

lists : making them

Let's talk about
2 ways
of creating a list

Way the first: use [ ] notation

Yup. Now your brain has to deal with 2 meanings of [ ].

completed_tasks = []

toggled_on = [True]

midday_temps_c = [-4, -14, 3, 2, 0]

weighted_marks = [2.4, 3.1, 4.0]

gamer_handles = ["skinny", "lady-bug", "rickNmorty", "D00MZDAY"]

an empty list

a list of length 1

a list of length 5

🙋🏻‍♂️❓🙋🏻‍♀️How else have you used [ ]?

lists : making them

Let's talk about
2 ways
of creating a list

Way the second: use split() 

a_string = "My dog has fleas."

split_s = a_string.split()

print(split_s) # ['My', 'dog', 'has', 'fleas.']

split() is an example of a method - it's a function that is "understood" by an object

You'll deal a lot more with objects in your second semester. In the meantime, just know that strings and lists have many useful methods they can use.

🙋🏻‍♂️❓🙋🏻‍♀️BTW - what string methods have you used so far?

You can always tell you're looking at a method because there's always a dot (.) in front of it.

lists : making them

Let's talk about
2 ways
of creating a list

some split() nuances

sentence = "  My    dog 		has         fleas.  "

split_result = sentence.split()

print(split_result) # predict!

lots of spaces - and even some tabs!

  1. By default, split() uses whitespace as a delimiter.
  2. Leading and trailing whitespace is ignored.
  3. Adjacent whitespace is considered a single delimiter.

lists : making them

Let's talk about
2 ways
of creating a list

some more split() nuances

some_numbers = "1,2,3,4"

split_result = some_numbers.split(",")

print(split_result)  # predict!
  1. split() can use any string as a delimiter, if you pass one in.
  2. Be careful - the items in the resulting list are always strings!

lists : making them

Let's talk about
2 ways
of creating a list

even MORE split() nuances

some_strings = "ready;willing;;able;"

split_result = some_strings.split(";")

print(split_result)  # predict!
  1. Be careful - once you pass in an argument, split() does NOT ignore leading/trailing delimiters and does NOT clump adjacent delimiters!

adjacent delimiters

trailing delimiter

lists : making them

Let's talk about
2 ways
of creating a list

predict what happens in these two cases

sentence = "1,2,3,4"

split_result = sentence.split()

print(split_result)  # predict!
sentence = "1, 2, 3, 4"

split_result = sentence.split(",")

print(split_result)  # predict!

lists : referring to items

lists : referring to items

Good news, everyone!

We can refer to items in a list just like we can with letters in a string - with [ ] 

"RHINO"[0] # "R"

response = "yes"
response[2] # What's this?
["Aliens", "Jaws", "Hereditary"][1] # "Jaws"

temps = [-2, 0, 5, -13]
temps[2] # What's this?

"I am a wild party".split()[3] # What's this?

The index operator with strings.

The index operator with lists.

🙋🏻‍♂️❓🙋🏻‍♀️What was the [ ] operator called?

lists : iterating through 'em

lists : iterating through

Good news, everyone!

We can iterate through items in a list just like we can with strings.

lists : iterating through

buncha_things = ["knock", "it", "off"]
number_of_things = len(buncha_things)

i = 0

while i < number_of_things:
    curr_thing = buncha_things[i]

    # do something with curr_thing

    i += 1

get upper bound

initialize index counter LCV

grab next thing in list

bump up LCV

while style

counted loop

🙋🏻‍♂️❓🙋🏻‍♀️Why not just call len(buncha_things) in the while?

Does this seem pretty "busy", or is it just me?

lists : iterating through

buncha_things = ["knock", "it", "off"]

for curr_thing in buncha_things:

    # do something with curr_thing

for style

so nice and quiet!

TASK

Make an app that takes in a sequence of integers separated by commas from a user and tells the user whether there are more odd numbers, or more even numbers, or the same number of both.


You should try NOT put everything in a main() - make 2 functions:

- a function that prompts the user and returns a list (of what)?

- a function that take in a list (of what?) and returns the number of even (or odd - your choice) numbers in that list

Numbers? 1,2,3,4,5

There are more odd numbers than even numbers in 1,2,3,4,5.
Numbers? 11,14

There are the same number of even and odd numbers in 11,14.
Numbers? 240

There are more even numbers than odd numbers in 240.

lists : lists of lists

Remember this?

112 -4 9
0 1 2

list of ints

0.2 1.0 -100.03 -4.81
0 1 2 3

list of floats

True True False True False
0 1 2 3 4

list of bools

"foo" "hi there"
0 1

list of strings

(ominous music plays)

["chicken", "beef"] ["mashed potato", "baked potato"] ["corn", "peas", "carrots"]
0 1 2

list of lists

If a list can hold anything...

...then couldn't it hold another list?!?

lists : lists of lists

Here we go again!

lists : lists of lists

print(f"this is {cruel(and_unusual(punishment))}")

This can hurt a bit - because any kind of nesting is challenging to the brain

if distance < 0:
  if distance < 10 and limit >= DA_LIMIT:
    if limit - fudge(factor) < 0:
      print("please stop")
    else:
      print("no, really")
  elif distance < 10 and limit < OTHER_LIMIT:
    print("I'm begging you")
  else:
    print("my brain feels funny")
else:
  print("<...passes out...>")
def display_number_triangle(
  lower_bound, 
  upper_bound, 
  step_size):
  
    line_starting_num = lower_bound
    
    while line_starting_num < upper_bound:
      
        curr_num = line_starting_num
        
        while curr_num < upper_bound:
            print(curr_num, end="")
            curr_num += step_size
            
        print()
        line_starting_num += step_size
choices = [["beef", "pork"], ["pasta", "potatoes"],["peas", "carrots"]]

lists : lists of lists

Good news, everyone!

We can still refer to items in a list of lists just like we can with a simple list - with [ ]

Bad news, everyone!

This can be confusing and frustrating.

lists : lists of lists

movies = [
    ["Aliens", ""],  # action genre
    [], # horror
    [], # comedy

]

Let's do some live practice!

Help me make a nested list of movies, then we'll practice our index skillz.

lec-16

By Jordan Pratt

Made with Slides.com

lec-16

list creation ([ ], split) | list iteration | list item reference | list of lists

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