Information Infrastructure II

1
Information Infrastructure II

• I211 Week 3
• Rajarshi Guha

2
Topics To Be Covered

• Lists
• Strings as lists string methods
• Tuples
• Dictionaries
• File operations

3
Lists

• A list is a vector of objects
• Numbers
• Strings
• Anything!

Elements of the list
1
A
ab
2.5

-2
Element 1
Element 0
Element 5
4
Creating a List

• Put values between
• represents an empty list
• Can be used to represent False (like 0)

gases 'He', 'Ne', 'Ar', 'Kr' print
gases 'He', 'Ne', 'Ar', 'Kr' alist
1.2345, "Hello World", 4567383, 'abcdefg'
print alist 1.2344999999999999, 'Hello World',
4567383, 'abcdefg'
5
Creating a List

• Since a list can hold anything, it can also
  contain another list!
• This willl allow us to use lists as 2D matrices

l 1, 3, 'a', 'b', 'c', 10, 'hello'
print l 1, 3, 'a', 'b', 'c', 10, 'hello'
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Accessing List Elements

• a.k.a list indexing
• Since a list is a vector of objects, we can
  access a list by indicating the position of the
  element
• We can use a number (such as 1, 2 or 10)
• We can use a variable containing the number
• Numbering starts from 0 !!!
• len( ) gives the length of the list

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Accessing List Elements
l 1, 3, 'a', 'b', 'c', 10, 'hello'
l0 1 l2 'a', 'b', 'c' x 4
lx 'hello len(l) 5

• If you go beyond the last element of the list,
  you get an error

l5 Traceback (most recent call last)
File "", line 1, in ? IndexError list
index out of range
8
Lists References

• Previously we learned that variables refer to an
  object
• Variables dont have types
• Two variables can refer to the same object
• Changing one variable doesnt change the other
• Lists are a little different

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Lists and References
What happensto L2?
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Negative Indices

• We can supply a negative index
• Indicates that we start from the end of the list

-5
-4
-3
-2
-1
alist 1, 'A', 'ab', 2.5, '' alist 1,
'A', 'ab', 2.5, '' alist-1 ''
alist-2 2.5
1
A
ab
2.5

alist
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List Slicing

• Slicing lets us access ranges from a list
• We write a slice as alist14
• Returns elements 1, 2, and 3 (not 4!!)

0
1
2
3
4
1
A
ab
2.5

alist
alist25
alist02
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List Slicing

• But going beyond the last index does not give an
  error
• Truncates out ofrange indices
• We can also use negative indices

0
1
2
3
4
1
A
ab
2.5

alist
alist210
alist1-2
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Slicing Shortcuts

• alistx returns all elements from 0 to x-1
• alistx returns all elements from x to the end
  of the list

alist3
1
A
ab
2.5

alist
alist3
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Modifying Lists

• Assign a new value to a position in the list by
  doing xi v
• But i must be a valid index

a 'p', 'q', 'r', 's', 't' a3
'hello' print a 'p', 'q', 'r', 'hello',
't' a-1 1234 print a 'p', 'q', 'r',
'hello', 1234
a 'p', 'q', 'r' a5 'hi' Traceback
(most recent call last) File "", line
1, in ? IndexError list assignment index out of
range
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Modifying Lists

• We can add items to the end of the listby using
  append()
• But you can only addone thing at a time

a 'p', 'q', 'r' a.append(123)
print a 'p', 'q', 'r', 123
a 'p', 'q', 'r' a.append('s',
't') Traceback (most recent call last) File
"", line 1, in ? TypeError append() takes
exactly one argument (2 given)
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Modifying Lists

• So what if we want to append more than one item
  to a list?
• Use the extend() method
• So the items we wantto append, should be placed
  in a list
• The original list is then extended with the new
  list

a 'p', 'q', 'r' a.extend( 's', 't'
) print a 'p', 'q', 'r', 's', 't'
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Deleting Elements

