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 Numbername = raw_input("Name:)
        phone = raw_input("Number:)
        numbers[name] = phone
    elif menu_choice == 3:
        print "Remove Name and Numbername = raw_input("Name:)
        if numbers.has_key(name):
            del numbers[name]
        else:
            print name,was not foundelif menu_choice == 4:
        print "Lookup Numbername = raw_input("Name:)
        if numbers.has_key(name):
            print "The number is,numbers[name]
        else:
            print name,was not foundelif menu_choice != 5:
        print_menu()

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 studentprint "2. Remove studentprint "3. Print gradesprint "4. Record gradeprint "5. Print Menuprint "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 foundelif menu_choice == 3: print_all_grades() elif menu_choice == 4: print "Record Gradename = raw_input("Student:) if students.has_key(name): grades = students[name] print "Type in the number of the grade to recordprint "Type a 0 (zero) to exitfor 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 Numberelse: print "Student not foundelif 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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