8 – Lists and tuples John R. Woodward

Lists The list is a most versatile datatype available in Python which can be written as a list of comma-separated values (items) between square brackets []. Good (and bad) thing about a list is that items in a list need not all have the same type. Creating a list - put different comma-separated values between square brackets. list1 = ['physics', 'chemistry', 1997, 2000]; list2 = [1, 2, 3, 4, 5 ]; list3 = ["a", "b", "c", "d"];

Accessing Values in Lists To access values in lists, use the square brackets for slicing along with the index or indices to obtain value available at that index. Following is a simple example: list1 = ['physics', 'chemistry', 1997, 2000]; list2 = [1, 2, 3, 4, 5, 6, 7 ]; print "list1[0]: ", list1[0] print "list2[1:5]: ", list2[1:5] it produces the following result: list1[0]: physics list2[1:5]: [2, 3, 4, 5]

Updating Lists You can update single or multiple elements of lists by giving the slice on the left-hand side of the assignment operator, and you can add to elements in a list with the append() method. Following is a simple example: list = ['physics', 'chemistry', 1997, 2000]; print "Value available at index 2 : " print list[2]; list[2] = 2001; print "New value available at index 2 : " print list[2];

Example list = [0,1,2,3,4,5,6] print list list[3] = 9 list[-1] = 0 #Output [0, 1, 2, 3, 4, 5, 6] [0, 1, 2, 9, 4, 5, 6] [0, 1, 2, 9, 4, 5, 0]

Del at item from a list print len(list) del list[3] print list #Output [0, 1, 2, 9, 4, 5, 0] 7 [0, 1, 2, 4, 5, 0] 6

Delete List Elements To remove a list element, you can use either the del statement list1 = ['physics', 'chemistry', 1997, 2000]; print list1; del list1[2]; print "After deleting value at index 2 : " #it produces following result: ['physics', 'chemistry', 1997, 2000] After deleting value at index 2 : ['physics', 'chemistry', 2000]

Delete a sub-list list = [1,2,3,4,5,6,7,8,9,0] print list del list[3:6] #Output [1, 2, 3, 4, 5, 6, 7, 8, 9, 0] [1, 2, 3, 7, 8, 9, 0]

Basic List Operations Python Expression Results Description len([1, 2, 3]) 3 Length [1, 2, 3] + [4, 5, 6] [1, 2, 3, 4, 5, 6] Concatenation ['Hi!'] * 4 ['Hi!', 'Hi!', 'Hi!', 'Hi!'] Repetition 3 in [1, 2, 3] True Membership for x in [1, 2, 3]: print x, 1 2 3 Iteration

Indexing Assuming following input: L = ['spam', 'Spam', 'SPAM!'] Python Expression Results Description L[2] 'SPAM!' Offsets start at zero L[-2] 'Spam' Negative: count from the right L[1:] ['Spam', 'SPAM!'] Slicing fetches sections

Built-in List Functions 1 Function with Description 1 cmp(list1, list2) Compares elements of both lists. 2 len(list) Gives the total length of the list. 3 max(list) Returns item from the list with max value. 4 min(list) Returns item from the list with min value. 5 list(seq) Converts a tuple into list.

Max – different data types print max([1,2,3,4,5,6,7]) print max([1,2.6,3,4.8]) print max(["a", "b", "c"]) print max(["a", "b", "c", 1, 2, 3]) #Outout 7 4.8 c

Cmp – compare two list (-1,0,1) print cmp(list1, list2) print cmp(list1, list3) print cmp(list3, list1) #output 1 -1

Built-in List Functions 2 Methods with Description list.append(obj) Appends object obj to list list.count(obj) Returns count of how many times obj occurs in list list.extend(seq) Appends the contents of seq to list list.index(obj) Returns the lowest index in list that obj appears list.insert(index, obj) Inserts object obj into list at offset index list.pop(obj=list[-1]) Removes and returns last object or obj from list list.remove(obj) Removes object obj from list list.reverse() Reverses objects of list in place list.sort([func]) Sorts objects of list, use compare func if given

APPEND myList1 = [1,2,3] myList2 = [10,20,30] print myList1 myList1.append(myList2) #OUTPUT [1, 2, 3] [10, 20, 30] [1, 2, 3, [10, 20, 30]]

EXTEND myList1 = [1,2,3] myList2 = [10,20,30] print myList1 myList1.extend(myList2) #OUTPUT [1, 2, 3] [10, 20, 30] [1, 2, 3, 10, 20, 30]

Count example list1 = [1,1,1,2,2,3] print list1.count(1) Outputs 3 2 1

b=a (points to same list) a = list1 b = a print a print b a[0] = 9

b=a (points to same list) a = list1 b = a print a print b a[0] = 9 [0, 1, 2, 3] [9, 1, 2, 3] The difference is in bold

b = a[:] copy a list list1 = [0,1,2,3] a = list1 b = a[:] print a print b a[0] = 9

b = a[:] copy a list list1 = [0,1,2,3] a = list1 b = a[:] print a print b a[0] = 9 OUTPUT [0, 1, 2, 3] [9, 1, 2, 3] [0, 1, 2, 3] (notice this has not changed to 9)

