diff -r 0b0620ae8f4d -r d12107cbe14b manipulating-lists.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/manipulating-lists.rst Sat Sep 18 00:42:37 2010 +0530 @@ -0,0 +1,180 @@ +Hello friends. Welcome to this spoken tutorial on Getting started with +strings. + +{{{ Show the slide containing the title }}} + +{{{ Show the slide containing the outline }}} + +We have already learnt a lot about Lists in Python. In this tutorial, +we will learn more about advanced features of Lists in Python. We will +see in detail how to concatenate two lists, slicing and striding of +lists, methods to sort and reverse the list. + +{{{ Shift to terminal and start ipython }}} + +To begin with let us start ipython, by typing:: + + ipython + +on the terminal + +We already know what Lists are in Python, how to access individual +elements in the list and some of the functions that can be run on the +lists like max, min, sum len and so on. Now let us learn some of the +basic operations that can be performed on Lists. + +We already know how to access individual elements in a List. But what +if we have a scenario where we need to get a part of the entire list +or what we call as a slice of the list? Python supports slicing on +lists. Let us say I have the list:: + + primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29] + +To obtain the all the primes between 10 and 20 from the above list of +primes we say:: + + primes[4:8] + +This gives us all the elements in the list starting from the element +with the index 4 which is 11 in our list upto the element with index 8 +in the list but not including the eigth element. So we obtain a slice +starting from 11 upto 19th. It is a very important to remember that +when ever we specify a range of elements in Python the start index is +included and end index is not included. So in the above case, 11 which +was the element with the index 4 was included but 23 which was the +element with index 8 was exluded. + +Generalizing, we can obtain a slice of the list "p" from the index +"start" upto the index "end" but excluding "end" with the following +syntax + +{{{ Show the slide containing p[start:stop] }}} + +By default the slice fetches all the elements between start and stop +including start but not stop. So as to say we obtain all the elements +between start and stop in steps of one. Python also provides us the +functionality to specify the steps in which the slice must be +obtained. Say we have:: + + num = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] + +If we want to obtain all the odd numbers less than 10 from the list +"num" we have to start from element with index 1 upto the index 10 in +steps of 2:: + + num[1:10:2] + +So if we don't specify the step it is by default 1. Similary there are +default values for start and stop indices as well. If we don't specify +the start index it is implicitly taken as the first element of the +list:: + + num[:10] + +This gives us all the elements from the beginning upto the 10th +element but not including the 10th element in the list "num". Similary +if the stop index is not specified it is implicitly assumed to be the +end of the list, including the last element of the list:: + + num[10:] + +gives all the elements starting from the 10th element in the list +"num" upto the final element including that last element. Now:: + + num[::2] + +gives us all the even numbers in the list "num". + +The other basic operation that we can perform on list is concatenation +of two or more lists. We can combine two lists by using the "plus" +operator. Say we have + +{{{ Read as you type }}}:: + + a = [1, 2, 3, 4] + b = [4, 5, 6, 7] + a + b + +When we concatenate lists using the "plus" operator we get a new +list. We can store this list in a new variable:: + + c = a + b + c + +It is important to observe that the "plus" operator always returns a +new list without touching anything in the existing lists which are the +operands of the concatenation operation. + +We know that list is a collection of data. Whenever we have a +collection we run into situations where we want to start the +collection. Lists support sort method which sorts the list inplace:: + + a = [5, 1, 6, 7, 7, 10] + a.sort() + +Now the contents of the list "a" will be:: + + a + [1, 5, 6, 7, 7, 10] + +Since the sort method sorts the list inplace the original list we had +is overwritten or replaced. We have no way to obtain the original list +back. One way to avoid this is to keep a copy of the original list in +another variable and run the sort method on the list. However Python +also provides a built-in function called sorted which sorts the list +which is passed as an argument to it and returns a new sorted list:: + + a = [5, 1, 6, 7, 7, 10] + sorted(a) + +We can store this sorted list another list variable:: + + sa = sorted(a) + +Similarly to perform certain operations on the list we would like to +reverse the list. Python provides reverse method which again reverses +the list inplace:: + + a = [1, 2, 3, 4, 5] + a.reverse() + +reverses the list "a" and stores the reversed list inplace i.e. in "a" +itself. Lets see the list "a":: + + a + [5, 4, 3, 2, 1] + +But again the original list is lost. If we want to obtain the reverse +of a list keeping the original list intact we can use the Python +built-in function reversed. reversed function returns a new list which +is the reverse of the list which was passed as the argument to the +reversed function:: + + a = [1, 2, 3, 4, 5] + reversed(a) + +We can also store this new reversed list in another list variable. + +{{{ Show summary slide }}} + +This brings us to the end of another session. In this tutorial session +we learnt + + * How to define strings + * Different types of defining a string + * String concatenation and repeatition + * Accessing individual elements of the string + * Immutability of strings + +{{{ Show the "sponsored by FOSSEE" slide }}} + +This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India + +Hope you have enjoyed and found it useful. +Thankyou + +.. Author : Madhu + Internal Reviewer 1 : [potential reviewer: Nishanth] + Internal Reviewer 2 : [potential reviewer: Amit] + External Reviewer : +