Dictionaries
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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):5This 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.
Next: Using Modules Up: Non-Programmers Tutorial For Python Previous: Boolean Expressions Contents Josh Cogliati jjc@honors.montana.edu Wikibooks Version: Wikibooks Non-programmers Python Tutorial Contents