COMPUTATION WITH STRINGS 1 DAY 2 - 8/27/14 LING 3820 & 6820 Natural Language Processing Harry Howard Tulane University

Course organization 27-Aug-2014NLP, Prof. Howard, Tulane University 2   The syllabus is coming.   Is there anyone here that wasn't here on Monday?

Can anyoone NOT get Spyder to do this? Installation of Python 27-Aug NLP, Prof. Howard, Tulane University

Test >>>  Be sure to try the other arithmetic operators, subtraction (-), multiplication (*), and division (/). Does division work the way you expect?  After you have tired of playing with math, play with some text: >>> word = 'msinairatnemhsilbatsesiditna' >>> 'anti' in word False >>> 'itna' in word True 27-Aug-2014NLP, Prof. Howard, Tulane University 4

A string is a sequence of characters delimited between single or double quotes.string §3. Computation with strings 27-Aug NLP, Prof. Howard, Tulane University

Examples 1. >>> monty = 'Monty Python' 2. >>> monty 3. 'Monty Python' 4. >>> doublemonty = "Monty Python" 5. >>> doublemonty 6. 'Monty Python' 7. >>> circus = 'Monty Python's Flying Circus' 8. File " ", line 1 circus = 'Monty Python's Flying Circus' 9. ^ SyntaxError: invalid syntax 10. >>> circus = "Monty Python's Flying Circus" 11. >>> circus 12. "Monty Python's Flying Circus" 13. >>> circus = 'Monty Python\'s Flying Circus' 14. >>> circus 15. "Monty Python's Flying Circus" 27-Aug-2014NLP, Prof. Howard, Tulane University 6

The + and * operators  A new string can be formed by combination or concatenation of two strings with + or repeating a string a number of times with *. Unfortunately, a character cannot be deleted with –:concatenation 1. >>> S = 'balloon' 2. >>> S+'!' 3. >>> S+! 4. >>> 'M'+S 5. >>> S*2 6. >>> S+'!'*2 7. >>> (S+'!')*2 8. >>> S-'n' 9. >>> S >>> S+'2' 27-Aug-2014NLP, Prof. Howard, Tulane University 7

Some string methods  Python supplies several methods that can be applied to strings to perform tasks. Some of them are illustrated below. The input code is given, without the corresponding output. It is up to you to type them in to see what they do: 1. >>> len(S) 2. >>> len(S+'!') 3. >>> len(S*2) 4. >>> sorted(S) 5. >>> len(sorted(S)) 6. >>> set(S) 7. >>> sorted(set(S)) 8. >>> len(set(S)) 27-Aug-2014NLP, Prof. Howard, Tulane University 8

Tokens vs. types  set(S) produces the set of characters in the string.  One useful property of sets is that they do not contain duplicate elements.  The process of removing repetitions performed by set() touches on a fundamental concept in language computation, that of the distinction between a token and a type.tokentype  A representation in which repetitions are allowed is said to consist of tokens, while one in which there are no repetitions is said to consist of types.  Thus set() converts the tokens of a string into types. There is one type of 'o' in 'balloon', but two tokens of 'o'. 27-Aug-2014NLP, Prof. Howard, Tulane University 9

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 has to be memorized. 27-Aug-2014NLP, Prof. Howard, Tulane University 10

Cleaning 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() 27-Aug-2014NLP, Prof. Howard, Tulane University 11

3.3. Finding your way around a string I will try to send you some practice for what we have done today. Next time