Introduction to Computational Linguistics

1. Introduction to Computational Linguistics Programming II

2. Resumé • calculator mode • arithmetic operators, simple and complex arithmetic expressions • saving, checking and running programs

3. Exercise 2.1 Write a program that • gets 2 string variables and 2 integer variables from the user, • concatenates (joins them together with no spaces) and displays the strings, • then multiplies the two numbers on a new line.

4. Answer s1 = raw_input("give me string 1 ") s2 = raw_input("give me string 2 ") n1 = input("give me number 1 ") n2 = input("give me number 2 ") print s1+ " " + s2, n1+n2

5. while loop • while <condition> : <statements> i=0 while (i<10) : print i i = i+1 • Notice that indentation is used to group items together

6. Exercise 2.2 • Modify the last program so that it prints the sum of all the numbers.

7. Exercise 2.2 sum=0 i=0 while (i<10) : print i i = i+1 sum = sum+i print sum

8. range([start,] stop[, step]) • Creates lists containing arithmetic progressions • most often used in for loops. • If the step argument is omitted, it defaults to 1. • If the start argument is omitted, it defaults to 0. • Observe the behaviour of the range function

9. Use of range function >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(1, 11) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>> range(0, 30, 5) [0, 5, 10, 15, 20, 25] >>> range(0, 10, 3) [0, 3, 6, 9] >>> range(0, -10, -1)[0, -1, -2, -3, -4, -5, -6, -7, -8, -9] >>> range(0) [] >>> range(1, 0) []

10. for loop • Basic shape of for statement is this for <variable> in <list>: do something • Examplesfor i in [1,2,3] : print ifor i in range(1,4): print ifor i in ["comp", "ling"]: print ifor i in ['comp', 'ling']: print i

11. Exercise 2.3 • calculate and print the sum of numbers in range(5,20).

12. Strings • Besides numbers, Python can also manipulate strings, which can be expressed in several ways. • They can be enclosed in single quotes or double quotes: >>> 'spam eggs‘'spam eggs' >>> 'doesn\'t‘"doesn't" >>> "doesn't“"doesn't" >>> ‘ "Yes," he said.' ‘ "Yes," he said.'

13. Strings • We can get at individual characters of a string using subscript notation>>> s = 'dog'>>>s[0]'d'>>>s[1]'o'>>>s[2]'g'

14. Strings • Strings can be concatenated (glued together) with the + operator, and repeated with *: • >>> word = 'Help' + 'A' • >>> word 'HelpA' • >>> '<' + word*5 + '>' '<HelpAHelpAHelpAHelpAHelpA>'

15. Slice notation I • Slice notation is two indices separated by a colon, and selects that part of a string which begins with the first index and which finishes just before the second>>> s = 'dog'>>> s[0:1]'d'>>> s[0:2]'do'

16. Slice notation II • Slice indices have useful defaults • an omitted first index defaults to zero • an omitted second index defaults to the size of the string being sliced. >>> s[:2] 'do' >>> word[2:] 'g' >>> word[0:] 'dog'

17. More Data Types • We have seen numbers and strings. • These are different types of data. • Each kind of data has characteristic operations. • Now we look at lists. • A list is a compound data type which is used to group other data types together. • example: range(3) = [0,1,2]

18. Lists • The list can be written as a list of comma-separated values (items) between square brackets. • List items need not all have the same type. >>> a = ['spam', 'eggs', 100, 1234]>>> a ['spam', 'eggs', 100, 1234] • Like string indices, list indices start at 0, and lists can be sliced, concatenated and so on:

19. Lists • Lists can be sliced, concatenated and so on: >>> a[0] 'spam' >>> a[3] 1234>>> a[-2] 100>>> a[1:-1] ['eggs', 100]>>> a[:2] + ['bacon', 2*2] ['spam', 'eggs', 'bacon', 4] >>> 3*a[:3] + ['Boe!']['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boe!']

20. Lists can be changed • Unlike strings, which are immutable, it is possible to change individual elements of a list>>> a[2] = a[2] + 23 • Assignment to slices is also possible, and this can even change the size of the list

21. Changing Lists # Replace some items: >>> a[0:2] = [1, 12] >>> a [1, 12, 123, 1234] # Remove some: >>>a[0:2] = [] >>> a[123, 1234] # Insert some: >>>a[1:1] = ['bletch', 'xyzzy'] >>> a [123, 'bletch', 'xyzzy', 1234]

22. The built-in function len( ) This function computes the number of elements in a list >>> a = [1,2,3]>>> len(a)8>>>len([1,1,1,1,1,1])6

23. Nesting Lists It is possible to nest lists (create lists containing other lists), for example: >>> q = [2, 3]>>> p = [1, q, 4]>>> p[1,[2,3],4]