1. Computation with strings 2 Day 3 - 9/02/16
LING 3820 & 6820
Natural Language Processing
Harry Howard
Tulane University

2. Course organization http://www.tulane.edu/~howard/NLP/
Schedule of assignments
Is there anyone here that wasn't here on Monday?
NLP, Prof. Howard, Tulane University
02-Sep-2016

3. Computer hygiene Refresh your browser!
NLP, Prof. Howard, Tulane University
02-Sep-2016

4. Scripts for the example code
How to use a script in Spyder
NLP, Prof. Howard, Tulane University
02-Sep-2016

5. There are two, so far 4.1. How to create a string
Note: The code for this and the next section can be downloaded as this script nlp4sec1-2.py, presumably to your Downloads folder, and then moved to pyScipts, from whence you can open it in Spyder and run each line one by one.
4.3. How to find your way around a string
Note: The code for this section is found at nlp4sec3.py.
NLP, Prof. Howard, Tulane University
02-Sep-2016

6. §4. Computation with strings
What is a string?
A string is a sequence of characters delimited between single or double quotes.
NLP, Prof. Howard, Tulane University
02-Sep-2016

7. Changes to text
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02-Sep-2016

8. 4.1.2. Object type >>> type('one') == type(1)
>>> str(1)
>>> str(1.0)
>>> '1' == str(1)
>>> '1.0' == str(1.0)
>>> type('1') == type(str(1))
>>> type('1.0') == type(str(1.0))
>>> int('1') >>> float('1.0')
>>> 1 == int('1')
>>> 1.0 == float('1.0')
>>> type(1) == type(int('1'))
>>> type(1.0) == type(float('1.0'))
NLP, Prof. Howard, Tulane University
02-Sep-2016

9. Practice 1
1. What types are output by len(), sort(), and set()?
>>> B = 'balloon'
>>> type(len(B))
<type 'int'>
>>> type(sorted(B))
<type 'list'>
>>> type(set(B))
<type 'set'>
NLP, Prof. Howard, Tulane University
02-Sep-2016

10. Practice 1, cont.
2. Write the code to perform the changes given below on these two strings:
>>> S = 'ABCDEFGH'
>>> s = 'abcdefgh'
a. Extract the first 3 characters of S and make them lowercase.
>>> S.strip('DEFGH').lower()
b. Extract the last 4 characters of s and make them uppercase.
>>> s.strip('abcd').upper()
c. Create a string from the first 4 characters of S and the last 4 characters of s and then switch its case.
>>> S.strip('EFGH')+s.strip('abcd').swapcase()
'ABCDEFGH'
>>> (S.strip('EFGH')+s.strip('abcd')).swapcase()
NLP, Prof. Howard, Tulane University
02-Sep-2016

11. Practice 1, cont.
3. Here are two real life strings to work with:
>>> mail =
>>> url = '
a. How would you strip out the user name and the server name from my address?
>>>
>>> 'howar'
>>>
b. Internet addresses start with the transfer protocol that the site uses. For web pages, this is usually the hypertext transfer protocol, http. How would you strip this information out to leave just the address of the book?
>>> url.strip('
c. Following up on (b), how would you extract just Tulane’s server address?
>>> url.lstrip('
NLP, Prof. Howard, Tulane University
02-Sep-2016

12. 4.2.5. Practice 1, cont. Now we mix types:
>>> day = 1
>>> month = 'Sept'
>>> year = 2016
Concatenate them into the string ‘Sept. 1, 2016’.
>>> month+'. '+str(day)+', '+str(year)
Then split the string ‘Nov. 1, 1957’ into its three parts.
>>> date = 'Nov. 1, 1957'
>>> m = date.strip('. 1, 1957')
>>> d = int(date.strip('Nov.').strip('1957').strip().strip(','))
>>> y = int(date.strip('Nov. 1').strip(',').strip())
NLP, Prof. Howard, Tulane University
02-Sep-2016

13. In a picture
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02-Sep-2016

14. 4.3.1.3. How to slice a string >>> E[0:3] >>> E[1:4]
NLP, Prof. Howard, Tulane University
02-Sep-2016

15. Practice 2
1. Write the code to perform the changes given below on these two strings, but try to using slicing rather than stripping:
>>> S = 'ABCDEFGH' >>> s = 'abcdefgh'
a. Make the first 3 characters of S lowercase.
>>> S[:3].lower()
b. Make the last 4 characters of s uppercase.
>>> s[4:].upper()
c. Create a string from the first 4 characters of S and the last 4 characters of s and then switch its case.
>>> S[:4]+s[4:].swapcase()'
'ABCDEFGH'
>>> (S[:4]+s[4:]).swapcase()
'abcdEFGH'
d. Concatenate both strings and slice out every even character.
>>> (S+s)[1::2]
e. Concatenate both strings and reverse the order of the characters.
>>> (S+s)[::-1]
f. Retrieve the index of ‘E’ and ‘h’.
>>> S.index('e')
>>> s.index('h')
NLP, Prof. Howard, Tulane University
02-Sep-2016

16. Others 4.3.4. Practice 2 NLP, Prof. Howard, Tulane University
02-Sep-2016

17. Next time
§4. practices 3, 4, 5, 6, 7 -- not all of which are finished yet.
NLP, Prof. Howard, Tulane University
02-Sep-2016