Dictionary Types

* Dictionary types with special properties
  * Default Dictionary
  * Counters
* Require the collections module
  * includes several dictionary types (including the two just mentioned)
* can use all dictionary operators and methods

---

Dictionary Example 1

    dname={'first': 'Kathryn', 'last': 'Janeway'}
    namelist=[dname['first'], dname['last']]
    name=''.join(namelist)
    print(f'Character Name: {name}')

    # Output
    Character Name: Kathryn Janeway

Dictionary Example 2 (how to handle non existing keys)

    dname={'first': 'Spock'}
    namelist=[dname['first'], dname['last']]
    name=''.join(namelist)
    print(f'Character Name: {name}')

    # Output
    KeyError:'last'

    dname={'first': 'Spock'}
    if 'last' not in dname: 
        dname['last']=''
        namelist=[dname['first'], dname['last']]
    name=''.join(namelist)
    print(f'Character Name: {name}')

    # Output
    # Character Name: Spock

Default Dictionary

    import collections
    collections.defaultdict(factory, dictionary=None)

* return a default object
* default values for nonexitent keys are generated by a factory function

### Default Dictionary Example1

    import collections
    #we create a default dictionary and pass two arguments to it
    dname = collections.defaultdict(str, {'first':'Spock'})
    # we default to an empty str

    namelist=[dname['first'], dname['last']]
    # dname will default to an empty string
    name=''.join(namelist)
    print(f'Character Name: {name}')
    # Output
    # Character Name: Spock

Default Dictionary Example2

    import collections
    d = collections.defaultdict(list)
    #we create a default dictionary that defaults an empty list
    mylist=[1,1,2,3,5,8,13,21,34]
    for n in mylist:
        if n%2==0:
            key='even'
        else:
            key='odd'
        d[key].append(n)
    print(d)
    # Output
    # defaultdict(<class'list'>, {'odd': [1,1,3,5,13,21], 'even':[2,8,34]})

if we would have not used defaultdictionary, we would have to explicitly creat a new list as a value.

Counter

* Store count of items from a sequence in a dictionary
  * items form a sequence and serce as dictionary keys.
  * the values are the counts of the keys.
  * dictionary key is the item.
  * dictionary vlaue is the count.

Dictionary Count Example

```python
s= "Berkeley"
# this is our sequence
d={}
# this iwll store our count information
# letter is the key and account is the value
for c in s:
    # each letter in s will be stored in c
    if c not in d:
        d[c]=0
    d[c]+=1
    #if the key does not exist we default the value to zero
print(d)

# Output
# {'B':1,'e':3,'r':1,'k':1,'l':1,'y':1,}
```

Default Dictionary Count Example

    import collections
    d = collections.defaultdict(int)
    # int: default new items to the value zero
    s= "Berkeley"
    for c in s:
        d[c]=d[c]+1
    print(d)

    # Output
    # defaultdict(<class'int'>, {'B':1,'e':3,'r':1,'k':1,'l':1,'y':1,})

Syntax

    import collections
    collections.Counter(iterable)

* return a Counter object form a given iterable
* it consist of items from iterable as keys and their counts as values

Counter Example

The default dict class already has reduced our code, 
but the counter class shrink our code to jsut one line (d = collections.Counter(s))
```python
import collections
s= "Berkeley"
d = collections.Counter(s)
print(d)

# Output
# Counter({'e':3,'B':1,'r':1,'k':1,'l':1,'y':1,})
```

Counter Update Method

Syntax

    counter.update(iterable)

• update counter object counts with a given iterable

Most common example

    import collections
    s= "Berkeley"
    d = collections.Counter(s)
    print(d)
    d.update(['k','e''y'])
    print(d)
    # Output
    # Counter({'e':3,'B':1,'r':1,'k':1,'l':1,'y':1})
    # Counter({'e':4,k':2,'y':2,'B':1,'r':1,''l':1})