- - Table of Contents - -
-- - Table of Contents - -
-- Python provides an intuitive way to represent a group items, called - - - Lists - - - . The -items of a - - - List - - - are called its elements. Unlike C/C++, elements can be of any -type. A - - - List - - - is represented as a list of comma-sepated elements with square -brackets around them: -
-- >>> a = [10, 'Python programming', 20.3523, 23, 3534534L] +Chapter. list_tuples ++ +- +in the subsequent chapters. +++Table of Contents
+ ++ +-++Table of Contents
+ ++ ++Python provides an intuitive way to represent a group items, called Lists. The +items of a List are called its elements. Unlike C/C++, elements can be of any +type. A List is represented as a list of comma-sepated elements with square +brackets around them:
++>>> a = [10, 'Python programming', 20.3523, 23, 3534534L] >>> a -[10, 'Python programming', 20.3523, 23, 3534534L] ---+------- - - 1.1.Common List Operations -
-- The following are some of the most commonly used operations on - - - Lists - - - . -
--+------- - - 1.1.1.Indexing -
-- Individual elements of a - - - List - - - can be accessed using an index to the element. -The indices start at 0. One can also access the elements of the - - - List - - - in reverse -using negative indices.: -
-- >>> a[1] +[10, 'Python programming', 20.3523, 23, 3534534L]++ +-The following are some of the most commonly used operations on Lists.
++ +-Individual elements of a List can be accessed using an index to the element. +The indices start at 0. One can also access the elements of the List in reverse +using negative indices.:
++>>> a[1] 'Python programming' >>> a[-1] -3534534L --- It is important to note here that the last element of the - - - List - - - has an index of --1. -
--+------- - - 1.1.2.Concatenating -
-- Two or more - - - Lists - - - can be concatenated using the + operator: -
-- >>> a + ['foo', 12, 23.3432, 54] +3534534L+It is important to note here that the last element of the List has an index of +-1.
++ +-Two or more Lists can be concatenated using the + operator:
++>>> a + ['foo', 12, 23.3432, 54] [10, 'Python programming', 20.3523, 'foo', 12, 23.3432, 54] >>> [54, 75, 23] + ['write', 67, 'read'] -[54, 75, 23, 'write', 67, 'read'] ---+------- - - 1.1.3.Slicing -
-- A - - - List - - - can be sliced off to contain a subset of elements of the - - - List - - - . Slicing +[54, 75, 23, 'write', 67, 'read'] +
+ +-A List can be sliced off to contain a subset of elements of the List. Slicing can be done by using two indices separated by a colon, where the first index is -inclusive and the second index is exclusive. The resulting slice is also a - - - List - - - .: -
-- >>> num = [1, 2, 3, 4, 5, 6, 7, 8, 9] +inclusive and the second index is exclusive. The resulting slice is also a List.: +++>>> num = [1, 2, 3, 4, 5, 6, 7, 8, 9] >>> num[3:6] [4, 5, 6] >>> num[0:1] [1] >>> num[7:10] -[7, 8, 9] --- The last example showed how to access last 3 elements of the - - - List - - - . There is a +[7, 8, 9]
The last example showed how to access last 3 elements of the List. There is a small catch here. The second index 10 actually refers to the 11th element of the - - - List - - - which is still valid, even though it doesn't exist because the second +List which is still valid, even though it doesn't exist because the second index is exclusive and tells the Python interpreter to get the last element of -the - - - List - - - . But this can also be done in a much easier way using negative indices: -
-- >>> num[-3:-1] -[7, 8, 9] --- Excluding the first index implies that the slice must start at the beginning of -the - - - List - - - , while excluding the second index includes all the elements till the -end of the - - - List - - - . A third parameter to a slice, which is implicitly taken as 1 +the List. But this can also be done in a much easier way using negative indices:
