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  • A standard data structure in computer science is theassociative arraywhich is also

  • called a “map.” In Python, this structure is called a “dictionary.” Dictionaries

  • are used when you have key/value pairs of data - an input which is mapped to an output.

  • If we had a dictionary that stored the quality of YouTube videos, then the value for this

  • lesson would be 17.8...

  • Suppose we are creating a new social network calledFriendfacewhere any life form

  • or intelligent agent is allowed to join.

  • Let us make our first post to Friendface... We will use this post to show you how to create

  • a dictionary.

  • To begin, we will collect several pieces of data for each post:

  • The user id... message... language... datetimeand the location from whence the post was

  • made.

  • Using dictionaries, we can store all of this data in a single object.

  • To create a dictionary, use the left brace, then list the key name, a colon, and the value.

  • If you have more than one key/value pair, you separate them using a comma.

  • Once you have finished entering your data, use a right brace to finish the dictionary.

  • We have created a dictionary calledpostwith 5 pieces of data. If you think of a dictionary

  • as a map, there are 5 inputs, and 5 outputs. In Python, inputs are called keys, and outputs

  • are called values.

  • Notice how the values have a variety of data types in this dictionary: an integer, three

  • strings... and a tuple of floats. Python is flexible: you can use all kinds of data types

  • for both keys and values. You are free to mix and match data to suit your needs.

  • If you use thetypefunction, you will seepostis an instance of thedict

  • class. This suggests we can use thedictconstructor to create a dictionary, and this

  • suggestion is correct. Reminder - we are using Python version 3 in this video.

  • Let’s create another Friendface post using thedictconstructor. This time, we

  • will only provide a few pieces of dataYou can see this made a dictionary.

  • We add additional pieces of data by using brackets.

  • The key name goes inside the brackets, and you use the equals sign to assign the value.

  • Notice that in the constructor, you did not have to put quotes around the key name, but

  • when you add new data using brackets, you DO use quotes.

  • If you print the dictionary, you will see all the data is there inside braces.

  • Just as you use brackets to store new data in a dictionary, you use brackets to access

  • data as well. Example: to see the message from the first post, use the key namemessage

  • inside brackets.

  • But do not be careless. Let’s see what happens if you try to access data that is not in the

  • dictionary. When we createdpost2’, we did not assign a location. If we try to access

  • this value, we get a Key Error.

  • How do we avoid such catastrophes? Simple. One way is to use theinoperator to

  • first check if a key is in the dictionary.

  • Another way is to try and retrieve the value, but handle the possibility of a KeyError.

  • To do this, type thetrycommand followed by a colon.

  • Next, attempt to print the location. If the dictionary does not have a ‘location

  • key, it will raise a KeyError. We handle this case by creating anexcept

  • block. Typeexceptand the possible errors you would like to handle. Here, we

  • want to handle the KeyError. If this occurs, the following code block will execute.

  • It works...

  • There is another way to access data in a dictionary and handle the possibility it does not have

  • a certain key. First, display the directory for thepost2’ dictionary. Here we see

  • a list of methods available to us. We will explore thegetmethod. To see what

  • this does, use the help function.

  • Thegetmethod lets you try and get the value for a specific key. If the dictionary

  • does not contain data for that key, you can specify a default value. We will use this

  • method to get thelocationfrom post2. If this post does not have a location, let’s

  • return None. If you print the value, you see the method

  • did returnNone.’

  • Turn your attention back to the original post.

  • A common task is to iterate over all the key/value pairs in a dictionary.

  • A straightforward way to do this is to loop over all the keys, then get the value for

  • each key. Thekeysmethod gives us an object we can loop over that contains all

  • the keys for the dictionary.

  • If you are using Python version 2, the output may look different.

  • This is because we are using Python 3, and in version 3, the print function became more

  • powerful.

  • The order of the data may be different for you. Do not panic. Dictionaries are not ordered

  • data. As long as you see all the data, everything is under control.

  • Another way to iterate over all the key/value pairs in a dictionary is to use theitems

  • method. This will give you both the key and the value in each step of the iteration.

  • There are a variety of methods for removing data from a dictionary.

  • Thepopandpopitemmethods allow you to remove a single item from a dictionary,

  • while theclearmethod will remove all data. After you complete this video, be sure

  • to experiment with these methods.

  • Look at me… I am a dictionary

A standard data structure in computer science is theassociative arraywhich is also

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Python Dictionaries|Python Tutorial|Pythonプログラミングを学ぶ (Python Dictionaries || Python Tutorial || Learn Python Programming)

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    林宜悉 に公開 2021 年 01 月 14 日
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