Lists and Tuples

1. Lists and Tuples Intro to Computer Science CS1510 Dr. Sarah Diesburg

2. Homework Assignment PA07 • Regular deadline is Monday - 3/31 • Remember • Follow the new design document and function commenting • Example design document • Example function commenting

3. The Python List Data Structure • A list an ordered sequence of items of any type. • myList = [1, [2,1], ‘hello’]

4. List Indexing • We can index into a list in much the same way as a string >>> myList = [1, [2,1], ‘hello’] >>> myList[2] ‘hello’ • Let’s play with some indexing problems. Break up into teams of 2 or 3.

5. Example Program myList = [[‘Penny’, 90], [‘Leonard’, 85], 2] print(myList[0]) print(myList[0][0][3] + myList[1][0][2]) print((myList[0][1] + myList[1][1]) / myList[2]) #What is the output of the three print statements?

6. List Methods • myList.append(x) • myList.extend(C) • myList.pop() • myList.insert(i,x) • myList.remove(x) • myList.sort() • myList.reverse() • myList.index(x) • myList.count(x) Which methods modify the list??

7. Sorting • Only lists have a built-in sorting method. Thus you often convert your data to a list if it needs sorting: myLst = list(‘xyzabc’) myLst  [‘x’,’y’,’z’,’a’,’b’,’c’] myLst.sort() # convert back to a string sortStr = ‘’.join(myLst)  ‘abcxyz’

8. Sorted Function • The sorted function will break a sequence into elements and sort the sequence, placing the results in a list: sortLst = sorted(‘hi mom’)  [‘ ‘,’h’,’i’,’m’,’m’,’o’]

9. Unusual Results myLst = [4, 7, 1, 2] myLst = myLst.sort() myLst None # what happened? What happened was the sort operation changed the order of the list in place (right side of assignment). Then the sort method returned None, which was assigned to the variable. The list was lost and None is now the value of the variable.

10. Anagram Example • Anagrams are words that contain the same letters in a different order. For example: ‘iceman’ and ‘cinema.’ • A strategy to identify anagrams is to take the letters of a word, sort those letters, then compare the sorted sequences. • Anagrams should have the same sequence.

11. Anagram Program Algorithm • Input 2 words to examine. • Sort the letters of each word into a new string. • Compare the resulting sorted strings

12. Tuples

13. 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

14. 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.

15. 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.

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