CLASS -11 COMPUTER SCIENCE
CHAPTER- 9 DICTIONARIES
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NOTES FOR COMPLETE UNDERSTANDING -
- Dictionary too is a collection of data like a string, list and tuple.
- It is a very versatile data type.
- It has key : value pair in it.
- Dictionaries are mutable data types and it is an unordered collection in the form of key : value .
- In Tuple and List, index of a value is used for indexing whereas in dictionary, key of a value is used.
- To create any dictionary, it is required to collect pairs of key:value in “{ }” .
Example: teachers={“Rashi”:”Mathematics”, “Arti”:”Physics” , ”Priyanka”:”Chemistry" , ”Sakshi”:”CS”}
Some examples of Dictionary are : Dict1= { } # this is an empty dictionary without any element.
NoOfDaysInMonth= { January”:31, ”February”:28, ”March”:31, ”April”:30, ”May”:31, ”June”:30, ”July”:31, ”August”:31, ”September”:30, ”October”:31, ”November”:30, ”December”:31}
The thing to note is that keys should always be of immutable type.
Note: Dictionary is also referred as associative array or mapping or hashes .
Accessing a Dictionary : To access a value from dictionary, we use key of it's key:value pair same as we use an index to access a value from a list or tuple. We get the key from the pair of Key: value. If we input following statement from above example-
- >>>print(teachers['Rashi']) We have selected key “Rashi” and on printing it, Mathmatics got printed.
Traversal of a Dictionary : For traversing a Dictionary, we use for loop. Example : >>>for k in d: print(d[k])
To access key and value we need to use keys( ) and values( ) function : d.keys( ) function will display only key. d.values ( ) function will display values of the dictionary only .
Features of Dictionary :
1. Unordered set: Dictionary is collection of key:value pairs in unordered manner .
2. Not a sequence: Unlike list, string and tuple , it is a collection of unordered elements whereas a sequence is a collection of indexed numbers to keep them in order.
3. Key of any key:value pair is used for indexing because according to Python key can only be of immutable type. Strings & numbers are immutable type of data and therefore can be used as a key. Key of a Dictionary should always be of immutable type like integer or string or tuple whereas value of a dictionary can be of both mutable and immutable type.
4. Keys should be unique : Because keys are used to identify values just like indices are used , so they should be unique.
5. Values of two unique keys can be same but this is not allowed for values.
6. Dictionary is mutable hence we can update value of any key.
7. Internally it is stored as a mapping. Its key : value are related to each other by an internal function called hash function. Such procedure of linking is knows as mapping. **Hash-function is a type of internal algorithm to link a variable and its value.
Working with Dictionary:
Dictionary initialization- For initialisation of dictionary we keep collection of pairs of key : value separated by comma (,) and then place this collection inside “{ }”.
Adding key:value pair to an empty dictionary. There exists 2 ways to create an empty dictionary : 1. Employee = { } 2. Employee = dict( ) 3. Creating a Dictionary with the pair of name and value: dict( ) constructor of python is used to create dictionary with the pairs of key and value. There are various methods for this-- I. By passing Key:value pair as an argument. II. By specifying Comma-separated key : value pair .
Updation in a Dictionary following syntax is used to update an element in >>>Dictionary [key ]=new value
Deleting an element from a Dictionary follows two syntaxes For deletion, key should be there otherwise python will give error. 1. del [ ]- it deletes the value and does not return deleted value. 2. .pop( ) it returns the deleted value after deletion.
Detection of an element from a Dictionary : Membership operator is used to detect presence of an element in a Dictionary. 1. in : It gives true on finding the key otherwise gives false. 2. not in : I t gives true on not finding the key otherwise gives false. *** in and not in does not process on values, they can only work with keys.
Pretty Printing of a Dictionary : To print a Dictionary in a beautified manner, we can import json module. After this following syntax of dumps( ) function will be used. json.dumps(< >,indent= ' < < >' )
Dictionary Function and Method :
1. len( ) Method : it specifies the length of dictionary.
2. clear( ) Method : it deletes every key:value pairs of the dictionary.
3. get( ) Method : it outputs value of the given key.
4. items( ) Method : it returns all items of a dictionary in the form of tuple of (key : value).
5. keys( ) Method : it outputs list of dictionary keys.
6. values( ) Method : it outputs list of dictionary values.
7. Update ( ) Method: This function of python is used to merge the pair of key : value of a dictionary into other dictionary. Change and addition in this is possible as per need.
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