Mutable Data Lists are mutable (can change) E.g. shoppingList[0] = “eggs” Will change the first item to “eggs”

Immutable Data A tuple does cannot change E.g. carRegistration = (2001, “X715KRW”,12112) These things typically do not change for a car (year of manufacture, Registration, chassis number) E.g. carRegistration = (2015, “JOHN 1”, 33452) You can reassign, if you buy a new car

Pre-computers Writing in pencil is like mutable data It can be erases and rewritten. Writing in ink is like immutable data Once is it written – you are stuck with it – like a Tattoo.

Tuples John R. Woodward

Tuples 1 A tuple is a sequence of immutable Python objects. Tuples are sequences, just like lists. The only difference is that tuples can't be changed i.e., tuples are immutable tuples use parentheses and lists use square brackets. Creating a tuple is as simple as putting different comma-separated values and optionally you can put these comma-separated values between parentheses also. tup1 = ('physics', 'chemistry', 1997, 2000); tup2 = (1, 2, 3, 4, 5 ); tup3 = "a", "b", "c", "d";

Tuples 2 The empty tuple is written as two parentheses containing nothing: tup1 = () To write a tuple containing a single value you have to include a comma, even though there is only one value: tup1 = (50,); ??? tup1 = (50);??? What would this mean Like string indices, tuple indices start at 0, and tuples can be sliced, concatenated and so on.

Types - Be careful tup1 = (50,) print tup1 print type(tup1) tup1 = (50)#no comma

Types - Be careful Output: (50,) <type 'tuple'> 50 print tup1 print type(tup1) tup1 = (50)#no comma Output: (50,) <type 'tuple'> 50 <type 'int'>

Same with Strings tup1 = ("Stirling") print tup1 print type(tup1)

Same with Strings OUTPUT Stirling <type 'str'> ('Stirling',) tup1 = ("Stirling") print tup1 print type(tup1) tup1 = ("Stirling",) OUTPUT Stirling <type 'str'> ('Stirling',) <type 'tuple'>

Updating Tuples Tuples are immutable. You are able to take portions of existing tuples to create new tuples tup1 = (12, 34.56); tup2 = ('abc', 'xyz'); tup3 = tup1 + tup2; print tup3; it produces the following result: (12, 34.56, 'abc', 'xyz')

Delete Tuple Elements Removing individual tuple elements is not possible. There is nothing wrong with putting together another tuple with the desired elements. To explicitly remove an entire tuple, just use the del statement. tup = ('physics', 'chemistry', 1997, 2000); print tup; del tup; print "After deleting tup : " print tup;#WHAT ERROR WOULD YOU GET

Basic Tuples Operations Python Expression Results Description len((1, 2, 3)) 3 Length (1, 2, 3) + (4, 5, 6) (1, 2, 3, 4, 5, 6) Concatenation ('Hi!',) * 4 ('Hi!', 'Hi!', 'Hi!', 'Hi!') Repetition 3 in (1, 2, 3) 4 not in (1, 2, 3) True Membership for x in (1, 2, 3): print x, 1 2 3 Iteration

Indexing L = ('spam', 'Spam', 'SPAM!') Python Expression Results Description L[2] 'SPAM!' Offsets start at zero L[-2] 'Spam' Negative: count from the right L[1:] ['Spam', 'SPAM!'] Slicing fetches sections

Built-in Tuple Functions cmp(tuple1, tuple2) Compares elements of both tuples. len(tuple) Gives the total length of the tuple. max(tuple) Returns item from the tuple with max value. min(tuple) Returns item from the tuple with min value. tuple(seq) Converts a list into tuple.

Accessing Values in Tuples To access values in tuple, use the square brackets. tup1 = ('physics', 'chemistry', 1997, 2000); tup2 = (1, 2, 3, 4, 5, 6, 7 ); print "tup1[0]: ", tup1[0] print "tup2[1:5]: ", tup2[1:5] it produces the following result: tup1[0]: physics tup2[1:5]: [2, 3, 4, 5]

EXAMPLE integers = (0,1,2,3,4,5,6,7,8,9) print integers subset1 = integers[1:3] print subset1

EXAMPLE integers = (0,1,2,3,4,5,6,7,8,9) print integers subset1 = integers[1:3] print subset1 (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) 1 9 (1, 2) OUTPUT

EXAMPLE subset1 = integers[1:8:2] print subset1

EXAMPLE output subset1 = integers[1:8:2] (1, 3, 5, 7) print subset1 () (8, 7, 6, 5) subset1 = integers[1:8:2] print subset1 subset1 = integers[8:1:2] subset1 = integers[8:4:-1] subset1 = integers[4:8:-1]