Dictionaries

List

• Lists map numbers to values
• Lists are dense, meaning numbers from 0 up to the list length all map to a value

Dictionaries

• Can also map numbers to values
• Can be sparse meaning they don’t need to map all possible values
• Created using curly braces {}

Example

values = {}

values[0] = "a"
values[2] = "b"

print(values[2])

Hashable Types

• An object is hashable if it has a hash value which never changes during its lifetime and can be compared to other objects
• Hashable objects which compare equal must have the same hash value
• Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally

Dictionary Keys

• Integers may be used as keys
• Strings may be used as keys
• Any other hashable type may be used as a key

Example

values = {}

values["a"] = 1
values["b"] = 2

print(values["a"])

Exercise

Create a program that builds a dictionary that maps the days of the week to the time you need to wake up on these days.

Printing Dictionaries

• Dictionaries can be printed directly
• They will show value mapping between curly braces {}

Example

values = {}

values["a"] = 1
values["b"] = 2

print(values)

Instantiating Dictionaries with Values

• The output format from the previous example can be used to create new lists directly

Example

values = {"a": 1, "b": 2}

print(values)

Whitespace

• Whitespace may be used inside dictionary declarations
• New lines may be used to organize declarations

Example

poketypes = {
    "Charmander": "fire",
    "Squirtle": "water",
    "Bulbasaur": "grass",
}

print(f"Squirtle is {poketypes['Squirtle']} type")

in

• The in operator can be used
• in will return True if the key exist in the dictionary

Example

hrs = {
    "Bonds": 762,
    "Aaron": 755,
    "Ruth": 714,
}

print("Pujols" in hrs)
print("Aaron" in hrs)
print(762 in hrs)

Exercise

Create a program that can count the number of occurrences of each word in a text

text = """
  In the beginning was the Word,
  and the Word was with God,
  and the Word was God.
"""

Solution