The dictionary is indexed ○ The values can be accessed

PROGRAMMING

PYTHON

Dictionary

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• Python allows to organize information/data in various ways – Strings, lists and tuples organize data into sequences.

– The strings, lists and tuples are only indexed by integer values.

– These are ordered collection of items.

– The set data-type is an unordered collection of items.

– The elements can’t be accessed in a unique order.

– The set elements cannot be accessed using indexing operators.

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The dictionary data-type is an unordered collection of items.

○ Each item consists of key-value pairs

○ It is ordered collection of references - Each value is referenced though key in the pair

The dictionary is mutable - we can change its state—its length and content.

The dictionary is indexed

○ The values can be accessed.

○ The dictionary elements are not only accessed by integer values.

○ The values can be accessed using float, string, tuple etc.

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• Dictionaries stores information in pairs – Similar to an actual dictionary

• Word and definition

• Python uses key and value

Other data types have only value as an element, a dictionary has a pair of entries

● A key

● A value

○ The items in dictionaries are accessed using keys and not using their position.

• A dictionary is an associative array (also known as hashes).

• Any key of the dictionary is associated (or mapped) to a value.

A^DVANTAGES

Dictionary is the most powerful data collection of Python.

Dictionaries allow us to do fast database-like operations in python.

The items in dictionaries are accessed using keys and not by the positions.

Dictionaries have different names in different languages

• Associative Arrays - Perl / Php

• Properties or Map or HashMap - Java

• Property Bag - C# / .Net

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Place items inside curly braces { } separated by comma.

The dictionary can also be create using dict() function.

• An item has a key and the corresponding value expressed as a pair.

• key: value

• Key and values are separated by a colon

• The values can be of any data type and can repeat.

• The keys must be of immutable type (integer, float, string, tuple with immutable elements, frozenset etc.)

• The keys must be unique.

Example:

age = {'Alice' : 25, 'Bob' :28}

OR

age = dict( {'Alice' : 25, 'Bob' :28} )

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○ >>> age = {'Alice' : 25, 'Bob' : 28, 'Alice' : 26}

○ >>> age

{'Bob': 28, 'Alice': 26}

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>>>dict1 = {'BOB':24, 'JOHN':23, 23:23, [1,2]:345}

Traceback (most recent call last):

File "<pyshell#2>", line 1, in <module>

dict1 = {'BOB':24, 'JOHN':23, 23:23, [1,2]:345}

TypeError: unhashable type: 'list'

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To access values, dictionary uses keys.

The key is used with

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● square brackets(Index Operator)

○It returns the value corresponding to the key.

○If the key is not found, It raises key error.

get() method.

○ It returns the value corresponding to the key.

○ If the key is not found, it returns None.

Example:

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age = {'Alice' : 25, 'Bob' :28}

age['Alice'] is 25

age.get(‘Bob’) returns 28

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age = {'Alice' : 25, 'Carol': ‘twenty-two’}

○ items(): It returns all items of the dictionary.

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age.items()

dict_items([ ('Alice', 25), ('Carol', 'twenty-two')]) keys(): It returns all keys of the dictionary.

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age.keys()

dict_keys([ 'Alice', 'Carol'])

values(): It returns all values of the dictionary.

● age.values()

dict_values([28, 25, 'twenty-two'])

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○ Dictionary is a mutable data-type, the items can be added/updated.

o The new item can be added using assignment operator or The value of existing items can be updated using assignment operator.

o If the key is already present, value gets updated, else a new key: value pair is added

o The update() can also be used. It updates the value if key exists, otherwise it adds new item. It can be used to add multiple elements in a dictionary.

>>> age = {'Alice' : 25, 'Bob' : 28}

>>> age1 = age

>>> age['Bob'] = 29

>>> age

{'Bob': 29, 'Alice': 25}

>>> age1

{'Bob': 29, 'Alice': 25}

>>> age[‘John'] = 20

>>> age

{'Bob': 29, 'Alice': 25, ‘John’:20}

age.update({‘Kayva’: 29, ‘Haider’:23})

{'Bob': 29, 'Alice': 25, ‘John’:20, ‘Kayva’: 29, ‘Haider’:23}

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Dictionaries are mutable, the items can also be removed.

pop() – This method removes a particular item. It takes the key, removes the item and returns the value.

popitem(): The method removes an arbitrary item and returns the item (key, value) form the dictionary. Raises an error, if dictionary is empty.

clear(): This method can be used to clear all items at once.

del: The del keyword can also be used to remove individual items or the entire dictionary itself.

>>> a = {'Alice' : 26, 'Carol' : 22, ‘John’:20, }

>>> a.pop('Carol’) ---- 22 or del a[‘Carol’]

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>>> a.popitem() --- ('Alice': 26)

>>> a.clear()

>>>del a

Dictionary: Method

>>> x = {1:2, 3:4}

>>> type(x)

<class 'dict'>

>>> dir(x)

['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__',

'__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values’]

>>> help(x)

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○ A = {1:3, 3:4, 5:6}

○ len(): Return the length (the number of items) in the dictionary. (len(A))

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sorted(): Return a new sorted list of keys in the dictionary. (sorted(A))

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copy() --- Return a shallow copy of the dictionary (A.copy()).

○ fromkeys(seq, v]) ----Return a new dictionary with keys from seq and value equal to v (defaults to None).

{}.fromkeys([1,2,3,4], 23), {}.fromkeys(‘Python’, 2), …..

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get(key, d) ----Return the value of key. If key does not exit, return d(defaults to None).

pop(key, d ) ---Remove the item with key and return its value or d if key is not found. If d is not provided and key is not found, raises KeyError.

Setdefault (key, v): Returns the value of the specified key. If the key does not exists; insert the key, with the specified value.

