Containers

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Class outline:

  • Lists
  • Containment
  • For statements
  • Ranges
  • List comprehensions
  • String literals

Lists

Lists

A list is a container that holds a sequence of related pieces of information.

The shortest list is an empty list, just 2 square brackets:


                    members = []
                    

Lists can hold any Python values, separated by commas:


                    members = ["Pamela", "Tinu", "Brenda", "Kaya"]

                    ages_of_kids = [1, 2, 7]

                    prices = [79.99, 49.99, 89.99]

                    digits = [2//2, 2+2+2+2, 2, 2*2*2]

                    remixed = ["Pamela", 7, 79.99, 2*2*2]
                    

List length

Use the global len() function to find the length of a list.


                    attendees = ["Tammy", "Shonda", "Tina"]

                    print(len(attendees))   #  3

                    num_of_attendees = len(attendees)
                    print(num_of_attendees)
                    

🤔 What could go wrong with storing the length?

Accessing list items (brackets)

Each list item has an index, starting from 0.


                    letters = ['A', 'B', 'C']
                    # Index:   0     1     2
                    

Access each item by putting the index in brackets:


                    letters[0]  # 'A'
                    letters[1]  # 'B'
                    letters[2]  # 'C'
                    letters[3]  # 🚫 Error!
                    

                    curr_ind = 1
                    letters[curr_ind] # 'B'
                    

Negative indices are also possible:


                    letters[-1]  # 'C'
                    letters[-2]  # 'B'
                    letters[-4]  # 🚫 Error!
                    

Accessing list items (function)

It's also possible to use a function from the operator module:


                    from operator import getitem

                    getitem(letters, 0)
                    

List concatenation

Add two lists together using the + operator:


                    boba_prices = [5.50, 6.50, 7.50]
                    smoothie_prices = [7.00, 7.50]
                    all_prices = boba_prices + smoothie_prices
                    

Or the add function:


                    from operator import add

                    boba_prices = [5.50, 6.50, 7.50]
                    smoothie_prices = [7.00, 7.50]
                    all_prices = add(boba_prices, smoothie_prices)
                    

List repetition

Concatenate the same list multiple times the * operator:


                    boba_prices = [5.50, 6.50, 7.50]

                    more_boba = boba_prices * 3
                    

Or the mul function:


                    from operator import mul

                    boba_prices = [5.50, 6.50, 7.50]
                    more_boba = mul(boba_prices, 3)
                    

All together now:


                    digits = [1, 9, 8, 4]
                    together = [6, 2, 4] + digits * 2 # [6, 2, 4, 1, 9, 8, 4, 1, 9, 8, 4]
                    together = add([2, 7], mul(digits, 2))
                    

Nested Lists

Since Python lists can contain any values, an item can itself be a list.


                    gymnasts = [ ["Brittany", 9.15, 9.4, 9.3, 9.2],
                                 ["Lea", 9, 8.8, 9.1, 9.5],
                                 ["Maya", 9.2, 8.7, 9.2, 8.8] ]
                    
  • What's the length of gymnasts? 3
  • What's the length of gymnasts[0]? 5

Accessing nested list items


                    gymnasts = [
                                    ["Brittany", 9.15, 9.4, 9.3, 9.2],
                                    ["Lea", 9, 8.8, 9.1, 9.5],
                                    ["Maya", 9.2, 8.7, 9.2, 8.8]
                                ]
                    

Access using bracket notation, with more brackets as needed:


                    gymnasts[0]    # ["Brittany", 9.15, 9.4, 9.3, 9.2]
                    gymnasts[0][0] # "Brittany"
                    gymnasts[1][0] # "Lea"
                    gymnasts[1][4] # 9.5
                    gymnasts[1][5] # 🚫 IndexError!
                    gymnasts[3][0] # 🚫 IndexError!
                    

Containment

Containment operator

Use the in operator to test if value is inside a container:


                    digits = [2, 8, 3, 1, 8, 5, 3, 0, 7, 1]

                    1 in digits # True

                    3 in digits # True

                    4 in digits # False

                    not (4 in digits) # True
                    

For statements

For loop

The for loop syntax:


                    for <value> in <sequence>:
                        <statement>
                        <statement>
                    

The for loop provides a cleaner way to write many while loops, as long as they are iterating over some sort of sequence.


                    def count(s, value):
                        total = 0
                        for element in s:
                            if element == value:
                                total = total + 1
                        return total
                    

For statement execution procedure


                    for <name> in <expression>:
                        <suite>
                    
  1. Evaluate the header <expression>, which must yield an iterable value (a sequence)
  2. For each element in that sequence, in order:
    1. Bind <name> to that element in the current frame
    2. Execute the <suite>

Looping through nested lists


                    gymnasts = [
                                    ["Brittany", 9.15, 9.4, 9.3, 9.2],
                                    ["Lea", 9, 8.8, 9.1, 9.5],
                                    ["Maya", 9.2, 8.7, 9.2, 8.8]
                                ]
                    

Use a nested for-in loop:


                    for gymnast in gymnasts:
                        for data in gymnast:
                            print(data, end="|")
                    

Remember what type of data is being stored in the loop variable!

