Introduction to Python

1
Introduction to Python

• Data Structures

2
Data Structures

• List
• Tuple
• Dictionary

3
Lists

• Construction
• Syntax elem1, elem2,
• Heterogeneous, ordered sequence
• Mutable
• Example
• gtgtgt list1 1, 'hello', 42j, 123.12
• gtgtgt list1
• 1, 'hello', (42j), 123.12
• gtgtgt list10 'a'
• gtgtgt list1
• 'a', 'hello', (42j), 123.12

4
Lists - Operations

• Concatenation ()
• Syntax list1 list2
• Example
• gtgtgt 1, 'a', 'b' 3, 4, 5
• 1, 'a', 'b', 3, 4, 5
• Repetition ()
• Syntax list number
• Example
• gtgtgt 23, 'x' 4
• 23, 'x', 23, 'x', 23, 'x', 23, 'x'

5
Indexing

• Indexing operator
• Positive indices count from the left
• Negative indices count from the right

0 1 2 3 4 5 6
a b c d e f g
-7 -6 -5 -4 -3 -2 -1
sequence0 a sequence-7 a sequence6
g sequence-1 g sequence2 c sequence-5
c
6
Slices

• Two indices separated by a colon
• Available for both strings and lists
• Example
• gtgtgt sequence 0, 1, 2, 3, 4, 5, 6, 7
• gtgtgt sequence14
• 1, 2, 3
• gtgtgt sequence2-1
• 2, 3, 4, 5, 6
• Missing Index implies end point
• gtgtgt sequence2
• 0, 1
• gtgtgt sequence3
• 3, 4, 5, 6, 7

7
Tuples

• Immutable version of list
• Syntax (elem1, elem2, )
• Items in tuple can not be altered
• Example
• gtgtgt tuple1 (1, 5, 10)
• gtgtgt tuple12 2
• Traceback (most recent call last)
• File "ltpyshell136gt", line 1, in ?
• tuple12 2
• TypeError object doesn't support item
  assignment

8
Built-in Function len

• Syntax len(object)
• Return the length of object
• Example
• gtgtgt list1 1, 2, 3, 4, 5
• gtgtgt len(list1)
• 5
• gtgtgt string1 "length of a string"
• gtgtgt len(string1)
• 18

9
Dictionaries

• Mapping
• Associate a key with a value
• Each key must be unique

keys
values
10
Dictionaries

• Construction
• Syntax key1 value1, key2 value2
• Unordered map
• Example
• gtgtgt dict1 'a' 1, 'b' 2
• gtgtgt dict1
• 'a' 1, 'b' 2
• gtgtgt dict1'a'
• 1
• gtgtgt dict1'b'
• 2

11
Controlling Flow

• Loops - while
• Syntax while ltboolean expressiongt
• body
• Execute body until looping condition is met
• Example
• gtgtgt i 0
• gtgtgt while i lt 4
• print i
• i 1
• 0
• 1
• 2
• 3

12
Controlling Flow

• Loops - for
• Syntax for ltvargt in ltsequencegt
• body
• Execute body once for every element in sequence
• Example
• gtgtgt for i in 0, 1, 2, 3
• print i
• 0
• 1
• 2
• 3

13
Built-in Function range

• Syntax range(start, stop, step)
• Generate a list of numbers from start to stop
  stepping every step
• start defaults to 0, step defaults to 1
• Example
• gtgtgt range(5)
• 0, 1, 2, 3, 4
• gtgtgt range(1, 9)
• 1, 2, 3, 4, 5, 6, 7, 8
• gtgtgt range(2, 20, 5)
• 2, 7, 12, 17

14
Controlling Flow

• Using range with for
• Generate list used by for with range
• Example
• gtgtgt for i in range(4)
• print i
• 0
• 1
• 2
• 3

15
Controlling Flow

• The continue statement
• Continue to next iteration of loop, skipping
  remainder of body
• Example
• gtgtgt for x in range(8)
• if x2 0
• continue
• print x

1
3
5
7
16
Controlling Flow

• The break statement
• Break out of inner most loop
• Example
• gtgtgt for number in range(10)
• if number 4
• print 'Breaking'
• break
• else
• print number
• 0
• 1
• 2
• 3
• Breaking

