# Learning Python 3 with the Linkbot/Dictionaries

This chapter is about dictionaries. Dictionaries have keys and values. The keys are used to find the values. Here is an example of a dictionary in use:
And here is my output:
This program is similar to the name list earlier in the chapter on lists. Here's how the program works. First the function print_menu is defined. print_menu just prints a menu that is later used twice in the program. Next comes the funny looking line numbers =. All that this line does is to tell Python that numbers is a dictionary. The next few lines just make the menu work. The lines
go through the dictionary and print all the information. The function numbers.keys returns a list that is then used by the for loop. The list returned by keys is not in any particular order so if you want it in alphabetic order it must be sorted. Similar to lists the statement numbers[x] is used to access a specific member of the dictionary. Of course in this case x is a string. Next the line numbers[name] = phone adds a name and phone number to the dictionary. If name had already been in the dictionary phone would replace whatever was there before. Next the lines
see if a name is in the dictionary and remove it if it is. The operator name in numbers returns true if name is in numbers but otherwise returns false. The line del numbers[name] removes the key name and the value associated with that key. The lines
check to see if the dictionary has a certain key and if it does prints out the number associated with it. Lastly if the menu choice is invalid it reprints the menu for your viewing pleasure.
A recap: Dictionaries have keys and values. Keys can be strings or numbers. Keys point to values. Values can be any type of variable (including lists or even dictionaries (those dictionaries or lists of course can contain dictionaries or lists themselves (scary right? :-) ))). Here is an example of using a list in a dictionary:
and here is a sample output:
Heres how the program works. Basically the variable students is a dictionary with the keys being the name of the students and the values being their grades. The first two lines just create two lists. The next line students = {'#Max': max_points} creates a new dictionary with the key {#Max} and the value is set to be [25, 25, 50, 25, 100] (since that's what max_points was when the assignment is made) (I use the key #Max since # is sorted ahead of any alphabetic characters). Next print_menu is defined. Next the print_all_grades function is defined in the lines:
Notice how first the keys are gotten out of the students dictionary with the keys function in the line keys = list(students.keys). keys is an iterable, and it is converted to list so all the functions for lists can be used on it. Next the keys are sorted in the line keys.sort. for is used to go through all the keys. The grades are stored as a list inside the dictionary so the assignment grades = students[x] gives grades the list that is stored at the key x. The function print_grades just prints a list and is defined a few lines later.
The later lines of the program implement the various options of the menu. The line students[name] = [0] * len(max_points) adds a student to the key of their name. The notation [0] * len(max_points) just creates a list of 0's that is the same length as the max_points list.
The remove student entry just deletes a student similar to the telephone book example. The record grades choice is a little more complex. The grades are retrieved in the line grades = students[name] gets a reference to the grades of the student name. A grade is then recorded in the line grades[which] = grade. You may notice that grades is never put back into the students dictionary (as in no students[name] = grades). The reason for the missing statement is that grades is actually another name for students[name] and so changing grades changes student[name].
Dictionaries provide an easy way to link keys to values. This can be used to easily keep track of data that is attached to various keys.

## Using a Dictionary to Store Musical Notes

Up to this point, our Linkbot programs have been playing musical notes on the Linkbot either by specifying a specific frequency to play, such as myLinkbot.setBuzzerFrequency(440) , or using an equation to calculate the frequency based on a keyboard key number. Wouldn't it be nice if we could just tell the Linkbot to play notes like "Do-Re-Mi", or "A-B-C"?
Perhaps we can use a dictionary to accomplish this task. We can use a dictionary that uses the strings "Do", "Re", "Mi", etc. as keys, and their actual frequencies as values. To find the actual values, we can use an equation or simply look them up online. A quick search shows these frequencies:
| Note Name | Note Frequency |
| Do | 261.63 |
| Re | 293.66 |
| Mi | 329.63 |
| Fa | 349.23 |
| Sol | 392.00 |
| La | 440 |
| Ti | 493.88 |
Now we can create and use a dictionary to play notes.
* Here, we define a helper function that helps us play notes. The function takes a linkbot object and a string note value such as "Do" as input arguments and plays that note on the linkbot for 0.5 seconds.
* This is where we create the dictionary that relates note names to note frequencies.
* This list defines the notes in a simple song we want to play.
* This loop takes each note in the list of notes that we constructed earlier and plays it using our playNote function.