Dictionary In Python

Jay Pithava

Dictionary



-  Dictionary is an unordered set of key and value pair.  
- It is an container that contains data, enclosed within curly braces. 
-  The key and the value is separated by a colon(:). This pair is known as item. 
-  Items are separated from each other by a comma(,).  
- Dictionary is mutable i.e., value can be updated. 
-  Key must be unique and immutable. Value is accessed by key. Value can be updated while key cannot be changed. 
- Dictionary is known as Associative array.

 Example: -
 stud={1:’Reena’, 2:’Seema’, 3:’Rahul’} 
     print stud 

Accessing Values:-
As index is not defined, a Dictionaries value can be accessed by their keys. 
Updation:-
New item can be added. The values can be changed.

Dictionary Operations:- 

- get()
- del() 
- cmp() 
- keys() 
- values() 
- items() 
- clear() 
- copy() 
- update() 
- has_key() 
- from_keys()

Dictionary: Operations: get(key):-

get(key):-
 Returns the value of the given key. If key is not present it returns none. 

Syntax:-
 dict.get(key, default = None)

The get() method takes maximum of two parameters:
key - key to be searched in the dictionary value (optional) - Value to be returned if the key is not found. The default value is None.

Return Value from get() The get() method returns:-    
- The value for the specified key if key is in dictionary.     
- None if the key is not found and value is not specified.   
- value if the key is not found and value is specified.

Example:-
person = {'name': 'Ramesh', 'age': 22}

print('Name: ', person.get('name')) 
print('Age: ', person.get('age'))

# value is not provided 
print('Salary: ', person.get('salary'))

# value is provided 
print('Salary: ', person.get('salary', 0.0))

Name:  Ramesh 
Age:  22 
Salary:  None 
Salary:  0.0

Dictionary: Operations: del():-

- Deletion: An item can be deleted using del statement from a dictionary using the key. Whole dictionary can also be deleted using del statement.
Syntax:-
 del  [key]
Example:-
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
del dict['Name'];    # remove entry with key 'Name' 
dict.clear();           # remove all entries in 
dict del dict ;        # delete entire dictionary




Dictionary: Operations: cmp():-

- cmp():-
 compares two dictionaries based on key and values. 

Syntax:-
cmp(dict1, dict2)
 returns 
-  0 if both dictionaries are equal, 
-  -1 if dict1 < dict2 and  
-   1 if dict1 > dic2.

dict1 = {'Name': 'Zara', 'Age': 7};
dict2 = {'Name': 'Mahnaz', 'Age': 27}; 
dict3 = {'Name': 'Abid', 'Age': 27}; 
dict4 = {'Name': 'Zara', 'Age': 7}; 
print "Return Value : %d" %  cmp (dict1, dict2) # -1 
print "Return Value : %d" %  cmp (dict2, dict3) #1 
print "Return Value : %d" %  cmp (dict1, dict4 #0


Dictionary: Operations: keys()

keys():- Return all the keys element of a dictionary.  

Syntax:-
dict.keys()

Example:-
dict = {'Name': 'Zara', 'Age': 7} 
print "Value : %s" %  dict.keys()
Output:-
Value : ['Age', 'Name']

Dictionary: Operations: values()

values():- Return all the values element of a dictionary. 

Syntax:-
dict.values()
Example:-
dict = {'Name': 'Zara', 'Age': 7} 
print "Value : %s" %  dict.values()
Output:-
Value : ['Zara', 7]

Dictionary: Operations: items()

items():-Return all the items(key-value pair) of a dictionary.  

Syntax:-
dict.items()

Example:-
dict = {'Name': 'Zara', 'Age': 7} 
print "Value : %s" %  dict.values()
Output:-
Value : [('Age', 7), ('Name', 'Zara')]

Dictionary: Operations: clear()

clear():- It is used to remove all items of a dictionary. It returns an empty dictionary.  

Syntax:-
dict.clear()

Example:-
dict = {'Name': 'Zara', 'Age': 7}; 
print "Len : %d" %  len(dict) 
dict.clear() 
print “Len : %d" %  len(dict)

Output:-
Len : 2 Len : 0

Dictionary: Operations: copy()

copy():- It returns an ordered copy of the data.  

Syntax:-
dict.copy()
 
Example:-
dict1 = {'Name': 'Zara', 'Age': 7}; 
dict2 = dict1.copy() 
print "New Dictionary : %s" %  str(dict2)
Output:-
New Dictionary : {'Age': 7, 'Name': 'Zara'}


Dictionary: Operations: update()

update(dictionary2): It is used to add items of dictionary2 to first dictionary.  

Syntax:-
dict.update(dict2)
Example:-
dict = {'Name': 'Zara', 'Age': 7} 
dict2 = {'Sex': 'female' }
dict.update(dict2) 
print "Value : %s" %  dict
Output:-
Value : { 'Name': 'Zara', 'Age': 7, 'Sex': 'female'}


Dictionary: Operations: has_key()

 has_key(key):- It returns a boolean value. True in case if key is present in the dictionary ,else false.  
Syntax:-
dict.has_key(key)
 
Example:-
dict = {'Name': 'Zara', 'Age': 7} 
print "Value : %s" %  dict.has_key('Age') 
print "Value : %s" %  dict.has_key('Sex')
Output:-
Value : True Value : False

Dictionary: Operations: fromkeys()

 fromkeys(sequence,value1):- creates a new dictionary with keys from seq and values set to value.  

Syntax:-
dict.fromkeys(seq[, value])
 
seq − list of values which would be used for dictionary keys preparation.  
value − Optional, if provided then value would be set to this value.


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