Computation with strings 1Day 2 - 8/27/14LING 3820 & 6820

Natural Language Processing

Harry Howard

Tulane University

Course organization

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http://www.tulane.edu/~howard/LING3820/

The syllabus is coming. http://www.tulane.edu/~howard/CompCu

ltEN/ Is there anyone here that wasn't here on

Monday?

Can anyoone NOT get Spyder to do this?

Installation of Python

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NLP, Prof. Howard, Tulane University

Test

>>> 237 + 9075 9312 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

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A string is a sequence of characters delimited between single or double quotes.

§3. Computation with strings

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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 "<stdin>", 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"

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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 –:

1. >>> S = 'balloon' 2. >>> S+'!' 3. >>> S+!4. >>> 'M'+S 5. >>> S*2 6. >>> S+'!'*2 7. >>> (S+'!')*2 8. >>> S-'n'9. >>> S+210.>>> S+'2'

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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))

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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.

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

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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.

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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()

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

Next time