++>>> num[-3:-1] +[7, 8, 9]+Excluding the first index implies that the slice must start at the beginning of +the List, while excluding the second index includes all the elements till the +end of the List. A third parameter to a slice, which is implicitly taken as 1 is the step of the slice. It is specified as a value which follows a colon after -the second index: -
-- >>> num[:4] +the second index: ++>>> num[:4] [1, 2, 3, 4] >>> num[7:] [8, 9] @@ -479,226 +131,77 @@ >>> num[3::2] [4, 6, 8] >>> num[::4] -[1, 5, 9] ---+------- - - 1.1.4.Multiplication -
-- A - - - List - - - can be multiplied with an integer to repeat itself: -
-- >>> [20] * 5 +[1, 5, 9]++ +-A List can be multiplied with an integer to repeat itself:
++>>> [20] * 5 [20, 20, 20, 20, 20] >>> [42, 'Python', 54] * 3 -[42, 'Python', 54, 42, 'Python', 54, 42, 'Python', 54] ---+------- - - 1.1.5.Membership -
-- - - in - - - operator is used to find whether an element is part of the - - - List - - - . It -returns - - - True - - - if the element is present in the - - - List - - - or - - - False - - - if it is not +[42, 'Python', 54, 42, 'Python', 54, 42, 'Python', 54] +
+ +-in operator is used to find whether an element is part of the List. It +returns True if the element is present in the List or False if it is not present. Since this operator returns a Boolean value it is called a Boolean -operator: -
-- >>> names = ['Guido', 'Alex', 'Tim'] +operator: ++>>> names = ['Guido', 'Alex', 'Tim'] >>> 'Tim' in names True >>> 'Adam' in names -False ---+------- - - 1.1.6.Length, Maximum and Minimum -
-- Length of a - - - List - - - can be found out using the len function. The max function +False +
+ +-Length of a List can be found out using the len function. The max function returns the element with the largest value and the min function returns the -element with the smallest value: -
-- >>> num = [4, 1, 32, 12, 67, 34, 65] +element with the smallest value: ++>>> num = [4, 1, 32, 12, 67, 34, 65] >>> len(num) 7 >>> max(num) 67 >>> min(num) -1 ---+------- - - 1.1.7.Changing Elements -
-- Unlike Strings - - - Lists - - - are mutable, i.e. elements of a - - - List - - - can be manipulated: -
-- >>> a = [1, 3, 5, 7] +1++ +-Unlike Strings Lists are mutable, i.e. elements of a List can be manipulated:
++>>> a = [1, 3, 5, 7] >>> a[2] = 9 >>> a -[1, 3, 9, 7] ---+------- - - 1.1.8.Deleting Elements -
-- An element or a slice of a - - - List - - - can be deleted by using the - - - del - - - statement: -
-- >>> a = [1, 3, 5, 7, 9, 11] +[1, 3, 9, 7]++ +-An element or a slice of a List can be deleted by using the del statement:
++>>> a = [1, 3, 5, 7, 9, 11] >>> del a[-2:] >>> a [1, 3, 5, 7] >>> del a[1] >>> a -[1, 5, 7] ---+------- - - 1.1.9.Assign to Slices -
-- In the same way, values can be assigned to individual elements of the - - - List - - - , -a - - - List - - - of elements can be assigned to a slice: -
-- >>> a = [2, 3, 4, 5] +[1, 5, 7]++ +-In the same way, values can be assigned to individual elements of the List, +a List of elements can be assigned to a slice:
++>>> a = [2, 3, 4, 5] >>> a[:2] = [0, 1] [0, 1, 4, 5] >>> a[2:2] = [2, 3] @@ -706,740 +209,230 @@ [0, 1, 2, 3, 4, 5] >>> a[2:4] = [] >>> a -[0, 1, 4, 5] --- The last two examples should be particularly noted carefully. The last but one -example insert elements or a list of elements into a - - - List - - - and the last example -deletes a list of elements from the - - - List - - - . -
--+------- - - 1.2.None, Empty Lists, and Initialization -
-- An - - - Empty List - - - is a - - - List - - - with no elements and is simply represented as -[]. A - - - None List - - - is one with all elements in it being - - - None - - - . It serves +[0, 1, 4, 5] +
The last two examples should be particularly noted carefully. The last but one +example insert elements or a list of elements into a List and the last example +deletes a list of elements from the List.
