Programming Essentials Mutator Method Explain the purpose of the mutator method.

Question

Programming Essentials

Mutator Method

Explain the purpose of the mutator method. What does it normally return and why?

Explain aliasing, how does it affect the mutation of an object?

Give an example of an object that behaves like a dictionary show a code example of how the python dictionary reflects the real world example.

Programming Essentials

Mutator Method

Explain the purpose of the mutator method. What does it normally return and why?

Explain aliasing, how does it affect the mutation of an object?

Give an example of an object that behaves like a dictionary show a code example of how the python dictionary reflects the real world example.

Explanation / Answer

Mutator Method

Just like any other object oriented programming language, Objects are used to store information. It has state and behavior. There are two kinds of methods in object oriented programming language: accessor and mutator methods.

These are like setter and getter methods.

Accessor method is used to access the current state of the object but don’t change it.

For example:

a = “Hello world”

b = a.upper()

print(b)

Now, here you can notice that upper method is used to change “a” to upper case and store it into “b” but it’s not changing object “a”.

Now, Mutator method is used to change the existing object.

For example:

sampleList= {10, 20, 30 }

sampleList.reverse()

print(sampleList)

This will print list in reverse manner, i.e. {30, 20, 10} to the screen. The change done by mutator method is permanent.

Aliasing in Python

Alias means another name of an entity. It is easy to make alias of data.

If data is immutable, alias doesn’t matter.

But if data is mutable then it can lead to unrecognizable bugs.

So, using mutator method on aliased objects can lead to bugs.

For example:

Lists are mutable in python.

x = [‘apple’]

y = x

x = x.append(‘orange’)

print first

[‘apple’, ‘orange’]

print second

[‘apple’, ‘orange’]

As both objects are having same reference to the value. Changing one object can explicitly modify second object as a side effect.

Python Dictionary

In python, it is called as “dicts”. In other languages, it is referred as “hashes” or “maps”.

It is use to data in key-value pair.

Let understand by comparing dictionary with list.

For example:

In case of list, we use index to store or identify its element.

>>> values = ['a', 'b', 'c', 'd']

>>> print values[1]

b

>>> values[1] = 'y'

>>> print values[1]

z

>>> values

['a', 'y', 'c', 'd']

In case of dictionary, we can use anything to associate one thing to another.

>>> values = {'name': 'Smith', 'age': 21, 'height': 5 * 12 + 6}

>>> print values['name']

Smith

>>> print values['age']

21

>>> print values['height']

66

>>> values['city'] = "New york"

>>> print values['city']

New york

To add values in dictionary, see below example

>>> values[1] = "Apple"

>>> values[2] = "Orange"

>>> print values[1]

Apple

>>> print values[2]

Orange

>>> values

{'city': 'New york', 2: 'Orange', 'name': 'Smith', 1: 'Orange', 'age': 21, 'height': 66}

To delete values from dictionary, see below example

>>> del values['city']

>>> del values[1]

>>> del values[2]

>>> values

{'name': 'Smith', 'age': 21, 'height': 66}

Example of Dictionary in python

Save it as sample.py and run as $ python sample.py

_______________________________________________________________________________________________

# create a mapping of state to abbreviation

states = {

    'Oregon': 'OR',

    'Florida': 'FL',

    'California': 'CA',

    'New York': 'NY',

    'Michigan': 'MI'

}

# create a basic set of states and some cities in them

cities = {

    'CA': 'San Francisco',

    'MI': 'Detroit',

    'FL': 'Jacksonville'

}

# add some more cities

cities['NY'] = 'New York'

cities['OR'] = 'Portland'

# print out some cities

print '-' * 10

print "NY State has: ", cities['NY']

print "OR State has: ", cities['OR']

# print some states

print '-' * 10

print "Michigan's abbreviation is: ", states['Michigan']

print "Florida's abbreviation is: ", states['Florida']

# do it by using the state then cities dict

print '-' * 10

print "Michigan has: ", cities[states['Michigan']]

print "Florida has: ", cities[states['Florida']]

# print every state abbreviation

print '-' * 10

for state, abbrev in states.items():

    print "%s is abbreviated %s" % (state, abbrev)

# print every city in state

print '-' * 10

for abbrev, city in cities.items():

    print "%s has the city %s" % (abbrev, city)

# now do both at the same time

print '-' * 10

for state, abbrev in states.items():

    print "%s state is abbreviated %s and has city %s" % (

        state, abbrev, cities[abbrev])

print '-' * 10

# safely get a abbreviation by state that might not be there

state = states.get('Texas')

if not state:

    print "Sorry, no Texas."

# get a city with a default value

city = cities.get('TX', 'Does Not Exist')

print "The city for the state 'TX' is: %s" % city

_______________________________________________________________________________________________

Real life example of Dictionary can be English word Dictionary like Oxford Dictionary,

Internet Address Lookup Table

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