1. Lists 2 Day 11 - 9/19/14 LING 3820 & 6820 Natural Language Processing
Harry Howard
Tulane University

2. Course organization http://www.tulane.edu/~howard/LING3820/
The syllabus is under construction.
NLP, Prof. Howard, Tulane University
19-Sept-2014

3. Review of Lists
A list in Python is a sequence of objects delimited by square brackets, [].
NLP, Prof. Howard, Tulane University
19-Sept-2014

4. Summary of string & list ops
operation
string S
list L
number of elements
len(S)
len(L)
alphabetical order
sorted(S)
sorted(L)
set formation
set(S)
set(L)
concatenation
S1+S2
L1+L2
duplication
S*2
L*2
count duplicates
S.count()
L.count()
find position
S.index()
L.index()
slice
S[]
L[]
conversion
S.split()
‘’.join(L)
NLP, Prof. Howard, Tulane University
19-Sept-2014

5. The inverse relation between splitting and joining
NLP, Prof. Howard, Tulane University
19-Sept-2014

6. §5. Lists
NLP, Prof. Howard, Tulane University
19-Sept-2014

7. Open Spyder
NLP, Prof. Howard, Tulane University
19-Sept-2014

8. 5.2.2. Strings and lists share the sequence type
Strings and lists share the count(), index() and [] operations because they are both members of Python’s sequence type
In a nutshell, a sequence type has an index associated with every one of its elements.
It is the indexation which endows the type with the ability to be counted, retrieved by position and sliced.
NLP, Prof. Howard, Tulane University
19-Sept-2014

9. Sets are not sequences
Sets are not indexed, so they do not permit counting (obviously, since all the duplicates are removed), retrieval by position or slicing.
All of the following operations produce an error:
# reassign L if it is not available:
L = 'Love looks not with the eyes, but with the mind.'.split()
>>> setL = set(L)
>>> setL.count('the')
>>> setL.index('with')
>>> setL[0]
Thus sets have almost the ‘opposite’ properties of lists, being unorderd and without duplicates.
NLP, Prof. Howard, Tulane University
19-Sept-2014

10. 5.3. How strings and lists differ
# reassign L if it is not available:
L = 'Love looks not with the eyes, but with the mind.'.split()
>>> L.append('Awesome!')
>>> L
>>> L.extend("You tell'em, Will!".split())
>>> L.insert(5,'bloodshot')
>>> L.remove('bloodshot')
>>> L.pop(5)
NLP, Prof. Howard, Tulane University
19-Sept-2014

11. None of these apply to strings
>>> S = ' '.join(L)
>>> S.append('Awesome!')
>>> S.extend("You tell'em, Will!".split())
>>> S.insert(5,'bloodshot')
>>> S.remove('not')
>>> S.pop(5)
NLP, Prof. Howard, Tulane University
19-Sept-2014

12. Lists are mutable Do you know why these methods fail on a string?
Recall our discussion of mutability.
We observed that, once a string is created, its constituency cannot be altered.
In Pythonese, such an object is called immutable.
What does the first block of code in this section say about the mutability of a list?
Well, ``L`` is clearly mutable; every method above changes its constituents.
So here we have the principle difference between the two types, strings are immutable while lists are not.
Thus if you need a data type whose elements will wax and wane over the course of your program, a list is the way to go.
NLP, Prof. Howard, Tulane University
19-Sept-2014

13. 5.3.1. How to reverse and randomize the order of a list
Given its mutability, not only can elements be added or subtracted from a list, but they also should be able to be moved around. Try this:
>>> L2 = "If you can't be with the one you love, love the one you're with.".split()
>>> L2.reverse()
>>> L2
>>> L2[::-1]
reverse(), it comes as no surprise, reverses the order of elements in a list. What is a surprise, though, is that it does so “in place”. That is, it reverses the list within the method, rather than sending the reversal as output to a new list. Thus the original order is destroyed. It is easy enough to recover, by reversing the reversal, or using the slice trick that you are reminded of in line 4.
NLP, Prof. Howard, Tulane University
19-Sept-2014

14. The original order
But what if you wanted to use the original order at the same time?
It seems logical to just assign the original list to a new list and reverse it, so that you wind up with two lists, one in the original order and one reversed. Don’t just sit there, try it out:
# recover the original order by reversing what you just did
>>> L2.reverse()
>>> L3 = L2
>>> L3.reverse()
>>> L3
>>> L2
NLP, Prof. Howard, Tulane University
19-Sept-2014

15. The official solution
# If necessary, recover the original order by reversing what you just did
>>> L2.reverse()
>>> from copy import copy
>>> L4 = copy(L2)
>>> L4.reverse()
>>> L4
>>> L2
NLP, Prof. Howard, Tulane University
19-Sept-2014

16. The sneaky solution >>> L5 = L2[:] >>> L5.reverse()
NLP, Prof. Howard, Tulane University
19-Sept-2014

17. Shuffling, again >>> L6 = copy(L2) >>> L6
>>> from random import shuffle
>>> shuffle(L6)
>>> L2
NLP, Prof. Howard, Tulane University
19-Sept-2014

18. Next time Q3
Unicode
NLP, Prof. Howard, Tulane University
19-Sept-2014