• Delete items at a specific position
• We can also delete using slices

a 'p', 'q', 'r', 's' del a2
print a 'p', 'q', 's'
a 'p', 'q', 'r', 's' del a13
print a 'p', 's'
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Useful List Methods
a b, a, c, d, d
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Testing Membership

• We just saw the find method
• A more English like way is to do
• x in y
• If the item denoted by x is present in the list
  y, the expression is True, otherwise False
• Checks on an elementby element basis

a 'p', 'q', 'r', 123 123 in
a True 'xyz' in a False 'p', 'q' in
a False
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Looping Over a List

• We can use the for x in y construct
• y is the list
• x will be each consecutive element
• Here we just print each element
• We can do anything withthe element

a 'p', 'q', 'r', 123 for x in a print
x p q r 123
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Looping With Lists

• So far weve seen different ways to loop
• while
• for
• How do we do the C for loop?
• for (i 0 i
• Use the range() function

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Looping With Lists

• range(0, n) returns a list with the numbers from
  0 to n-1
• This can be shortened to range(n), which always
  starts from 0
• We could also do range(p, q)
• What would we get?
• Since we get back a list, we can then use
  thefor x in y method we just saw
• We can use incrementsother than 1 by
  doingrange(p, q, step)

for x in range(0,4) print x 0 1 2 3
range(3, 15, 2) 3, 5, 7, 9, 11, 13
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List Within Lists

• A list can contain anything
• So we can add lists to lists

a 1,2,3, 'a', 'b', 'c', 'd', 'ab',
2.5 print a 1, 2, 3, 'a', 'b', 'c',
'd', 'ab', 2.5 a0 1, 2, 3 a3 2.5
ab
2.5
1
2
3
a
b
c
d
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Tuples

• A tuple is a list that cant be changed

a (1, 2, 'hello') a (1, 2, 'hello')
a1 2 a0 3 Traceback (most recent call
last) File "", line 1, in ? TypeError
object doesn't support item assignment
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Multi-valued Assignment

• Ordinarily we do a x
• In Python we can do a, b x, y
• This does a x b y all in one go
• Nice for swapping a, b b, a
• Nice for looping through lists of lists

a 1,2, 3,4, 5,6 for x,y in a
print x,y ... 1 2 3 4 5 6
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Strings as Lists

• A string is a list that cant be changed
• All the indexingrules can be usedon a string
• But we cantmodify elements of the string

a 'this is a string' a0 't'
a04 'this' a-5-1 'trin'
a1 'b' Traceback (most recent call
last) File "", line 1, in ? TypeError
object doesn't support item assignment
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String Methods

• A method is a function tied to a specific object
• An object can be a string, list, etc.
• Well look at string methods
• Later on well write our own
• We call a method by object.method()

This could be a string or list or any type of
object
This is a method on that object
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What Can We Do With a String?

• Replace portions of it
• Convert to lower or upper case
• Find a smaller string within a larger one
• Clear white space at the start and end of a
  string
• Split a string in different ways

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Examples
a "this Is A Longer string"
a.lower() 'this is a longer string'
a.upper() 'THIS IS A LONGER STRING'
a.find("onger") 11 a.replace("Longer",
"shorter") 'this Is A shorter string'
a.split(" ") 'this', 'Is', 'A', 'Longer',
'string'
In each case we get a new string. The old
string is unchanged
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Dictionaries

• In many cases we would like to associate a name
  with a value
• Names and ages (John 24)
• Counts of birds (Sparrow 3)
• A list wont let us do this (easily)
• So we use a dictionary

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Dictionaries

• Dictionaries are basically collections of key,
  value pairs
• Keys are usually strings, butcan actually be any
  object
• Values can be any object
• Keys are not sorted!