Sequence unpacking in for statements


                    pairs = [[1, 2], [2, 2], [3, 2], [4, 4]]
                    same_count = 0

                    for x, y in pairs:
                        if x == y:
                            same_count = same_count + 1
                    

Each name is bound to a value, like in multiple assignment.

Ranges

The range type

A range represents a sequence of integers.


                    ... -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5...
                                    range(-2, 3)
                    

If just one argument, range starts at 0 and ends just before it:


                    for num in range(6):
                        print(num)       # 0, 1, 2, 3, 4, 5
                    

If two arguments, range starts at first and ends just before second:


                    for num in range(1, 6):
                        print(num)       # 1, 2, 3, 4, 5
                    

List comprehensions

List comprehension syntax

A way to create a new list by "mapping" an existing list.

Short version:


                    [<map exp> for <name> in <iter exp>]
                    

                    odds = [1, 3, 5, 7, 9]
                    evens = [(num + 1) for num in odds]
                    

Long version (with filter):


                    [<map exp> for <name> in <iter exp> if <filter exp>]
                    

                    temps = [60, 65, 71, 67, 77, 89]
                    hot = [temp for temp in temps if temp > 70]
                    

List comprehension execution procedure


                    [<map exp> for <name> in <iter exp> if <filter exp>]
                    
  • Add a new frame with the current frame as its parent
  • Create an empty result list that is the value of the expression
  • For each element in the iterable value of <iter exp>:
    • Bind <name> to that element in the new frame from step 1
    • If <filter exp> evaluates to a true value, then add the value of <map exp> to the result list

                    letters = ['a', 'b', 'c', 'd', 'e', 'f', 'm', 'n', 'o', 'p']
                    word = [letters[i] for i in [3, 4, 6, 8]]
                    

Exercise: Divisors


                    def divisors(n):
                        """Returns all the divisors of N.

                        >>> divisors(12)
                        [1, 2, 3, 4, 6]
                        """
                    

Exercise: Divisors (solution)


                    def divisors(n):
                        """Returns all the divisors of N.

                        >>> divisors(12)
                        [1, 2, 3, 4, 6]
                        """
                        return [x for x in range(1, n) if n % x == 0]
                    

Exercise: Frontloaded


                    def front(s, f):
                        """Return S but with elements chosen by F at the front.

                        >>> front(range(10), lambda x: x % 2 == 1)  # odds in front
                        [1, 3, 5, 7, 9, 0, 2, 4, 6, 8]
                        """
                    

Exercise: Frontloaded (solution)


                    def front(s, f):
                        """Return S but with elements chosen by F at the front.

                        >>> front(range(10), lambda x: x % 2 == 1)  # odds in front
                        [1, 3, 5, 7, 9, 0, 2, 4, 6, 8]
                        """
                        return [e for e in s if f(e)] + [e for e in s if not f(e)]
                    

String literals

What's in a string?

Representing data:


                    '2,400' '2.400' '1.2e-5'
                    

Representing language:


                    """Se lembra quando a gente
                    Chegou um dia a acreditar
                    Que tudo era pra sempre
                    Sem saber
                    Que o pra sempre sempre acaba"""
                    

Representing programs:


                    'curry = lambda f: lambda x: lambda y: f(x, y)'
                    

String literals: 3 forms

Single quoted strings and double quoted strings are equivalent:


                    '您好, I am a string, hear me roar 🦁!'
                    "I've got an apostrophe"
                    

Multi-line strings automatically insert new lines:


                    """The Zen of Python
                    claims, Readability counts.
                    Read more: import this."""
                    # 'The Zen of Python\nclaims, Readability counts.\nRead more: import this.'
                    

The \n is an escape sequence signifying a line feed.

Strings are similar to lists


                    alfabeto = 'abcdefghijklmnñopqrstuvwxyz'

                    len(alfabeto)  # 27

                    alfabeto[13] + "andu" # ñandu

                    alfabeto + ' ¡Ya conoces el ABC!'
                    

Differences between strings & lists

A single-character string is the same as the character itself.


                    initial = 'P'
                    initial[0] == initial  # True
                    

The in operator will match substrings:


                    'W' in 'Where\'s Waldo'      # True
                    'Waldo' in  'Where\'s Waldo' # True