Keywords: easytut

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# 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:

    def print_menu():
        print '1. Print Phone Numbers'
        print '2. Add a Phone Number'
        print '3. Remove a Phone Number'
        print '4. Lookup a Phone Number'
        print '5. Quit'
        print
    numbers = {}
    menu_choice = 0
    print_menu()
    while menu_choice != 5:
        menu_choice = input("Type in a number (1-5):")
        if menu_choice == 1:
            print "Telephone Numbers:"
            for x in numbers.keys():
                print "Name: ",x," \tNumber: ",numbers[x]
            print
        elif menu_choice == 2:
            print "Add Name and Number"
            name = raw_input("Name:")
            phone = raw_input("Number:")
            numbers[name] = phone
        elif menu_choice == 3:
            print "Remove Name and Number"
            name = raw_input("Name:")
            if numbers.has_key(name):
                del numbers[name]
            else:
                print name," was not found"
        elif menu_choice == 4:
            print "Lookup Number"
            name = raw_input("Name:")
            if numbers.has_key(name):
                print "The number is",numbers[name]
            else:
                print name," was not found"
        elif menu_choice != 5:
            print_menu()

And here is my output:

    1. Print Phone Numbers
    2. Add a Phone Number
    3. Remove a Phone Number
    4. Lookup a Phone Number
    5. Quit

    Type in a number (1-5):2
    Add Name and Number
    Name:Joe
    Number:545-4464
    Type in a number (1-5):2
    Add Name and Number
    Name:Jill
    Number:979-4654
    Type in a number (1-5):2
    Add Name and Number
    Name:Fred
    Number:132-9874
    Type in a number (1-5):1
    Telephone Numbers:
    Name:  Jill     Number:  979-4654
    Name:  Joe      Number:  545-4464
    Name:  Fred     Number:  132-9874

    Type in a number (1-5):4
    Lookup Number
    Name:Joe
    The number is 545-4464
    Type in a number (1-5):3
    Remove Name and Number
    Name:Fred
    Type in a number (1-5):1
    Telephone Numbers:
    Name:  Jill     Number:  979-4654
    Name:  Joe      Number:  545-4464

    Type in a number (1-5):5

This program is similar to the name list earlier in the the chapter on lists. Heres 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 line does is tell Python that `numbers` is a dictionary. The next few lines just make the menu work. The lines:

    for x in numbers.keys():
        print "Name: ",x," \tNumber: ",numbers[x]

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:

    if numbers.has_key(name):
        del numbers[name]

see if a name is in the dictionary and remove it if it is. The function `numbers.has_key(name)` returns true if `name` is in `numbers` but other wise returns false. The line `del numbers[name]` removes the key `name` and the value associated with that key. The lines:

    if numbers.has_key(name):
        print "The number is",numbers[name]

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:

    max_points = [25,25,50,25,100]
    assignments = ['hw ch 1','hw ch 2','quiz   ','hw ch 3','test']
    students = {'#Max':max_points}

    def print_menu():
        print "1. Add student"
        print "2. Remove student"
        print "3. Print grades"
        print "4. Record grade"
        print "5. Print Menu"
        print "6. Exit"

    def print_all_grades():
            print '\t',
            for i in range(len(assignments)):
                print assignments[i],'\t',
            print
            keys = students.keys()
            keys.sort()
            for x in keys:
                print x,'\t',
                grades = students[x]
                print_grades(grades)

    def print_grades(grades):
        for i in range(len(grades)):
            print grades[i],'\t\t',
        print

    print_menu()
    menu_choice = 0
    while menu_choice != 6:
        print
        menu_choice = input("Menu Choice (1-6):")
        if menu_choice == 1:
            name = raw_input("Student to add:")
            students[name] = [0]*len(max_points)
        elif menu_choice == 2:
            name = raw_input("Student to remove:")
            if students.has_key(name):
                del students[name]
            else:
                print "Student: ",name," not found"
        elif menu_choice == 3:
            print_all_grades()

        elif menu_choice == 4:
            print "Record Grade"
            name = raw_input("Student:")
            if students.has_key(name):
                grades = students[name]
                print "Type in the number of the grade to record"
                print "Type a 0 (zero) to exit"
                for i in range(len(assignments)):
                    print i+1,' ',assignments[i],'\t',
                print
                print_grades(grades)
                which = 1234
                while which != -1:
                    which = input("Change which Grade:")
                    which = which-1
                    if 0 <= which < len(grades):
                        grade = input("Grade:")
                        grades[which] = grade
                    elif which != -1:
                        print "Invalid Grade Number"
            else:
                print "Student not found"
        elif menu_choice != 6:
            print_menu()

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
    #Max    25              25              50              25              100

    Menu Choice (1-6):6
    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
    #Max    25              25              50              25              100
    Bill    25              24              45              23              95

    Menu Choice (1-6):6

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 thats 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:

    def print_all_grades():
            print '\t',
            for i in range(len(assignments)):
                print assignments[i],'\t',
            print
            keys = students.keys()
            keys.sort()
            for x in keys:
                print x,'\t',
                grades = students[x]
                print_grades(grades)

Notice how first the keys are gotten out of the `students` dictionary with the `keys` function in the line `keys = students.keys()` . `keys` is a list so all the functions for lists can be used on it. Next the keys are sorted in the line `keys.sort()` since it is a list. `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 array 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 a easy way to link keys to values. This can be used to easily keep track of data that is attached to various keys.

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