collections code example

Example 1: Python Ordered Dictionary

from collections import OrderedDict

# Remembers the order the keys are added!
x = OrderedDict(a=1, b=2, c=3)

Example 2: collection framework

LIST: Can store duplicate values,
      Keeps the insertion order. 
      It allows multiple null values, 
      Also we can read a certain value by index.
- ArrayList not syncronized, array based class 
- LinkedList not synchronized, doubly linked
- Vector is synchronized, thread safe

SET: Can only store unique values, 
     And does not maintain order
- HashSet can have null, order is not guaranteed
- LinkedHashSet can have null and keeps the order 
- TreeSet sorts the order and don't accept null 

QUQUE : Accepts duplicates, 
        Doesn't have index num,
        First in first our order.  
       
MAP : is a (key-value format) 
      and keys are always unique, 
      and value can be duplicated. 
- HashTable don't have null key, sychronized(thread-safe)
- LinkedHashMap can have null key, keeps order
- HasHMap can have null key, order is not guaranteed
- TreeMap doesn't have null key and keys are sorted

Example 3: python ordereddict

>>> # regular unsorted dictionary
>>> d = {'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}

>>> # dictionary sorted by key
>>> OrderedDict(sorted(d.items(), key=lambda t: t[0]))
OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])

>>> # dictionary sorted by value
>>> OrderedDict(sorted(d.items(), key=lambda t: t[1]))
OrderedDict([('pear', 1), ('orange', 2), ('banana', 3), ('apple', 4)])

>>> # dictionary sorted by length of the key string
>>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])

Example 4: counter most_common

most_common([n])¶
Return a list of the n most common elements and their counts from the most common to the least. If n is omitted or None, most_common() returns all elements in the counter.
Elements with equal counts are ordered arbitrarily:

>>> Counter('abracadabra').most_common(3)
[('a', 5), ('r', 2), ('b', 2)]

Example 5: how to define a collection in java

List<String> list = Arrays.asList("Lars", "Simon");

Example 6: collections

LIST: Can store duplicate values,Keeps the insertion order.
- ArrayList not syncronized, array based class 
- LinkedList not synchronized, doubly linked
- Vector is synchronized, thread safe
SET: Can only store unique values,And does not maintain order
- HashSet can have null, order is not guaranteed
- LinkedHashSet can have null and keeps the order 
- TreeSet sorts the order and don't accept null 
QUQUE :Accepts duplicates, Doesn't have index num,
First in first our order.        
MAP:is a (key-value format) 
and keys are always unique, 
and value can be duplicated. 
- HashTable don't have null key, sychronized(thread-safe)
- LinkedHashMap can have null key, keeps order
- HasHMap can have null key, order is not guaranteed
- TreeMap doesn't have null key and keys are sorted
Examples
 ■ I have used TreeSet to print dropdown list in for 
 non duplicate values and ascending order 
■ I have used HashMaps to compare values from 
a database with expected values