Transcript document

Introduction to
Computational Linguistics
Programming II
Resumé
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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=0
while (i<10) :
print i
i = i+1
 Notice that indentation is used to group
items together
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Exercise 2.2
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Modify the last program so that it prints
the sum of all the numbers.
Exercise 2.2
sum=0
i=0
while (i<10) :
print i
i = i+1
sum = sum+i
print sum
range([start,] stop[, step])
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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
>>>
>>>
>>>
>>>
>>>
[0,
>>>
>>>
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)
-1, -2, -3, -4, -5, -6, -7, -8, -9]
range(0) []
range(1, 0) []
for loop
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Basic shape of for statement is this
for <variable> in <list>: do something
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Examples
for
for
for
for
i
i
i
i
in
in
in
in
[1,2,3] : print i
range(1,4): print i
["comp", "ling"]: print i
['comp', 'ling']: print i
Exercise 2.3
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calculate and print the sum of numbers
in range(5,20).
Strings
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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
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We can get at individual characters of a
string using subscript notation
>>> s = 'dog'
>>>s[0]
'd'
>>>s[1]
'o'
>>>s[2]
'g'
Strings
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Strings can be concatenated (glued
together) with the + operator, and
repeated with *:
>>> word = 'Help' + 'A'
>>> word 'HelpA'
>>> '<' + word*5 + '>'
'<HelpAHelpAHelpAHelpAHelpA>'
Slice notation I
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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
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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
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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
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The list can be written as a list of commaseparated 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
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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
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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]