Non-Programmer's Tutorial for Python 3/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 = int(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 = input("Name: ")
phone = input("Number: ")
numbers[name] = phone
elif menu_choice == 3:
print("Remove Name and Number")
name = input("Name: ")
if name in numbers:
del numbers[name]
else:
print(name, "was not found")
elif menu_choice == 4:
print("Lookup Number")
name = input("Name: ")
if name in numbers:
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 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
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 name in numbers:
del numbers[name]
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
if name in numbers:
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', end=' ')
for i in range(len(assignments)):
print(assignments[i], '\t', end=' ')
print()
keys = list(students.keys())
keys.sort()
for x in keys:
print(x, '\t', end=' ')
grades = students[x]
print_grades(grades)
def print_grades(grades):
for i in range(len(grades)):
print(grades[i], '\t', end=' ')
print()
print_menu()
menu_choice = 0
while menu_choice != 6:
print()
menu_choice = int(input("Menu Choice (1-6): "))
if menu_choice == 1:
name = input("Student to add: ")
students[name] = [0] * len(max_points)
elif menu_choice == 2:
name = input("Student to remove: ")
if name in students:
del students[name]
else:
print("Student:", name, "not found")
elif menu_choice == 3:
print_all_grades()
elif menu_choice == 4:
print("Record Grade")
name = input("Student: ")
if name in students:
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', end=' ')
print()
print_grades(grades)
which = 1234
while which != -1:
which = int(input("Change which Grade: "))
which -= 1 #same as which = which - 1
if 0 <= which < len(grades):
grade = int(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): 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
#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',end=" ")
for i in range(len(assignments)):
print(assignments[i], '\t',end=" ")
print()
keys = list(students.keys())
keys.sort()
for x in keys:
print(x, '\t',end=' ')
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 = 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.