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• We loop through the key-value pairs in a dictionary using

*two* iteration variables

• Each iteration, the first variable is the key and the the second variable is the corresponding value for the key.

Example

dict1 = {'Bob': 29, 'Alice': 25, ‘John’:20, ‘Kayva’: 29,

‘Haider’:23}

for key, value in dict1.items():

print(key, value)

I ^TERATION T ^HROUGH K ^EYS /V ^ALUES

Example

dict1 = {'Bob': 29, 'Alice': 25, ‘John’:20, ‘Kayva’: 29,

‘Haider’:23}

for key in dict1.keys():

print(key)

for value in dict1.values():

print(value)

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• Even though dictionaries are not stored in order, we can write a for loop that goes through all the entries in a

dictionary - actually it goes through all of the keys in the dictionary and looks up the values

counts = { 'chuck' : 1 , 'fred' : 42, 'jan': 100}

for key in counts:

print( key, counts[key])

...

...

jan 100 chuck 1 fred 42

>>>

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The general pattern to count the words in a line of text is to split the line into words, then loop through the words and use a dictionary to track the count of each word independently.

counts = dict()

text = input('Enter a line of text:') words = text.split()

print('Words:', words) print('Counting...') for word in words:

counts[word] = counts.get(word,0) + 1 print('Counts', counts)

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python wordcount.py

Enter a line of text: the clown ran after the car and the car ran into the tent and the tent fell down on the clown and the car

Words: ['the', 'clown', 'ran', 'after', 'the', 'car', 'and', 'the', 'car', 'ran',

'into', 'the', 'tent', 'and', 'the', 'tent', 'fell', 'down', 'on', 'the', 'clown', 'and', 'the', 'car']

Counting...

Counts {'and': 3, 'on': 1, 'ran': 2, 'car': 3, 'into': 1, 'after': 1, 'clown': 2, 'down': 1, 'fell': 1, 'the': 7, 'tent': 2}

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Characters

Frequency of Each Characters

counts = dict()

text = input('Enter a line of text:')

print('Counting...') for letter in text:

counts[letter] = counts.get(letter, 0) + 1 print('Counts', counts)

Pangram

A pangram, or holo-alphabetic sentence, is a sentence that contains every letter of the alphabet at least once.

The pangram string are as follows

• “The quick brown fox jumps over the lazy dog,”

• “Sphinx of black quartz, judge my vow”

• ”Jackdaws love my big sphinx of quartz”

• ”Pack my box with five dozen liquor jugs”:

• ”The quick onyx goblin jumps over the lazy dwarf”:

• ”Cwm fjord bank glyphs vext quiz”:

• ”How razorback-jumping frogs can level six piqued gymnasts!”

• ”Cozy lummox gives smart squid who asks for job pen”

Pangram

counts = dict()

string = input('Enter a line of text:') text = string.casefold()

for letter in text:

counts[letter] = counts.get(letter,0) + 1 List1 = []

for a in counts.keys():

if a.isalpha() == False:

List1.append(a) for a in List1:

counts.pop(a) if len(counts) == 26:

print('The string:', text, 'is pangram') else:

print('The string:', text, 'is not pangram') print('Counts', counts)

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• It is possible to create a list of keys/values

>>> A = { 'chuck' : 1 , 'fred' : 42, 'jan': 100}

>>> B = list(A.items())

>>> C = list(A.keys())

>>> D = list(A.values())

>>> B = tuple(A.items())

>>> C = tuple(A.keys())

>>> D = tuple(A.values())

>>> B = set(A.items())

>>> C = set(A.keys())

>>> D = set(A.values())

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It is possible to create a dictionary using two lists.

>>> A = ['TajMahal', 'Effiel Tower', 'Statue of Liberty']

>>> B = ['India', ‘France', 'America']

>>> B = zip(A, B)

>>> B

<zip object at 0x104e1d588>

>>> C = list(B)

>>> C

[('TajMahal', 'India'), ('Effil Tower', ‘France'), ('Statue of Liberty', 'America')]

>>> D = dict(C)

>>> D

{'TajMahal': 'India', 'Effil Tower': ‘France', 'Statue of Liberty': 'America'} Or

>>> dict(zip(A, B))

But, What happens when one of the list has one extra elements The extra element will be discarded.

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>>> dict(zip(list1, list2))

>>> dict(zip(srt1, str2))

>>> dict(zip(tup1, tup2))

>>> dict(zip(set1, set2))

>>> dict(zip(dict1, dict2))

Programs

• Write a Python script to sort (ascending and descending) a dictionary by value.

• Write a Python script to concatenate three dictionaries to create a new one.

• Write a Python script to merge two Python dictionaries.

• Write a Python script to create a dictionary where the keys are numbers between 1 and 15 (both included) and the values are square of keys.

• Write a Python program to map two lists into a dictionary.

• Write a Python program to remove duplicates values from Dictionary.

• Write a python program to maintain information about each student of Bsc-II.

• Write a Python program to replace dictionary values with their sum.

Programs

• Write a Python program to combine two dictionary adding values for common keys.

d1 = {'a': 100, 'b': 200, 'c':300}

d2 = {'a': 300, 'b': 200, 'd':400}

Sample output: Counter({'a': 400, 'b': 400, 'd': 400, 'c': 300})

• Write a Python program to create a dictionary from a string.

Sample string : 'w3resource'

Expected output: {'3': 1, 's': 1, 'r': 2, 'u': 1, 'w': 1, 'c': 1, 'e': 2, 'o':

1}

• Write a Python program to create a dictionary from two lists without losing duplicate values.

• Write a Python program to replace dictionary values (integers) with their sum.

Thank You