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Sequences • A sequence (same as array in e.g. Java) is a list of elements – The range function returns a sequence >>> range(3, 12) >>> [3, 4, 5, 6, 7, 8, 9, 10, 11] >>> • Sequence elements – Referred to by subscripts; the first is always zero – Obtain one by using sequenceName[ subscript ] – Can also be referred to negatively • -1 would be the last element of the sequence! – Strings are sequences in Python 1 Sequences Name sequence (c) Position number of the element within sequence c Fig. 5.1 c[ 0 ] -45 c[ -12 ] c[ 1 ] 6 c[ -11 ] c[ 2 ] 0 c[ -10 ] c[ 3 ] 72 c[ -9 ] c[ 4 ] 1543 c[ -8] c[ 5 ] -89 c[- 7 ] c[ 6 ] 0 c[ -6 ] c[ 7 ] 62 c[ -5 ] c[ 8] -3 c[ -4 ] c[ 9 ] 1 c[- 3 ] c[ 10 ] 6453 c[ -2 ] c[ 11 ] -78 c[ -1 ] Sequence with elements and indices. 2 Creating Sequences • Creations – Strings • Use quotes: string1 = "" – Lists • Use brackets; separate multiple items with a comma • list1 = [] • list2 = [ 1, 7, 66 ] – Tuples • Use parentheses; separate multiple items with a comma • tuple1 = () • tuple2 = ( 19, 27 ) • this is a one element tuple/singleton: singleton = 1, • may ignore parentheses: tuple3 = 5,10,15 3 Using Lists and Tuples • Differences – Lists are mutable, tuples are immutable >>> >>> aList = [3, 6, 9] >>> aList[2] = 141 >>> aList [3, 6, 141] >>> >>> aTuple = (3, 6, 9) >>> aTuple[2] = 141 Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: object doesn't support item assignment >>> – Tuples and lists can both contain the same data 4 Adding items to a list aList = [] # create empty list # add values to list for number in range( 1, 11 ): aList += [ number ] Adds the values 1 to 10 to the list (by adding a new list with one element to the existing list in each round) • Using the + operator, you need to add a list to a list: - aList += [ number ] • Using the append method, you add an element to the list: - aList.append( number ) for i in .. ? Two ways of accessing all elements in a sequence startlist = [‘John’,‘George’, …,‘Wayne’] # list with 10.000 items for i in range( len( values ) ): # here we get the subscript, so we can output things like # ‘Bobby has index 8427’: print “%s has index %d” %( startlist[i], i ) for item in values: # here we get the item directly, but not the subscript print item Sequence unpacking - assigning values to multiple variables in one statement •# create sequences aString = "abc" aList = [ 1, 2, 3 ] aTuple = "a", "A", 1 Creates a string, a list and a tuple # unpack sequences to variables print "Unpacking string..." first, second, third = aString print "String values:", first, second, third Unpacks the string into characters print "\nUnpacking list..." first, second, third = aList print "List values:", first, second, third print "\nUnpacking tuple..." first, second, third = aTuple print "Tuple values:", first, second, third # swapping two values x = 3 y = 4 Unpacks the list into elements Unpacks the tuple into elements Sequence unpacking can also be used to swap two items (neat!) print "\nBefore swapping: x = %d, y = %d" % ( x, y ) x, y = y, x # swap variables print "After swapping: x = %d, y = %d" % ( x, y ) Unpacking string... String values: a b c Unpacking list... List values: 1 2 3 Unpacking tuple... Tuple values: a A 1 Before swapping: x = 3, y = 4 After swapping: x = 4, y = 3 Sequence Slicing • Allows access to a portion of a string or other sequence theSequence[ start:end ] • Returns new sequence with the portion of the original sequence from the starting position up to (but not including) the ending position >>> >>> aTuple = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) >>> print aTuple[2:4] (2, 3) >>> print aTuple[1:9] (1, 2, 3, 4, 5, 6, 7, 8) >>> print aTuple[-3:-1] (7, 8) >>> >>> aString = ‘Sibyl saw Basil lying there' >>> print aString[12:18] sil ly >>> >>> print aString[0:len(aString)/2] # printing half the string Sibyl saw Bas 8 List Methods Method Purp o se append( item ) Inserts item at the end of the list. count( element ) Returns the number of occurrences of element in the list. extend( newList ) Inserts the elements of newList at the end of the list. index( element ) Returns the index of the first occurrence of element in the list. If element is not in the list, a ValueError exception occurs. [Note: We discuss exceptions in Chapter 12, Exception Handling.] insert(index, item) Inserts item at position index. pop( [index] ) Parameter index is optional. If this method is called without arguments, it removes and returns the last element in the list. If parameter index is specified, this method removes and returns the element at position index. remove( element ) Removes the first occurrence of element from the list. If element is not in the list, a ValueError exception occurs. reverse() Reverses the contents of the list in place (rather than creating a reversed copy). sort( [comparefunction] ) Sorts the content of the list in place. The optional parameter compare-function is a function that specifies the compare criteria. The compare-function takes any two elements of the list (x and y) and returns -1 if x should appear before y, 0 if the orders of x and y do not matter and 1 if x should appear after y. [Note: We discuss sorting in Section 5.9.] Fig. 5.12 List m ethod s. 9 gcd_function_test.py Testing your function • Sequence unpacking • for/else construct 10 gcd_function2_test.py New implementation of function, same test 11 Dictionaries • Dictionaries – – – – Mapping that consists of unordered key-value pairs Each value is referenced though its key Curley braces ({}) are used to create a dictionary Creating a dictionary: { key1:value1, … } – Keys must be immutable values such as strings, numbers and tuples – Values can be of any Python data type 12 Creates an empty dictionary emptyDictionary = {} print "The value of emptyDictionary is:", emptyDictionary # create and print a dictionary with initial values grades = { "John": 87, "Bill": 76, "Laura": 92, "Edwin": 89 } print "\nAll grades:", grades # access and modify an existing dictionary print "\nBill's current grade:", grades[ "Bill" ] grades[ "Bill" ] = 90 print "Bill's new grade:", grades[ "Bill" ] # add to an existing dictionary grades[ "Michael" ] = 93 print "\nDictionary grades after modification:" print grades # delete entry from dictionary del grades[ "John" ] print "\nDictionary grades after deletion:" print grades Creates a grades dictionary using names as the key and their grade as the value Alters and displays the new grade for Bill Adds a new name to the grades dictionary (simply set the value of the new key) Removes the name john from the dictionary with the del keyword emptyDictionary = {} print "The value of emptyDictionary is:", emptyDictionary # create and print a dictionary with initial values grades = { "John": 87, "Bill": 76, "Laura": 92, "Edwin": 89 } print "\nAll grades:", grades # access and modify an existing dictionary print "\nBill's current grade:", grades[ "Bill" ] grades[ "Bill" ] = 90 print "Bill's new grade:", grades[ "Bill" ] # add to an existing dictionary grades[ "Michael" ] = 93 print "\nDictionary grades after modification:" print grades # delete entry from dictionary del grades[ "John" ] print "\nDictionary grades after deletion:" print grades The value of emptyDictionary is: {} All grades: {'Edwin': 89, 'John': 87, 'Bill': 76, 'Laura': 92} Note: unordered! (‘Michael’ not inserted at the end) Bill's current grade: 76 Bill's new grade: 90 Dictionary grades after modification: {'Edwin': 89, 'Michael': 93, 'John': 87, 'Bill': 90, 'Laura': 92} Dictionary grades after deletion: {'Edwin': 89, 'Michael': 93, 'Bill': 90, 'Laura': 92} Dictionary Methods Method De sc rip tio n clear() Deletes all items from the dictionary. copy() Creates and returns a shallow copy of the dictionary (the elements in the new dictionary are references to the elements in the original dictionary). get( key [, returnValue] ) Returns the value associated with key. If key is not in the dictionary and if returnValue is specified, returns the specified value. If returnValue is not specified, returns None. has_key( key ) Returns 1 if key is in the dictionary; returns 0 if key is not in the dictionary. items() Returns a list of tuples that are key-value pairs. keys() Returns a list of keys in the dictionary. popitem() Removes and returns an arbitrary key-value pair as a tuple of two elements. If dictionary is empty, a Key- Error exception occurs. [Note: We discuss exceptions in Chapter 12, Exception Handling.] This method is useful for accessing an element (i.e., print the key-value pair) before removing it from the dictionary. 15 dictionary shallowCopy deepCopy Shallow vs. Deep Copy >>> dictionary = { "listKey" : [ 1, 2, 3 ] } >>> shallowCopy = dictionary.copy() # make a shallow copy >>> >>> dictionary[ "listKey" ].append( 4 ) >>> >>> print dictionary {'listKey': [1, 2, 3, 4]} >>> >>> print shallowCopy {'listKey': [1, 2, 3, 4]} >>> from copy import deepcopy >>> deepCopy = deepcopy( dictionary ) >>> >>> dictionary[ "listKey" ].append( 5 ) >>> >>> print dictionary {'listKey': [1, 2, 3, 4, 5]} >>> >>> print shallowCopy {'listKey': [1, 2, 3, 4, 5]} >>> >>> print deepCopy {'listKey': [1, 2, 3, 4]} # make a deep copy 16 Passing Lists to Functions • Original list can be changed by the function • Items in the list that are immutable (numbers or strings) cannot be changed by the function when passed individually • In general: mutable objects can be changed when passed to a function (lists, dictionaries), immutable objects cannot (strings, tuples, numbers) 17 def modifyList( aList ): # multiply all elements in the list by 2 for i in range( len( aList ) ): No type aList[ i ] *= 2 def modifyElement( element ): # multiply single element by 2 element *= 2 aList = [ 1, 2, 3, 4, 5 ] modifyList( aList ) Passes the entire list, the changes in the function will affect the list print "\n\nThe values of the modified list are:" declaration, so function body assumes it gets a list! NB: good documentation helps you use your function as intended, no compiler to help you for item in aList: print item, print "aList[ 3 ] before modifyElement:", aList[ 3 ] modifyElement( aList[ 3 ] ) print "aList[ 3 ] after modifyElement:", aList[ 3 ] Passes a single element, it will not permanently be modified in the list Passes a slice of the list, print "aList[ 2:4 ] before modifyList:", aList[ 2:4 ] no permanent change to list modifyList( aList[ 2:4 ] ) print "aList[ 2:4 ] after modifyList:", aList[ 2:4 ] def modifyList( aList ): # multiply all elements in the list by 2 for i in range( len( aList ) ): aList[ i ] *= 2 def modifyElement( element ): element *= 2 aList = [ 1, 2, 3, 4, 5 ] modifyList( aList ) The values of the modified list are: 2 4 6 8 10 aList[ 3 ] before modifyElement: 8 aList[ 3 ] after modifyElement: 8 aList[ 2:4 ] before modifyList: [6, 8] aList[ 2:4 ] after modifyList: [6, 8] print "\n\nThe values of the modified list are:" for item in aList: print item, print "aList[ 3 ] before modifyElement:", aList[ 3 ] modifyElement( aList[ 3 ] ) print "aList[ 3 ] after modifyElement:", aList[ 3 ] print "aList[ 2:4 ] before modifyList:", aList[ 2:4 ] modifyList( aList[ 2:4 ] ) print "aList[ 2:4 ] after modifyList:", aList[ 2:4 ]