Lesson 14 Notes: Dictionaries (When a list just won't do)

• Remember Lists?
  • Lists are collections of items (strings, integers, or even other lists).
  • Each item in the list has an assigned index value.
  • list = ["first", "second", "third"]
  • list[0] == "first"
  • list[1] == "second"
  • list[2] == "third"
• List Limitations
  • To get a value out of a list, you must know its index position.
  • If you remove an item from a list, the index may change for the other items in the list
  .
• Dictionary Concept
  • Dictionaries are collections of items that have a "key" and a "value".
  • They are just like lists, except instead of having an assigned index number, you make up the index.
  • list = ["first", "second", "third"]
  • dictionary = {0:"first", 1:"second", 2:"third"}
• Creating a Dictionary
  • Dictionaries are enclosed by {}
  • To create a dictionary, provide the key and the value.
  • A colon is placed between key and value (key:value)
  • Each key must be unique.
  • Each key:value pair is separated by a comma.
  • espanol = {'uno':'one', 'dos':'two'}
• Sample Dictionary
  nihon_go = {}
nihon_go["ichi"] = "one"
nihon_go["ni"] = "two"
nihon_go["san"] = "three"
print nihon_go
print nihon_go
{'ichi':'one', 'ni':'two', 'san':'three'}
{'ichi':'one', 'ni':'two', 'san':'three'}
• Using a Dictionary
  • To get a value out of a dictionary, you must enter the key.
  • Dictionaries only work one way. You must provide the key to get a value. You cannot provide the value and get the key.
  • Example:
    nihon = {'ichi':'one', 'ni':'two'}
print nihon['ichi']
'one'
    
  • NOTE: This dictionary will translate Japanese to English, but not English to Japanese
• Editing a Dictionary
  • To change a value pair, reassign it:
    nihon["ichi"] = 1
  • To add a key value pair, enter it like so:
    nihon["shi"] = "Four"
  • To remove a value pair, use del
    del nihon["ichi"]
  • To see if a key exists, use has_key() method
    nihon.has_key("ichi")
  • To copy whole dictionary, use the copy() method
    japanese = nihon.copy()
• Dictionaries are Mutable
  • Dictionaries are mutable. You do not have to reassign the dictionary to make changes to it.
• Uses of Dictionaries
  • Dictionaries are useful whenever you have to items that you wish to link together.
  • Making substitutions (replace all x with y)
  • Storing results for quick lookup
  • Program menus
  • Creating mini databases of information
• Sample Code: A Menu
  import string
def add(num1,num2):
    print num1+num2
def mult(num1,num2):
    print num1*num2
num1 = input("Enter first number: ")
num2 = input("Enter second number: ")
menu = {'A':add, 'M':mult}
print "[A] to add, [M] to multiply: "
choice = string.upper(raw_input())
menu[choice](num1,num2)
• Review
  • How is a dictionary different from a list?
  • How do you create a dictionary?
  • How do you access an entry in a dictionary?
  • How do you add an entry? Delete an entry?
  • What situations might be good for using a dictionary instead of a list?
  
  Restricted access