Key
Value
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Using Dictionaries

• Create an empty dictionary by d
• You get values by specifying the key for the
  value
• Keys are unique!
• You can get all thekeys by callingd.keys()
• You can get all the values by callingd.values()

d john 24, mary 22, joe 26
djohn 24 d'mark' Traceback (most
recent call last) File "", line 1, in
? KeyError 'mark'
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Dictionaries are Unsorted

• When we create a list, the elements remain in the
  order we specified
• Dictionary keys are unsorted
• So the order in which we specify the elements may
  not be the order in which get them
• So dont depend on theordering of the keys

d a 1, b 2, c 3 d 'a' 1,
'c' 3, 'b' 2
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Looping Over a Dictionary

• Looping over a dictionary implies that we loop
  over the keys
• Different from lists! There, we loop over
  indices, not values
• Two ways
• We can get thekeys and loopover them
• Loop over the dictionary, whichautomatically
  loopsover the keys

d a 1, b 2, c 3, d 4
method 1 For key in d.keys() print Key ,
key, Value , dkey method 2 For key in
d print Key , key, Value , dkey
Allows youto loop overthe keys insorted order
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Assigning Deleting

• If the key exists then the old value is
  overwritten
• If the key does not exist it creates a new entry
• To delete an entry, do del dk
• Wont work if the key, k, doesntexist

d a 1, b 2 d 'a' 1, 'b'
2 da 2 d a 2, b 2
dc 3 d 'a' 2, 'c' 3, 'b' 2
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Files

• Typing lists and text is very boring!
• Nicer to read it from a file
• Whats involved?
• Opening a file
• Reading from a file
• Writing to a file
• What do we get as a result?

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Using a File

• We use the built-in function open
• Takes 2 arguments
• Name of the file
• Whether to read (r) or write (w)
• We get back a file object (which has certain
  methods)

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Reading from a File
f open('christmas.txt', 'r') contents
f.read() len(contents) 180929
Reads in each byte of the file
f open('christmas.txt', 'r') contents
f.readlines() len(contents) 4184
Reads in each line of the file
f open('christmas.txt', 'r') aline
f.readline() print aline CONTENTS
Reads in one line of the file
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What Are Bytes Lines?

• Stave IV The Last of the Spirits
• Stave V The End of It
• STAVE I MARLEY'S GHOST
• MARLEY was dead to begin with. There is no doubt
• whatever about that. The register of his burial
  was
• signed by the clergyman, the clerk, the
  undertaker,
• and the chief mourner. Scrooge signed it and
• Scrooge's name was good upon 'Change, for
  anything he
• chose to put his hand to. Old Marley was as dead
  as a
• door-nail.

A single byte. In general thismeans a
singlecharacter.But this is notalways the
case!
A single lineA line can be blank. A line
usually ends with a \n character. So a blank
line is usually just a \n
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Reading Single Bytes
f open('christmas.txt', 'r') contents
f.read() len(contents) 180929

• If we dont specifyan argument toread() it will
  read allthe bytes in the file.
• If we want the first 10 bytes, we can call
  read(10)
• We get back a list - each element is a single byte

41
Reading All the Lines

• readlines() readsthe whole file
• Returns a list
• Each element is a line of the file
• We can print each line by looping over the list

f open('christmas.txt', 'r') contents
f.readlines() len(contents) 4184
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Reading a Single Line
f open('christmas.txt', 'r') aline
f.readline() print aline CONTENTS

• readline() reads1 line from the file
• If you call it again it reads the next line
• If there are no more lines, it will return None
  (equivalent to False)

• We can read the whole file, line by line

43
Reading Lines One By One

• We can read a file line by line in two ways
• Call readline() in a loop, exit when we get None
• Loop over the file !

f open('christmas.txt', 'r') line
f.readline() while line line f.readline()
if not line break print line
f open('christmas.txt', 'r') for line
in f ... print line
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Writing to a File

• Very similar to reading
• Open the file
• Write text
• Close the file

f open('afile.txt', 'w') f.write("The
first line in the file") f.write("The second
line in the file") f.close()
Does this do whatwe expect it to do?