Lists Intro

Where we learn about:

  • What lists are and why we use them
  • Creating lists and accessing elements (indexing)
  • Basic list operations (length, accessing items)

Part 1 Warmup (5 minutes)

1.1

for i in range(4):
    print(i, "Mississippi")

a. How many times does this loop run?

b. What’s the first value of i?

c. What’s the last value of i?

1.2

x = 5
y = x + 2
x = 7
print(y)

a. What value is in y when it gets printed?

b. Does changing x after setting y affect y’s value?

1.3

A few lessons ago, we looked at this problem:

# We want to say hello to 5 friends:
# Alice, Bob, Charlie, Diana, and Eddie

count = 0
while count < 5:
    print("Hello", "???")
    count = count + 1

Then we learned a way to solve it using for ... in:. How would you fill in the blanks to make the program below work?

friends = ["Alice", "Bob", "Charlie", "Diana", "Eddie"]

for friend in ______:
    print("Hello", _____)

Part 2. Predictions (15 minutes)

2.1

# Program A
numbers = [1, 2, 3]
print("First number:", numbers[0])
print("All numbers:", numbers)

# Output:
# First number: 1
# All numbers: [1, 2, 3]

# Program B
names = ["Al", "Bob", "Cal"]
print("Winning name:", names[2])
print("All names:", names)

# Output:
# Winning name: Cal
# All names: ['Al', 'Bob', 'Cal']

Question: What will Program C output?

# Program C
grades = ["A", "B", "C"]
print("Uhhhh:", grades[0])
print("Grades:", grades)
print("OK:", grades[1])

# Output:
# ...

2.2

# Program A
nums = [10, 20, 30, 40]
print("Last:", nums[-1])
print("Second to last:", nums[-2])
# Output:
# Last: 40
# Second to last: 30

# Program B
words = ["apple", "banana", "cherry"]
print("Last fruit:", words[-1])
print(words[-2])
# Output:
# Last fruit: cherry
# banana

Question: What will Program C output?

# Program C
colors = ["red", "green", "blue"]
print(colors[-1])
print(colors[-3])
# Output:
# ...

2.3

# Program A
scores = [75, 80, 85]
print("Before:", scores)
scores[0] = 95
print("After:", scores)
# Output:
# Before: [75, 80, 85]
# After: [95, 80, 85]

# Program B
names = ["Al", "Bob", "Cal"]
print("Before:", names)
names[1] = "Ben"
print("After:", names)
# Output:
# Before: ['Al', 'Bob', 'Cal']
# After: ['Al', 'Ben', 'Cal']

Question: What will Program C output?

# Program C
pets = ["cat", "dog", "fish"]
print("Before:", pets)
pets[2] = "bird"
print("After:", pets)
# Output:
# ...

2.4

# Program A
fruits = ["apple", "banana"]
print("Is apple included?", "apple" in fruits)
print("Is orange included?", "orange" in fruits)
# Output:
# Is apple included? True
# Is orange included? False

# Program B
nums = [1, 2, 3]
print("Is 2 in the list?", 2 in nums)
print(4 in nums)
# Output:
# Is 2 in the list? True
# False

Question: What will Program C output?

# Program C
colors = ["red", "blue"]
print("red" in colors)
print("green" in colors)
# Output:
# ...

Part 3. Explanation (15 minutes)

A list is an ordered collection of items

  • Items can be numbers, strings, or other types
  • Items are stored in sequence
  • Items can be accessed by their position (index)

List indexing

  • Indexes start at 0
  • Last item is at length-1
  • Negative indexes count from the end (-1 is last item)

Basic list operations

  • Create: my_list = [item1, item2, item3]
  • Access: my_list[index]
  • Modify: my_list[index] = new_value
  • Length: len(my_list)
  • Check membership: if item in my_list:

Part 4. Practice

4.1 Approaching

  • Create a list called weekdays containing the names of the five weekdays.
  • Then print the list.
  • Then use list indexing to print a sentence that tells us your favorite weekday.

4.2 Proficient

temperatures = [72, 75, 73, 71, 74]
# 1. Print the number of temperatures
# 2. Print whether 76 is in the list
# 3. Change the last temperature to 77
# 4. Print the first three temperatures only
# 5. Check if 71 and 77 are both in the list

names = ["Alice", "Bob", "Charlie", "Diana"]
# 1. Add your name to the beginning
# 2. Replace "Charlie" with "Chuck"
# 3. Check if the list has exactly 4 names
# 4. Print the second letter of the first name
# 5. Print the last letter of the last name

4.3 Distinguished

# Create a set of list representing a tic-tac-toe board
# ["-", "-", "-"]
# ["-", "X", "-"]
# ["-", "-", "-"]
# Then change the center position to "O"
# Print the board so it looks like this:
#
# - - -
# - O -
# - - -

4.4 Super Extra Chellenge

songs = ["Song1", "Song2", "Song3", "Song4"]
durations = [3.5, 4.0, 3.0, 4.5]
# Write code to:
# 1. Print each song with its duration
# 2. Find the total playlist duration
# 3. Find the longest song
# 4. Add a new song and duration
# 5. Remove the shortest song