Lists and For Loops

Lists

• list: an ordered collection of items defined with [ ]
• index: numeric position (with first item at index 0)
• len(x): number of items in x
• Negative indices count backwards, from the end
• in / not in: test inclusion/exclusion

colors = ["red", "blue", "green", "yellow"]
print(colors[0])   # red
print(colors[1])   # blue
print(colors[-1])  # yellow
print(len(colors)) # 4
print("red" in colors)  # True
print("purple" in colors)  # False

Strings as Sequences

• Strings are sequences of characters: we can use len(), indexing, and membership on strings too

word = "hello"
print(word[0])  # "h"
print(word[-2])  # "l"
print(len(word))  # 5
print("h" in word)  # True

range()

• range(stop): starts at 0, stops before stop
• range(start, stop): starts at start, stops before stop
• range(start, stop, step): starts at start, stops before stop, counting by step

for i in range(5):
    print(i)  # 0..4

for i in range(2, 7):
    print(i)  # 2..6

for i in range(1, 10, 2):
    print(i)  # 1, 3, 5, 7, 9

For Loops

• The loop variable takes each value of the iterable in turn
• We can use for with any of our iterable types: range(...), lists, strings

for count in range(3):
    print(count)

colors = ["red", "blue", "green", "yellow"]
for color in colors:
    print(color)

word = "hello"
for letter in word:
    print(letter)

Building Lists with append()

• list.append(x) adds an item to the end of a list
• This is useful for building lists while the program is running

items = []
for i in range(3):
    items.append(i)
print(items)  # [0, 1, 2]

Random Selection

• We can get random items from a list in multiple ways:
  • if we need the index, we can use our familiar random.randint to get a random index, then use than index to get an item
  • if we don't need the index, we can use random.choice to get a random item directly

import random
colors = ["red", "blue", "green", "yellow"]

# If we need the index:
random_index = random.randint(0, len(colors) - 1)
print(colors[random_index])

# If we don't need the index:
print(random.choice(colors))