++ +-An Empty List is a List with no elements and is simply represented as +[]. A None List is one with all elements in it being None. It serves the purpose having a container list of some fixed number of elements with -no value: -
-- >>> a = [] +no value: ++>>> a = [] >>> a [] >>> n = [None] * 10 >>> n -[None, None, None, None, None, None, None, None, None, None] ----------- - - 1.3.Nested Lists -
-- As mentioned earlier, a List can contain elements of any data type. This also -implies a - - - List - - - can have a - - - Lists - - - themselves as its elements. These are -called as - - - Nested Lists - - - . There is no limit on the depth of the - - - Nested Lists - - - : -
-- >>> a = [1, [1, 2, 3], 3, [1, [1, 2, 3]], 7] ---+------- - - 1.4.List Methods -
-- A method is a function that is coupled to an object. More about objects +[None, None, None, None, None, None, None, None, None, None] +
+ ++As mentioned earlier, a List can contain elements of any data type. This also +implies a List can have a Lists themselves as its elements. These are +called as Nested Lists. There is no limit on the depth of the Nested Lists:
++>>> a = [1, [1, 2, 3], 3, [1, [1, 2, 3]], 7]++ +-A method is a function that is coupled to an object. More about objects and its methods are discussed in Advanced Python module. In general, a -method is called like: -
-- object.method(arguments) --- For now, it is enough to know that a list of elements is an object and -so - - - List - - - methods can be called upon them. Also some of the methods change -the - - - List - - - in-place, meaning it modifies the existing list instead of creating +method is called like:
++object.method(arguments)+For now, it is enough to know that a list of elements is an object and +so List methods can be called upon them. Also some of the methods change +the List in-place, meaning it modifies the existing list instead of creating a new one, while other methods don't. It must be noted as we run through -the - - - List - - - methods. -
-- Some of the most commonly used - - - List - - - methods are as follows: -
---------- - - 1.4.1.append -
-- The - - - append - - - method is used to append an object at the end of the list: -
-- >>> prime = [2, 3, 5] +the List methods. +Some of the most commonly used List methods are as follows:
++ +-The append method is used to append an object at the end of the list:
++>>> prime = [2, 3, 5] >>> prime.append(7) >>> prime -[2, 3, 5, 7] --- It is important to note that append changes the - - - List - - - in-place. -
--+------- - - 1.4.2.count -
-- The - - - count - - - method returns the number of occurences of a particular element -in a list: -
-- >>> [1, 4, 4, 9, 9, 9].count(9) +[2, 3, 5, 7]+It is important to note that append changes the List in-place.
++ +-The count method returns the number of occurences of a particular element +in a list:
++>>> [1, 4, 4, 9, 9, 9].count(9) 3 >>> tlst = ['Python', 'is', 'a', 'beautiful', 'language'] >>> tlst.count('Python') -1 ---+------- - - 1.4.3.extend -
-- The - - - extend - - - method extends the list on which it is called by the list supplied -as argument to it: -
-- >>> a = [1, 2, 3] +1++ +-The extend method extends the list on which it is called by the list supplied +as argument to it:
++>>> a = [1, 2, 3] >>> b = [4, 5, 6] >>> a.extend(b) -[1, 2, 3, 4, 5, 6] --- This is an in-place method. This method is equivalent to using the + operator, but -using the + operator returns a new list. -
--+------- - - 1.4.4.index -
-- The - - - index - - - method returns the index position of the element in the list -specified as argument: -
-- >>> a = [1, 2, 3, ,4, 5] +[1, 2, 3, 4, 5, 6]+This is an in-place method. This method is equivalent to using the + operator, but +using the + operator returns a new list.
++ +-The index method returns the index position of the element in the list +specified as argument:
++>>> a = [1, 2, 3, ,4, 5] >>> a.index(4) -3 ---+------- - - 1.4.5.insert -
-- The - - - insert - - - method is used to insert an element specified as the second -argument to the list at the position specified by the first argument: -
-- >>> a = ['Python', 'is', 'cool'] +3++ +-The insert method is used to insert an element specified as the second +argument to the list at the position specified by the first argument:
++>>> a = ['Python', 'is', 'cool'] >>> a.insert(2, 'so') >>> a -['Python', 'is', 'so', 'cool'] --- The - - - insert - - - method changes the - - - List - - - in-place. -
--+------- - - 1.4.6.pop -
-- The - - - pop - - - method removes an element from the list. The index position +['Python', 'is', 'so', 'cool'] +
The insert method changes the List in-place.
++ +-The pop method removes an element from the list. The index position of the element to be removed can be specified as an argument to the - - - pop - - - method, if not it removes the last element by default: -
-- >>> a = [1, 2, 3, 4, 5] +pop method, if not it removes the last element by default: ++>>> a = [1, 2, 3, 4, 5] >>> a.pop() >>> a 5 >>> a.pop(2) >>> a -3 --- The - - - pop - - - method changes the - - - List - - - in-place. -
--+------- - - 1.4.7.remove -
-- The - - - remove - - - method removes the first occurence of an element supplied as a -parameter: -
-- >>> a = [1, 2, 3, 4, 2, 5, 2] +3+The pop method changes the List in-place.
