How to find the last element of list in Python assume sam is the name of list

Python | How to get the last element of list

In Python, how do you get the last element of a list?

To just get the last element,

• without modifying the list, and
• assuming you know the list has a last element (i.e. it is nonempty)

pass -1 to the subscript notation:

>>> a_list = ['zero', 'one', 'two', 'three'] >>> a_list[-1] 'three'

How to get the last element of a list in Python?

Python sequence, including list object allows indexing. Any element in list can be accessed using zero based index. If index is a negative number, count of index starts from end. As we want last element in list, use -1 as index.

>>> L1=[1,2,3,4,5] >>> print (L1[-1]) 5

Get last item of a list using negative indexing

List in python supports negative indexing. So, if we have a list of size “S”, then to access the Nth element from last we can use the index “-N”. Let’s understand by an example,
Suppose we have a list of size 10,

sample_list = [1, 2, 3, 4, 5, 6, 7, 8, 9]
To access the last element i.e. element at index 9 we can use the index -1,
# Get last element by accessing element at index -1 last_elem = sample_list[-1] print('Last Element: ', last_elem)
Output:
Last Element: 9
Similarly, to access the second last element i.e. at index 8 we can use the index -2.
last_elem = sample_list[-2]
Output:
Last Element: 8
Using negative indexing, you can select elements from the end of list, it is a very efficient solution even if you list is of very large size. Also, this is the most simplest and most used solution to get the last element of list. Let’s discuss some other ways,

Overview — Creating a List in Python

There are many ways of creating a list in Python. Let’s get a quick overview in the following table:

CodeDescription

[]	Square bracket: Initializes an empty list with zero elements. You can add elements later.
[x1, x2, x3, … ]	List display: Initializes an empty list with elements x1, x2, x3, … For example, [1, 2, 3] creates a list with three integers 1, 2, and 3.
[expr1, expr2, ... ]	List display with expressions: Initializes a list with the result of the expressions expr1, expr2, … For example, [1+1, 2-1] creates the list [2, 1].
[expr for var in iter]	List comprehension: applies the expression expr to each element in an iterable.
list(iterable)	List constructor that takes an iterable as input and returns a new list.

You can play with some examples in our interactive Python shell:

Exercise: Use list comprehension to create a list of square numbers.

Introduction¶

Pysam is a python module that makes it easy to read and manipulate mapped short read sequence data stored in SAM/BAM files. It is a lightweight wrapper of the htslib C-API.

This page provides a quick introduction in using pysam followed by the API. See Working with BAM/CRAM/SAM-formatted files for more detailed usage instructions.

To use the module to read a file in BAM format, create a AlignmentFile object:

Once a file is opened you can iterate over all of the read mapping to a specified region using fetch(). Each iteration returns a AlignedSegment object which represents a single read along with its fields and optional tags:

To give:

You can also write to a AlignmentFile:

An alternative way of accessing the data in a SAM file is by iterating over each base of a specified region using the pileup() method. Each iteration returns a PileupColumn which represents all the reads in the SAM file that map to a single base in the reference sequence. The list of reads are represented as PileupRead objects in the PileupColumn.pileups property:

The above code outputs:

Commands available in csamtools are available as simple function calls. For example:

corresponds to the command line:

Analogous to AlignmentFile, a TabixFile allows fast random access to compressed and tabix indexed tab-separated file formats with genomic data:

TabixFile implements lazy parsing in order to iterate over large tables efficiently.

More detailed usage instructions is at Working with BAM/CRAM/SAM-formatted files.