COMPUTATION WITH STRINGS 2 DAY 2 - 8/29/14 LING 3820 & 6820 Natural Language Processing Harry Howard Tulane University

Course organization 29-Aug-2014NLP, Prof. Howard, Tulane University 2   The syllabus is under construction.   Is there anyone here that wasn't here on Wednesday?  I didn't put together any practice, because we have done too little.  I will you some practice to do over the weekend.

Computer hygiene  You must turn your computer off every now and then, so that it can clean itself.  By the same token, you should close applications every now and then. 29-Aug NLP, Prof. Howard, Tulane University

What is a string? What is an escape character? What do these do: +, *, len(), sorted(), set()? What is the difference between a type & a token? Does Python know what you mean? Review 29-Aug NLP, Prof. Howard, Tulane University

A string is a sequence of characters delimited between single or double quotes.string §3. Computation with strings 29-Aug NLP, Prof. Howard, Tulane University

Open Spyder 29-Aug NLP, Prof. Howard, Tulane University

Method notation  The material aggregated to a method in parentheses is called its argument(s).  In the examples above, the argument S can be thought of linguistically as the object of a noun: the length of S, the alphabetical sorting of S, the set of S. But what if two pieces of information are needed for a method to work, for instance, to count the number of o’s in otolaryngologist?  To do so, Python allows for information to be prefixed to a method with a dot: >>> S.count('o')  The example can be read as “in S, count the o’s”, with the argument being the substring to be counted, 'o', and the attribute being the string over which the count progresses, or more generally:  attribute.method(argument)  What can be attribute and argument varies from method to method and so must be memorized. 29-Aug-2014NLP, Prof. Howard, Tulane University 7

How to clean up a string  There is a group of methods for modifying the properties of a string, illustrated below. You can guess what they do from their names: >>> S = 'i lOvE yOu' >>> S >>> S.lower() >>> S.upper() >>> S.swapcase() >>> S.capitalize() >>> S.title() >>> S.replace('O','o') >>> S.strip('i') >>> S2 = ' '+S+' ' >>> S2 >>> S2.strip() 29-Aug-2014NLP, Prof. Howard, Tulane University 8

3.3. How to find your way around a string 29-Aug NLP, Prof. Howard, Tulane University

index() or rindex()  You can ask Python for a character’s index with the index() or rindex() methods, which take the string as an attribute and the character as an argument: 1. >>> S = 'otolaryngologist' 2. >>> S.index('o') 3. >>> S.rindex('o') 4. >>> S.index('t') 5. >>> S.rindex('t') 6. >>> S.index('l') 7. >>> S.rindex('l') 8. >>> S.index('a') 9. >>> S.rindex('a') 29-Aug-2014NLP, Prof. Howard, Tulane University 10

find() & rfind()  Python also has a method find(), which appears to do the same thing as index() : 1. >>> S.find('o') 2. >>> S.rfind('o') 3. >>> S.find('t') 4. >>> S.rfind('t') 5. >>> S.find('l') 6. >>> S.rfind('l') 7. >>> S.find('a') 8. >>> S.rfind('a') 29-Aug-2014NLP, Prof. Howard, Tulane University 11

index() or find()  Where they differ lies in how they handle null responses: 1. >>> S.find('z') >>> S.index('z') 4. Traceback (most recent call last): 5. File " ", line 1, in 6. ValueError: substring not found 29-Aug-2014NLP, Prof. Howard, Tulane University 12

How to find substrings  These two methods can also find substrings: 1. >>> S.find('oto') 2. >>> S.index('oto') 3. >>> S.find('ist') 4. >>> S.index('ist') 5. >>> S.find('ly') 6. >>> S.index('ly') 29-Aug-2014NLP, Prof. Howard, Tulane University 13

Limiting the search to a substring  index() and find() allow optional arguments for the beginning and end positions of a substring, in order to limit searching to a substring’s confines: 1. >>> S.index('oto', 0, 3) 2. >>> S.index('oto', 3) 3. >>> S.find('oto', 0, 3) 4. >>> S.find('oto', 3)  index/find(string, beginning, end) 29-Aug-2014NLP, Prof. Howard, Tulane University 14

Zero-based indexation 29-Aug NLP, Prof. Howard, Tulane University

0 = 1  You probably thought that the first character in a string should be given the number 1, but Python actually gives it 0, and the second character gets 1.  There are some advantages to this format which do not concern us here, but we will mention a real- world example.  In Europe, the floors of buildings are numbered in such a way that the ground floor is considered the zeroth one, so that the first floor up from the ground is the first floor, though in the USA, it would called the second floor. 29-Aug-2014NLP, Prof. Howard, Tulane University 16

In a picture 29-Aug-2014NLP, Prof. Howard, Tulane University 17

Finding characters given a position 1. >>> S = 'abcdefgh' 2. >>> S[2] 3. >>> S[5] 4. >>> S[2:5] 5. >>> S[-6] 6. >>> S[-3] 7. >>> S[-6:-3] 8. >>> S[-6:-3] == S[2:5] 9. >>> S[-6:5] 10. >>> S[5:-6] 29-Aug-2014NLP, Prof. Howard, Tulane University 18

More slicing  If no beginning or end position is mentioned for a slice, Python defaults to the beginning or end of the string: 1. >>> S[2:] 2. >>> S[-2:] 3. >>> S[:2] 4. >>> S[:-2] 5. >>> S[:]  The result of a slice is a string object, so it can be concatenated with another string or repeated: 1. >>> S[:-1] + '!' 2. >>> S[:2] + S[2:] 3. >>> S[:2] + S[2:] == S 4. >>> S[-2:] * 2 29-Aug-2014NLP, Prof. Howard, Tulane University 19

Extended slicing  Slice syntax allows a mysterious third argument, by appending an additional colon and integer. What do these do?: 1. >>> S[::1] 2. >>> S[::2] 3. >>> S[::3] 4. >>> S[::4] 29-Aug-2014NLP, Prof. Howard, Tulane University 20

All three arguments together  Of course, you can still use the first two arguments to slice out a substring, which the third one steps through: 1. >>> S[1:7:1] 2. >>> S[1:7:2] 3. >>> S[1:7:3] 4. >>> S[1:7:6]  Thus the overall format of a slice is:  string[start:end:step] 29-Aug-2014NLP, Prof. Howard, Tulane University 21

How to reverse a string 1. >>> S[::-1] 2. >>> S[::-2] 3. >>> S[::-3] 4. >>> S[::-4] 29-Aug-2014NLP, Prof. Howard, Tulane University 22

The rest of §3 I will send you some practice for what we have done this week. Next time 29-Aug-2014NLP, Prof. Howard, Tulane University 23