Non-Programmer's Tutorial for Python 2.6/Dictionaries

This chapter is about dictionaries. If you open a dictionary, you should notice every entry consists of two parts, a word and the word's definition. The word is the key to finding out what a word means, and what the word means is considered the value for that key. In Python, dictionaries have keys and values. Keys are used to find values. Here is an example of a dictionary in use:

And here is my output:

1. Print Dictionary 2. Add definition 3. Remove word 4. Lookup word 5. Quit Type in a number (1-5): 2 Add definition Word: Python Definition: A snake, a programming language, and a British comedy. Type in a number (1-5): 2 Add definition Word: Dictionary Definition: A book where words are defined. Type in a number (1-5): 1 Definitions: Python: A snake, a programming language, and a British comedy. Dictionary: A book where words are defined. Type in a number (1-5): 4 Lookup Word Word: Python The definition of Python is: A snake, a programming language, and a British comedy. Type in a number (1-5): 3 Remove Word Word: Dictionary Dictionary was removed. Type in a number (1-5): 1 Definitions: Python: A snake, a programming language, and a British comedy. Type in a number (1-5): 5

This program is similar to the name list from the earlier chapter on lists (note that lists use indexes and dictionaries don't). Here's how the program works:


 * First the function  is defined.   just prints a menu that is later used twice in the program.
 * Next comes the funny looking line .  All that line does is tell Python that   is a dictionary.
 * The next few lines just make the menu work.


 * This goes through the dictionary and prints all the information. The function  returns a list that is then used by the   loop.  The list returned by   is not in any particular order so if you want it in alphabetic order it must be sorted.  Similar to lists the statement   is used to access a specific member of the dictionary.  Of course in this case   is a string.
 * Next the line  adds a word and definition to the dictionary.  If   is already in the dictionary   replaces whatever was there before.


 * See if name is in words and remove it if it is. The expression   returns true if   is a key in   but otherwise returns false.  The line   removes the key   and the value associated with that key.


 * Check to see if words has a certain key and if it does prints out the definition 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:

1. Add student 2. Remove student 3. Print grades 4. Record grade 5. Print Menu 6. Exit Menu Choice (1-6): 3 hw ch 1        hw ch 2         quiz            hw ch 3         test Menu Choice (1-6): 5 1. Add student 2. Remove student 3. Print grades 4. Record grade 5. Print Menu 6. Exit Menu Choice (1-6): 1 Student to add: Bill Menu Choice (1-6): 4 Record Grade Student: Bill Type in the number of the grade to record Type a 0 (zero) to exit 1  hw ch 1     2   hw ch 2     3   quiz        4   hw ch 3     5   test 0              0               0               0               0  Change which Grade: 1 Grade: 25 Change which Grade: 2 Grade: 24 Change which Grade: 3 Grade: 45 Change which Grade: 4 Grade: 23 Change which Grade: 5 Grade: 95 Change which Grade: 0 Menu Choice (1-6): 3 hw ch 1        hw ch 2         quiz            hw ch 3         test Bill   25              24              45              23              95 Menu Choice (1-6): 6
 * 1) Max   25              25              50              25              100
 * 1) Max   25              25              50              25              100

Here's how the program works. Basically the variable  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  creates a new dictionary with the key { } and the value is set to be, since thats what   was when the assignment is made (I use the key   since   is sorted ahead of any alphabetic characters). Next  is defined. Next the  function is defined in the lines:

Notice how first the keys are gotten out of the  dictionary with the   function in the line. is a list so all the functions for lists can be used on it. Next the keys are sorted in the line  since it is a list. is used to go through all the keys. The grades are stored as a list inside the dictionary so the assignment  gives   the list that is stored at the key. The function  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  adds a student to the key of their name. The notation  just creates a list of 0's that is the same length as the   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  gets a reference to the grades of the student. A grade is then recorded in the line. You may notice that  is never put back into the students dictionary (as in no  ). The reason for the missing statement is that  is actually another name for   and so changing   changes.

Dictionaries provide a easy way to link keys to values. This can be used to easily keep track of data that is attached to various keys.