Class Notes: Dictionaries

  1. Quick Example [Pre-reading]
  2. Creating Dictionaries [Pre-reading]
  3. Using Dictionaries [Pre-reading]
  4. Properties of Dictionaries
    1. Dictionaries Map Keys to Values [Pre-reading]
    2. Keys are Sets [Pre-reading]
    3. Values are Unrestricted [Pre-reading]
    4. Dictionaries are Very Efficient
  5. Some Worked Examples Using Dictionaries

  1. Quick Example [Pre-reading]

    2.   
    # A dictionary is a data structure that maps keys to values in the same way
# that a list maps indexes to values. However, keys can be any immutable value!

stateMap = { 'pittsburgh':'PA', 'chicago':'IL', 'seattle':'WA', 'boston':'MA' }
city = input("Enter a city name --> ").lower()
if (city in stateMap):
    print(city.title(), "is in", stateMap[city])
else:
    print("Sorry, never heard of it.")

    Another Example: 
    counts = dict()
while True:
    n = int(input("Enter an integer (0 to end) --> "))
    if (n == 0): break
    if (n in counts):
        counts[n] += 1
    else:
        counts[n] = 1
    print("I have seen", n, "a total of", counts[n], "time(s)")
print("Done, counts:", counts)

  2. Creating Dictionaries [Pre-reading]

    3.  

  3. Using Dictionaries [Pre-reading] 

    4. # We can interact with dictionaries in a similar way to lists/sets
d = { "a" : 1, "b" : 2, "c" : 3 }

print(len(d)) # prints 3, the number of key-value pairs

print("a" in d) # prints True
print(2 in d) # prints False - we check the keys, not the values
print(2 not in d) # prints True
print("a" not in d) # prints False

print(d["a"]) # finds the value associated with the given key. Crashes if the key is not in d
print(d.get("z", 42)) # finds the value of the key if the key is in the dictionary,
# or returns the second (default) value if the key is not in d

d["e"] = "wow" # adds a new key-value pair to the dictionary, or updates the value of a current key
del d["e"] # removes the key-value pair specified from the dictionary. Crashes if the key is not in d

for key in d:
    print(key, d[key]) # we can iterate over the keys, then print out the keys or corresponding values

  4. Properties of Dictionaries [Pre-reading]

    5.  

    1. Dictionaries Map Keys to Values [Pre-reading] 

      2. ages = dict()
key = "fred"
value = 38
ages[key] = value  # "fred" is the key, 38 is the value
print(ages[key])

    2. Keys are Sets [Pre-reading]

      • Keys are unordered 
        d = dict()
d[2] = 100
d[4] = 200
d[8] = 300
print(d)  # unpredictable order

      • Keys are unique 
        d = dict()
d[2] = 100
d[2] = 200
d[2] = 400
print(d)  # { 2:400 }

      • Keys must be immutable 
        d = dict()
a = [1] # lists are mutable, so...
d[a] = 42 # Error: unhashable type: 'list'

    3. Values are Unrestricted [Pre-reading] 

      4. # values may be mutable
d = dict()
a = [1,2]
d["fred"] = a
print(d["fred"])
a += [3]
print(d["fred"]) # sees change in a!

# but keys may not be mutable
d[a] = 42       # TypeError: unhashable type: 'list'

    4. Dictionaries are Very Efficient

      5. As mentioned above, a dictionary's keys are stored as a set. This means that finding where a key is stored takes constant time. This lets us look up a dictionary's value based on a key in constant time too!

  5. Some Worked Examples Using Dictionaries