Tuples and Dictionaries

1. Tuples and Dictionaries Intro to Computer Science CS1510, Section 2 Dr. Sarah Diesburg

2. Homework Assignments • PA07 regular deadline is today - 11/4 • New homework PA08 assigned today • Elise is available to help with homework in-person or in email • Email: fahrenke@uni.edu • 3-4pm M/W in WRT 339

3. Tuples • Last time we finished up talking about lists • We made an anagram program • Today we will briefly go over tuples • Like lists, but immutable!

4. Tuples • Tuples are easy: they are simply immutable lists. • They are designated with a comma: • Not the parenthesis! myTuple = (1,’a’,3.14,True) newTuple = (,) #Empty tuple anotherTuple = 2,3 #Tuple without parenthesis

5. The Question is, Why? • The real question is, why have an immutable list, a tuple, as a separate type? • An immutable list gives you a data structure with some integrity, some permanency, if you will. • You know you cannot accidentally change one.

6. Lists and Tuples • Everything that works with a list works with a tuple except methods that modify the list/tuple. • Thus indexing, slicing, len, print all work as expected. • However, none of the mutable methods work: append, extend, remove.

7. Anagram Example with Tuples • Let’s see if we can modify our anagram program to use tuples

8. Chapter 9Dictionaries

9. More Data Structures • We have seen the string and list data structures and their uses. • In particular, the dictionary is an important, very useful part of Python as well as generally useful to solve many problems.

10. What is a Dictionary? • In data structure terms, a dictionary is better termed an associative array or associative list or a map. • You can think if it as a list of pairs, where the first element of the pair, the key, is used to retrieve the second element, the value. • Thus we map a key to a value.

11. Key Value Pairs • The key acts as a “lookup” to find the associated value. • Just like a dictionary, you look up a word by its spelling to find the associated definition. • A dictionary can be searched to locate the value associated with a key.

12. Python Dictionary • Use the { } marker to create a dictionary • Use the : marker to indicate key:value pairs: contacts= {‘bill’: ‘353-1234’, ‘rich’: ‘269-1234’, ‘jane’: ‘352-1234’} print (contacts) {‘jane’: ‘352-1234’, ‘bill’: ‘353-1234’, ‘rich’: ‘369-1234’}

14. Keys and Values • Key must be immutable: • strings, integers, tuples are fine • lists are NOT • Value can be anything.

15. Collections but not a Sequence • Dictionaries are collections, but they are not sequences like lists, strings or tuples: • there is no order to the elements of a dictionary • in fact, the order (for example, when printed) might change as elements are added or deleted. • So how to access dictionary elements?

16. Access Dictionary Elements Access requires [ ], but the key is the index! myDict={} • an empty dictionary myDict[‘bill’]=25 • added the pair ‘bill’:25 print(myDict[‘bill’]) • prints 25

17. Dictionaries are Mutable • Like lists, dictionaries are a mutable data structure: • you can change the object via various operations, such as index assignment myDict = {‘bill’:3, ‘rich’:10} print (myDict[‘bill’] ) # prints 3 myDict[‘bill’] = 100 print (myDict[‘bill’])# prints 100

18. Again, Common Operators Like others, dictionaries respond to these: • len(myDict) • number of key:value pairs in the dictionary • element in myDict • boolean, is element a key in the dictionary • for key in myDict: • iterates through the keys of a dictionary

19. Lots of Methods • myDict.items() – all the key/value pairs • myDict.keys() – all the keys • myDict.values() – all the values • key in myDict does the key exist in the dictionary • myDict.clear() – empty the dictionary • myDict.update(yourDict) – for each key in yourDict, updates myDict with that key/value pair

20. Dictionaries are Iterable for key in myDict: print(key) • prints all the keys for key,value in myDict.items(): print(key,value) • prints all the key/value pairs for value in myDict.values(): print(value) • prints all the values

21. Doing something with this • Write a function called letterCount that: • takes in a string as a parameter • prints a table of the letters of the alphabet (in alphabetical order ) together with the number of times each letter occurs. • Case should be ignored.