Introduction to Computational Linguistics

Programming II

Resumé

calculator mode arithmetic operators, simple and

complex arithmetic expressions saving, checking and running

programs

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.

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

while loop

while <condition> : <statements>i=0while (i<10) :

print i i = i+1 Notice that indentation is used to

group items together

Exercise 2.2

Modify the last program so that it prints the sum of all the numbers.

Exercise 2.2

sum=0i=0while (i<10) :

print i i = i+1 sum = sum+iprint sum

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

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) []

for loop

Basic shape of for statement is thisfor <variable> in <list>: do something

Examples

for 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

Exercise 2.3

calculate and print the sum of numbers in range(5,20).

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.'

Strings We can get at individual characters

of a string using subscript notation>>> s = 'dog'>>>s[0]'d'>>>s[1]'o'>>>s[2]'g'

Strings

Strings can be concatenated (glued together) with the + operator, and repeated with *:

>>> word = 'Help' + 'A' >>> word 'HelpA' >>> '<' + word*5 + '>'

'<HelpAHelpAHelpAHelpAHelpA>'

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'

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'

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]

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:

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!']

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

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]

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

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]