Values and Data Types

VALUES AND DATA TYPES

Value:

Value can be any letter ,number or string.

Eg, Values are 2, 42.0, and 'Hello, World!'. (These values belong to different datatypes.)

Data type:

Every value in Python has a data type.

It is a set of values, and the allowable operations on those values.

Python has four standard data types:

Numbers:

v   Number data type stores Numerical Values.

v   This data type is immutable [i.e. values/items cannot be changed].

v   Python supports integers, floating point numbers and complex numbers. They are defined as,

Sequence:

v   A sequence is an ordered collection of items, indexed by positive integers.

v   It is a combination of mutable (value can be changed) and immutable (values cannot be changed) data types.

v   There are three types of sequence data type available in Python, they are

1.              Strings

2.              Lists

3.              Tuples

1. Strings

Ø   A String in Python consists of a series or sequence of characters - letters, numbers, and special characters.

Ø   Strings are marked by quotes:

·  single quotes (' ') Eg, 'This a string in single quotes'

·  double quotes (" ") Eg, "'This a string in double quotes'"

·                 triple quotes(""" """) Eg, This is a paragraph. It is made up of multiple lines and sentences."""

Ø   Individual character in a string is accessed using a subscript (index).

Ø   Characters can be accessed using indexing and slicing operations

Strings are immutable i.e. the contents of the string cannot be changed after it is created.

Indexing:

·                 Positive indexing helps in accessing the string from the beginning

·                 Negative subscript helps in accessing the string from the end.

·                 Subscript 0 or –ve n(where n is length of the string) displays the first element.

Example: A[0] or A[-5] will display “H”

·                 Subscript 1 or –ve (n-1) displays the second element.

Example: A[1] or A[-4] will display “E”

Operations on string:

i.                 Indexing

ii.              Slicing

iii.            Concatenation

iv.            Repetitions

v.              Member ship

2. Lists

v   List is an ordered sequence of items. Values in the list are called elements / items.

v   It can be written as a list of comma-separated items (values) between square brackets[ ].

v   Items in the lists can be of different data types.

Operations on list:

Indexing

Slicing

Concatenation

Repetitions

Updation, Insertion, Deletion

3. Tuple:

v   A tuple is same as list, except that the set of elements is enclosed in parentheses instead of square brackets.

v   A tuple is an immutable list. i.e. once a tuple has been created, you can't add elements to a tuple or remove elements from the tuple.

v   Benefit of Tuple:

v   Tuples are faster than lists.

v   If the user wants to protect the data from accidental changes, tuple can be used.

v   Tuples can be used as keys in dictionaries, while lists can't.

Basic Operations:

Altering the tuple data type leads to error. Following error occurs when user tries to do.

>>> t[0]="a"

Trace back (most recent call last):

File "<stdin>", line 1, in <module>

Type Error: 'tuple' object does not support item assignment

Mapping

-This data type is unordered and mutable.

-Dictionaries fall under Mappings.

Dictionaries:

v   Lists are ordered sets of objects, whereas dictionaries are unordered sets.

v   Dictionary is created by using curly brackets. i,e. {}

v   Dictionaries are accessed via keys and not via their position.

v   A dictionary is an associative array (also known as hashes). Any key of the dictionary is associated (or mapped) to a value.

v   The values of a dictionary can be any Python data type. So dictionaries are

unordered key-value-pairs(The association of a key and a value is called a key-value pair )

Dictionaries don't support the sequence operation of the sequence data types like strings, tuples and lists.

If you try to access a key which doesn't exist, you will get an error message:

>>> words = {"house" : "Haus", "cat":"Katze"}

>>> words["car"]

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

KeyError: 'car'