1. CSE 231 Lab 8

2. Dictionaires vs lists Creating Dictionaries Dictionaries – Methods
Topics to cover
Dictionaires vs lists
Creating Dictionaries
Dictionaries – Methods

3. Dictionaries - Visualizing
Insertion-ordered collection of Python objects (Python >=3.6)
What is a key? What is a value?
Should be immutable
Any objects is allowed
A key is unique, there are no duplicates. Think of a word in a dictionary. There is only one entry for each word. You won’t find “key” in the dictionary twice.

4. Dictionaries vs Lists

5. Given a dictionary, D, where: D[“key”] = “value” print(D[“key”])
What happens?
Given a dictionary, D, where:
D[“key”] = “value”
print(D[“key”])
“value”
The above format will print the value associated with the key in the dictionary.
The key is “key”, the value is “value”, so this will print “value”

6. Create and update a dictionary
Are dictionaries mutable?
Creating a Dictionary
M = {}
M = dict() #preferred
M = { 200: "EE", 100: "ME"}
Adding/updating a value in a Dictionary
M[500] = "CS"

7. Patterns Ints: Strings: Lists: x = 0 # initialize at zero
x += 1 # increment the counter
Strings:
s = ‘’ # initialize with empty string
s += ch # concatenate characters
Lists:
L = [] # initialize with empty list
L.append(value) # append values to the list

8. Patterns Dictionaries: D = { } # initialize with empty dictionary
S = “aabacdbacd” # we have a string, we want to count all the characters
for ch in S:
if ch in D: # check to see if the key exists in the dictionary
D[ch] += 1 # increment the value if it exists
else:
D[ch] = 1 # set the value to 1 if it doesn’t exist

9. Dictionaries – Methods
.items() # Returns a set-like object of all the key-value pairs. .keys() # Returns a set-like object of all the keys. .values() # Returns a set-like pbject of all the values. .pop(key) # Removes entire associated with key and returns it’s value.

10. more Useful Methods & functions
D = M.get( 200 )
returns the value if key exists, else returns None
E = M.pop( "Mike", None )
pops (and returns) the value if the key exists, returns the second argument otherwise
del M[600]
deletes if the key exist, KeyError if it doesn't
D = dict(L) # assuming L = [('a',1),('b',2),('c',3)]
D = {'a': 1, 'b': 2, 'c': 3}

11. Iteration for key in M.keys(): for key in M: for value in M.values():
for key, value in M.items():

12. D = {“one” : 1, “two” : 2, ”three” : 3} for x,y in D.items():
What happens?
Given a dictionary D,
D = {“one” : 1, “two” : 2, ”three” : 3}
for x,y in D.items():
print(x,y)
one 1
two 2
three 3
x and y will give each key and the corresponding value from the dictionary, so this statement will print every key-value pair in the dictionary

13. What happens?
Given a dictionary D,
D = {}
Print(D[‘a’])
KeyError

14. Other useful methods and function
D = {“one” : 1, “two” : 2, ”three” : 3}
list(D) # returns a list of all the keys used in the dictionary
['one', 'two', 'three']
sorted(D) # in insertion order (if you want it sorted, just use sorted instead)
['one', 'three', 'two']
Performing list(d) on a dictionary returns a list of all the keys used in the dictionary, in insertion order (if you want it sorted, just use sorted(d) instead). To check whether a single key is in the dictionary, use the in keyword.