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Introducing Dictionaries

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Accessing Keys and Values

4:07 with Ashley Boucher

Discover how to access data from a Python dictionary

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Lexicographical ordering is very similar to alphabetical ordering, but it considers additional characters besides the letters of the alphabet.

Some of the basic rules in Python are that:

1) Uppercase letters come earlier than lowercase letters. This means that A < Z < a < z.

2) Numbers come earlier than letters. This means that 0 < 9 < A < a.

3) Space characters come before all printable characters.

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    In this video, we're gonna talk about how to access keys and 0:00
    values from inside a Python dictionary. 0:03
    Do you remember how I mentioned previously that unlike sequences, 0:05
    which are indexed by numbers, dictionaries are indexed by their keys? 0:09
    We can use that difference to transfer what we know about accessing values in 0:13
    sequences, to accessing values in dictionaries. 0:17
    You see here in my workspace, I have a simple Python list. 0:19
    Without any context, the data stored in this list doesn't make a lot of sense, 0:22
    which is why soon, we're gonna convert it to a dictionary. 0:26
    But, for right now, let's refresh how we get values out of this list. 0:29
    If I want the first element in the list, I'll use the code course, 0:33
    then square brackets, and then the index of the value I want to grab, 0:38
    in this case, let's say zero. 0:43
    The same syntax is used when pulling values from dictionaries. 0:45
    Except instead of putting a numerical index inside the square brackets, 0:48
    we put the value's key. 0:52
    So first let's back up and convert this list to a dictionary. 0:54
    To do that, I'll first change the square brackets here to curly braces. 0:57
    Then for each value in this I'll add a key, which is kinda like a label 1:04
    Now to access that first value again, 1:24
    I'll just change this code here to reflect the correct index. 1:26
    And now I'll wrap that in a print statement, and then I'll save and 1:32
    run the file to show the output. 1:35
    Let's save. 1:40
    Yep, it printed out Ashley. 1:51
    So that the basics of accessing an individual value from a dictionary. 1:53
    It's very similar to accessing an individual value from a sequence. 1:57
    It's important to know that if you attempt to access a value with a non existing key, 2:01
    Python won't just skip over it, you will get an error. 2:05
    Now, Python also has ways to grab a list of all the keys or 2:08
    all the values from a dictionary. 2:11
    And even a built-in function to sort those lists into lexicographical order. 2:14
    You might have learned about lexicographical ordering before, but 2:18
    check the teacher's notes for more info. 2:21
    So let's move down to the terminal where we can work a little more efficiently with 2:22
    our dictionary. 2:26
    I'm gonna start the interpreter and copy the dictionary down there. 2:26
    So, first, to get a list of all the keys in the dictionary, 2:38
    I'll use the keys method. 2:41
    To use that I type the name of the dictionary which is course then dot and 2:43
    then keys, the name of the method, and then parens, 2:47
    just like a regular function call. 2:50
    When I hit Enter, it will show the return value from the keys method. 2:52
    Yep, here's a list of every key, teacher, title and level. 2:58
    Similarly, we can access a list of every value in the dictionary using the values 3:02
    method. 3:07
    That's course.values(). 3:07
    And yes, that returned the list of every value, Ashley, 3:12
    Introducing Dictionaries and Beginner. 3:15
    Both of these methods returned just plain lists that can be iterated over using 3:17
    a for loop. 3:21
    Now, if for some reason we needed these lists sorted, 3:22
    we could wrap them in a call to the built in python function, sorted. 3:25
    Which will return a copy of the past list back in lexicographical order, 3:28
    which in this case, just means it will be sorted alphabetically. 3:32
    Let's clear this and take a look. 3:36
    That returned a list of the alphabetized keys. 3:47
    And then to do it again for values. 3:50
    There we go. 3:59
    Okay, now that we covered accessing keys and values, 4:00
    join me in the next video to learn about updating and mutating dictionaries. 4:03

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