++ +-The remove method removes the first occurence of an element supplied as a +parameter:
++>>> a = [1, 2, 3, 4, 2, 5, 2] >>> a.remove(2) >>> a -[1, 3, 4, 2, 5, 2] ---+------- - - 1.4.8.reverse -
-- The - - - reverse - - - method reverses elements in the list. It is important to note -here that - - - reverse - - - method changes the list in-place and doesn't return any -thing: -
-- >>> a = ['guido', 'alex', 'tim'] +[1, 3, 4, 2, 5, 2]++ +-The reverse method reverses elements in the list. It is important to note +here that reverse method changes the list in-place and doesn't return any +thing:
++>>> a = ['guido', 'alex', 'tim'] >>> a.reverse() >>> a -['tim', 'alex', 'guido'] ---+------- - - 1.4.9.sort -
-- The - - - sort - - - method is used to sort the elements of the list. The - - - sort - - - method -also sorts in-place and does not return anything: -
-- >>> a = [5, 1, 3, 7, 4] +['tim', 'alex', 'guido']++ +-The sort method is used to sort the elements of the list. The sort method +also sorts in-place and does not return anything:
++>>> a = [5, 1, 3, 7, 4] >>> a.sort() >>> a -[1, 3, 4, 5, 7] --- In addition to the sort method on a - - - List - - - object we can also use the built-in - - - sorted - - - function. This function takes the - - - List - - - as a parameter and returns -a sorted copy of the list. However the original list is left intact: -
-- >>> a = [5, 1, 3, 7, 4] +[1, 3, 4, 5, 7]+In addition to the sort method on a List object we can also use the built-in +sorted function. This function takes the List as a parameter and returns +a sorted copy of the list. However the original list is left intact:
++>>> a = [5, 1, 3, 7, 4] >>> b = sorted(a) >>> b [1, 3, 4, 5, 7] >>> a -[5, 1, 3, 7, 4] ---+------- - - 2.Tuples -
-- - - Tuples - - - are sequences just like - - - Lists - - - , but they are immutable. In other -words - - - Tuples - - - provides a way to represent a group of items, where the group -of items cannot be changed in any way. The syntax of a - - - Tuple - - - is also very -similar to - - - List - - - . A - - - Tuple - - - is represented with the list of items, called -elements of the - - - Tuple - - - separated by comma, with the entire list being enclosed -in parenthesis. It is not compulsory to use parenthesis around a - - - Tuple - - - but -it may be necessary in some of the cases: -
-- >>> a = 1, 2, 3 +[5, 1, 3, 7, 4]++ ++ -Tuples are sequences just like Lists, but they are immutable. In other +words Tuples provides a way to represent a group of items, where the group +of items cannot be changed in any way. The syntax of a Tuple is also very +similar to List. A Tuple is represented with the list of items, called +elements of the Tuple separated by comma, with the entire list being enclosed +in parenthesis. It is not compulsory to use parenthesis around a Tuple but +it may be necessary in some of the cases:
++>>> a = 1, 2, 3 >>> a (1, 2, 3) >>> b = 1, >>> b -(1,) --- It is interesting to note the second example. Just a value followed by a comma -automatically makes that an element of a - - - Tuple - - - with only one element. It is +(1,) +
It is interesting to note the second example. Just a value followed by a comma +automatically makes that an element of a Tuple with only one element. It is also important to note that, irrespective of input having a parenthesis, the -output always has a parenthesis. -
-- The first example is also known as - - - Tuple packing - - - , because values are being -packed into a tuple. It is also possible to do - - - Tuple unpacking - - - which is more +output always has a parenthesis.