17
Using Data Structures

• Data structures also have methods
• Use built-in function dir to list all available
  methods
• Example
• gtgtgt lst 1, 3, 2
• gtgtgt dir(lst)
• '__add__', '__class__', '__contains__',
  '__delattr__', '__delitem__', '__delslice__',
  '__doc__', '__eq__', '__ge__', '__getattribute__',
  '__getitem__', '__getslice__', '__gt__',
  '__hash__', '__iadd__', '__imul__', '__init__',
  '__le__', '__len__', '__lt__', '__mul__',
  '__ne__', '__new__', '__reduce__', '__repr__',
  '__rmul__', '__setattr__', '__setitem__',
  '__setslice__', '__str__', 'append', 'count',
  'extend', 'index', 'insert', 'pop', 'remove',
  'reverse', 'sort'

18
Strings - Methods

• split
• Syntax string.split(sep)
• Returns a list of strings
• Example
• gtgtgt text "1 2 4 5 1"
• gtgtgt text.split()
• '1', '2', '4', '5', '1'
• gtgtgt test "a, b, c, d, e"
• gtgtgt test.split(',')
• 'a', ' b', ' c', ' d', ' e'

19
Strings - Methods

• strip
• Syntax string.strip()
• Remove leading and trailing white space (tabs,
  new lines, etc)
• Example
• gtgtgt padded " stuff "
• gtgtgt padded.strip()
• 'stuff'
• gtgtgt padded
• ' stuff '
• gtgtgt unpadded padded.strip()
• gtgtgt unpadded
• 'stuff'

20
Lists - Methods

• append
• Syntax list.append(element)
• Add element to end of list
• Example
• gtgtgt list1 3, '10', 2
• gtgtgt list1.append('new')
• gtgtgt list1
• 3, '10', 2, 'new'

21
Lists - Methods

• pop
• Syntax list.pop(index)
• Remove and return item at position index from
  list
• Default is to remove last item
• Example
• gtgtgt list1 3, '10', 2, 9, 11
• gtgtgt list1.pop()
• 11
• gtgtgt list1
• 3, '10', 2, 9

22
Lists - Methods

• insert
• Syntax list.insert(index, element)
• Insert element into list at position index
• Example
• gtgtgt list2 0, 1, 2, 3, 4, 5
• gtgtgt list2.insert(3, 'new')
• gtgtgt list2
• 0, 1, 2, 'new', 3, 4, 5

23
Lists - Methods

• remove
• Syntax list.remove(element)
• Removes the first occurrence of element in list
• Example
• gtgtgt list2 0, 1, 3, 4, 3, 5
• gtgtgt list2.remove(3)
• gtgtgt list2
• 0, 1, 4, 3, 5

24
Lists - Methods

• sort
• Syntax list.sort(cmpfunc)
• Sort list in place
• Example
• gtgtgt list3 4, 12, 3, 9
• gtgtgt list3.sort()
• gtgtgt list3
• 3, 4, 9, 12

25
Lists - Methods

• reverse
• Syntax list.reverse()
• Reverse elements of list in place
• Example
• gtgtgt list3 4, 12, 3, 9
• gtgtgt list3.reverse()
• gtgtgt list3
• 9, 3, 12, 4

26
Dictionaries - Methods

• keys
• Syntax dict.keys()
• Return a list of all the keys in dict
• Example
• gtgtgt dict1 1 'a', 9 'cat', 2 2, 1
• gtgtgt dict1.keys()
• 1, 2, 9

27
Dictionaries - Methods

• has_key
• Syntax dict.has_key(key)
• Determines if key is in dict
• Example
• gtgtgt dict1 'x' 1, 'y' 2
• gtgtgt dict1.has_key('x')
• 1
• gtgtgt dict1.has_key('w')
• 0

28
Dictionaries - Methods

• values
• Syntax dict.values()
• Returns a list of all values in dict
• Example
• gtgtgt dict1 (1,2) 'a', (3,4) 'b', (9,8,7)
  'c'
• gtgtgt dict1.values()
• 'a', 'c', 'b'