lect15

’’’ Dictionaries

New Tool: DICTIONARIES
Otherwise known as TABLES or MAPS
- Works where a KEY maps to a VALUE
- Use dictionaries for two main reasons:
  - Gives us more precise indexing than Lists
  - L[‘someString’] - allows us to index elements based on some key
  - Performance is MUCH better than searching through Lists.
    - keys are HASHED to a specific index value
      - keys are passed through some math equation and an index is computed
      - Provides DIRECT ACCESS to the value given a key.
    - Details of hashing is not discussed in this class, but you may see it sometime… ‘’’ ‘’’ Syntax for Dictionaries {:,:, ... , :valuen>} '''

D = {} # Empty dictionary. Notice the curly braces instead of []

D = {‘Jane’:18, ‘Jen’:19, ‘Joe’:20, ‘John’:21}

’’’ print(D) print(“Jane’s age is:”, D[‘Jane’]) print(len(D))

Simple way to get key / value

for x in D: print(x) # Prints the keys print(D[x]) # Prints the values ‘’’ ‘’’ Restrictions on using Dictionaries

Keys must be an immutable type (int, str, namedtuples, - not lists for example).
Values can be anything (mutable or immutable objects)
For our purposes, KEYS are UNIQUE. Don’t define something like {‘Joe’:17, ‘Joe’:18}.
Python is actually OK with duplicate keys and it will return the last key/value in the dictionary.
Again, for our purposes, we should never use duplicate key values (kinda defeats the purpose of dictionaries). ‘’’

’’’ dictionary methods D.pop(key) # remove and return the value D.update(D2) # combine values in D and D2 D.get(key) # returns item if it exists. If not, returns None or a default value D.keys() # dict_keys([ LIST OF KEYS ]) D.values() # dict_values([ LIST OF VALUES ]) D.items() # dict_items([ LIST OF KEY,VALUE ]) ‘’’ ‘’’ value = D.pop(‘John’) print(D) print(value) D.update({‘Jess’:25,’Jerry’:27}) print(D)

What if something is not in the dictionary and we try to access it?

#print(D[“CS8”]) #ERROR - “CS8” key doesn’t exist. print(D.get(“CS8”))

Can define the default return value if key doesn’t exist

print(D.get(“CS8”, “not in dictionary!”))

print(D.keys()) print(D.values()) ‘’’ ‘’’ for item in D.keys(): print(item) ‘’’ ‘’’ keys = D.keys() #print(keys[2]) # ERROR - Not a list! print(list(keys)[2]) # OK ‘’’

Example of adding to a dictionary

D = {} print(D) D[‘CS8’] = “UCSB” print(D) D[‘CS16’] = “UCSB” print(D) ‘’’

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