Lists and Tuples

1
Lists and Tuples

• Intro to Computer Science
• CS1510
• Dr. Sarah Diesburg

2
Homework Assignment PA07

• Regular deadline is Monday - 11/3
• 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
• gtgtgt myList 1, 2,1, hello
• gtgtgt myList2
• hello
• Lets play with some indexing problems. Break up
  into teams of 2 or 3.

5
Example Program

• myList Penny, 90, Leonard, 85, 2
• print(myList0)
• print(myList003 myList102)
• print((myList01 myList11) / myList2)
• 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

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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

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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

1. Input 2 words to examine.
2. Sort the letters of each word into a new string.
3. 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

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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.

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Anagram Example with Tuples

• Lets see if we can modify our anagram program to
  use tuples