Difference between revisions of "Python-3.4.3/C3/Dictionaries/English"

Objectives

• Create dictionaries
• Add and delete data from dictionaries
• Retrieve data from dictionaries
• Check for presence of keys and
• Iterate over elements.

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3 and
• IPython 5.1.0

Pre-requisite

• use basic data types and operators

• If not, see the relevant Python tutorials on this website.

Overview of dictionaries

Dictionary is an unordered collection of items which has key:value pairs.

Dictionary is used to look up for a specific key and retrieve the corresponding value.

Overview of dictionaries

The values of a dictionary can be of any data type.

But the keys must be of immutable data type such as strings, numbers or tuples.

Open the terminal.

From here onwards, remember to press the Enter key after typing every command on the terminal.

Highlight curly braces

Type, empty is equal to open and close curly braces

Note that unlike lists, curly braces are used to define a dictionary.

'age': 16,

'gender': 'male',

'class': 10}

Type as shown.

• Each key:value pair is separated by a comma,
• Each key is separated from its value by a colon, and
• The whole thing is enclosed in curly braces.

Here, we have defined four entries in the dictionary student.

The keys are name, age, gender, class.

Accessing elements

dictionary_name[key]

The value of a dictionary can be accessed using the corresponding key.

The syntax is:

dictionary underscore name inside square brackets key

Type,

print inside brackets student inside square brackets inside single quotes name

As expected it printed the output as raj.

Type,

print inside brackets student inside square brackets inside single quotes class

It displays the output as 10.

Highlight KeyError

Type, student inside square brackets inside single quotes height

The interpreter gave us the KeyError: 'height'

This is because, key 'height' is not present in the student dictionary.

This is about creating dictionaries.

<<PAUSE>>

First let us add an element height to the dictionary student.

Type, student inside square brackets inside single quotes height is equal to 6.2

Highlight 'height': 6.2

Type, student

You can see that key-value 'height': 6.2 is added to the dictionary student.

Type,

student inside square brackets inside single quotes class is equal to 11

Highlight class

As you can see, the value for class key is changed to 11 .

Type,

del student inside square brackets inside single quotes age

You can see that now key-value 'age': 16 is deleted from student.

<<PAUSE>>

For that we can use the method in.

It will return False if key is not present.

Note that we can check only for the presence of the keys in dictionaries and not values.

Keys of a dictionary are not ordered.

Hence, slicing and striding are not valid on dictionaries.

Type, inside single quotes school in student

We get False, since the key 'school' is not present in student.

We get True, as expected.

Retrieve keys and values

• Method keys() is used for getting a list of keys
• Method values() is used for getting a list of values.

Highlight the output

Type, student dot keys open and close brackets

It returns the list of keys of the dictionary student.

Highlight the output

student dot values open and close brackets

It returns the list of values of the dictionary student.

Highlight the output

Type,

student dot items open and close brackets

It returns a list of tuples of key-value pairs of the dictionary student.

The order of items of the list returned by keys(), values() and items() cannot be predicted.

In dictionaries, they are not in the order in which we inserted them.

Try this exercise and then resume the video.

Exercise 1

Hint: use the method items and for loop.

for key, val in student.items():

print (key, val)

It displays the keys and values one by one.

Summary

In this tutorial, we have learnt to,

• Create dictionaries
• Access elements of a dictionary using keys
• Add elements to a dictionary by assigning a value to a key

Summary

• Delete elements from a dictionary by using the function del
• Retrieve the keys and values by using the methods keys() and values() respectively and
• Iterate over elements of a dictionary using a for loop.

Self assessment questions

1. Dictionary d is defined as shown:

How do you retrieve the value 'b'?

1. Delete value 'b' from the dictionary d

Solution of self assessment questions

1. d inside square brackets 2 gives the value 'b'
2. del d inside square brackets 2 deletes the value ‘b’