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

Computation with strings 2Day 2 - 8/29/14LING 3820 & 6820

Natural Language Processing

Harry Howard

Tulane University

Course organization

29-Aug-2014NLP, Prof. Howard, Tulane University

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

The syllabus is under construction. http://www.tulane.edu/~howard/CompCultEN/

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 e-mail 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.

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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

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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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Open Spyder

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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 must be memorized.


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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()


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3.3. How to find your way around a string

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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')


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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')


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index() or find()

Where they differ lies in how they handle null responses:

1. >>> S.find('z')

2. -1

3. >>> S.index('z')

4. Traceback (most recent call last):

5. File "<stdin>", line 1, in <module>

6. ValueError: substring not found


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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')


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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)


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3.3.2. Zero-based indexation

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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.


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In a picture


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Finding characters given a position1. >>> 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]


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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


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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]


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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]


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How to reverse a string

1. >>> S[::-1] 2. >>> S[::-2] 3. >>> S[::-3] 4. >>> S[::-4]


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The rest of §3I will send you some practice for what we have done this week.

Next time


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

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