+The first example is also known as Tuple packing, because values are being +packed into a tuple. It is also possible to do Tuple unpacking which is more interesting. It is better to understand that by example. Say we have a -co-ordinate pair from which we need to separate x and y co-ordinates: -
-- >>> a = (1, 2) +co-ordinate pair from which we need to separate x and y co-ordinates: +++>>> a = (1, 2) >>> x, y = a >>> x 1 >>> y -2 --- - - Tuple unpacking - - - also has several other use-cases of which the most interesting +2
Tuple unpacking also has several other use-cases of which the most interesting one is to swap the values of two variables. Using programming languages like C would require anywhere around 10 lines of code and an extra temporary variable to do this (including all the #include stuff). Python does it in the most intuitive way in just one line. Say we want to swap the co-ordinates in the -above example: -
-- >>> x, y = y, x +above example: ++>>> x, y = y, x >>> x 2 >>> y -1 ----------- - - 2.1.Common Tuple Operations -
-- There is no need to introduce all the - - - Tuple - - - operations again, since - - - Tuples - - - support the following operations that - - - List - - - supports in exactly the same way: -
----
-- -
-- Indexing -
-- -
-- Concatenating -
-- -
-- Slicing -
-- -
-- Membership -
-- -
-- Multiplication -
-- -
-- Length, Maximum, Minimum -
-- The following examples illustrate the above operations: -
-- >>> a = (1, 2, 3, 4, 5, 6) +1++ +-There is no need to introduce all the Tuple operations again, since Tuples +support the following operations that List supports in exactly the same way:
+++
- +
Indexing
- +
Concatenating
- +
Slicing
- +
Membership
- +
Multiplication
- +
Length, Maximum, Minimum
The following examples illustrate the above operations:
++>>> a = (1, 2, 3, 4, 5, 6) >>> a[5] 6 >>> b = (7, 8, 9) @@ -1457,278 +450,73 @@ >>> max(a) 6 >>> min(a) -1 --- However the following - - - List - - - operations are not supported by - - - Tuples - - - because - - - Tuples - - - cannot be changed once they are created: -
----
-- -
-- Changing elements -
-- -
-- Deleting elements -
-- -
-- Assigning to slices -
-- Similarity to - - - Lists - - - leads to the questions like, why not - - - Lists - - - only? Why do -we even want - - - Tuples - - - ? Can we do the same with - - - Lists - - - ? And the answer is - - - Yes - - - we can do it, but - - - Tuples - - - are helpful at times, like we can return Tuples from +1 +
However the following List operations are not supported by Tuples because +Tuples cannot be changed once they are created:
+++
- +
Changing elements
- +
Deleting elements
- +
Assigning to slices
Similarity to Lists leads to the questions like, why not Lists only? Why do +we even want Tuples? Can we do the same with Lists? And the answer is Yes +we can do it, but Tuples are helpful at times, like we can return Tuples from functions. They are also returned by some built-in functions and methods. And -also there are some use cases like co-ordinate among other things. So - - - Tuples - - - are helpful. -
--+------- - - 3.Additional Syntax -
-- The following additional syntax are introduced to make it easier to operate on - - - Lists - - - . -
--+------- - - 3.1.range() -
-- The - - - range - - - function takes at least one argument and 2 additional optional +also there are some use cases like co-ordinate among other things. So Tuples +are helpful.
++ +-The following additional syntax are introduced to make it easier to operate on +Lists.
++ +-The range function takes at least one argument and 2 additional optional arguments. If two or more arguments are specified, the range function returns a list of natural numbers starting from the first argument passed to it to the second argument. The third argument, if specified is used as a step. Suppose -only one argument is specified, then - - - range - - - function returns a list of natural -numbers starting from 0 upto the argument specified: -
-- >>> range(5, 10, 2) +only one argument is specified, then range function returns a list of natural +numbers starting from 0 upto the argument specified: ++>>> range(5, 10, 2) [5, 7, 9] >>> range(2, 15) [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] >>> range(12) -[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] ---+------- - - 3.2.for -
-- The - - - for - - - keyword is used as a part of the looping construct. Unlike for loops +[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] +
+ +-The for keyword is used as a part of the looping construct. Unlike for loops in other languages, Python's for is used to iterate through the elements of -sequences like - - - Lists - - - , - - - Tuples - - - , - - - Dictionaries - - - , etc. The syntax of the for loop -consists of - - - for - - - , followed by a variable to hold the individual or the current -element of the list during iteration and - - - in - - - , followed by the sequence and a -semicolon(':') The next line which is part of the - - - for - - - loop, i.e the statements -that are part of the loop should start with a new intend: -
-- >>> names = ['Guido', 'Alex', 'Tim'] +sequences like Lists, Tuples, Dictionaries, etc. The syntax of the for loop +consists of for, followed by a variable to hold the individual or the current +element of the list during iteration and in, followed by the sequence and a +semicolon(':') The next line which is part of the for loop, i.e the statements +that are part of the loop should start with a new intend: ++>>> names = ['Guido', 'Alex', 'Tim'] >>> for name in names: ... print "Name =", name ... Name = Guido Name = Alex -Name = Tim ---+------- - - 4.Conclusion -
-- This section on - - - Lists - - - and - - - Tuples - - - introduces almost all the necessary -machinary required to work on - - - Lists - - - and - - - Tuples - - - . Topics like how to +Name = Tim +