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. 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]

    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