Python Programming/Decision Control

Python, like many other computer programming languages, uses Boolean logic for its decision control. That is, the Python interpreter compares one or more values in order to decide whether to execute a piece of code or not, given the proper syntax and instructions.

Decision control is then divided into two major categories, conditional and repetition. Conditional logic simply uses the keyword if and a Boolean expression to decide whether or not to execute a code block. Repetition builds on the conditional constructs by giving us a simple method in which to repeat a block of code while a Boolean expression evaluates to true.

Boolean Expressions

Here is a little example of boolean expressions (you don't have to type it in):

a = 6
b = 7
c = 42
print (1, a == 6)
print (2, a == 7)
print (3, a == 6 and b == 7)
print (4, a == 7 and b == 7)
print (5, not a == 7 and b == 7)
print (6, a == 7 or b == 7)
print (7, a == 7 or b == 6)
print (8, not (a == 7 and b == 6))
print (9, not a == 7 and b == 6)

With the output being:

1 True
2 False
3 True
4 False
5 True
6 True
7 False
8 True
9 False

What is going on? The program consists of a bunch of funny looking print statements. Each print statement prints a number and an expression. The number is to help keep track of which statement I am dealing with. Notice how each expression ends up being either True or False; these are built-in Python values. The lines:

print (1, a == 6)
print (2, a == 7)

print out True and False respectively, just as expected, since the first is true and the second is false. The third print, print (3, a == 6 and b == 7), is a little different. The operator and means if both the statement before and the statement after are true then the whole expression is true otherwise the whole expression is false. The next line, print (4, a == 7 and b == 7), shows how if part of an and expression is false, the whole thing is false. The behavior of and can be summarized as follows:

expression	result
true and true	true
true and false	false
false and true	false
false and false	false

Note that if the first expression is false Python does not check the second expression since it knows the whole expression is false.

The next line, print (5, not a == 7 and b == 7), uses the not operator. not just gives the opposite of the expression (The expression could be rewritten as print (5, a != 7 and b == 7)). Here's the table:

expression	result
not true	false
not false	true

The two following lines, print (6, a == 7 or b == 7) and print (7, a == 7 or b == 6), use the or operator. The or operator returns true if the first expression is true, or if the second expression is true or both are true. If neither are true it returns false. Here's the table:

expression	result
true or true	true
true or false	true
false or true	true
false or false	false

Note that if the first expression is true Python doesn't check the second expression since it knows the whole expression is true. This works since or is true if at least one of the expressions are true. The first part is true so the second part could be either false or true, but the whole expression is still true.

The next two lines, print (8, not (a == 7 and b == 6)) and print (9, not a == 7 and b == 6), show that parentheses can be used to group expressions and force one part to be evaluated first. Notice that the parentheses changed the expression from false to true. This occurred since the parentheses forced the not to apply to the whole expression instead of just the a == 7 portion.

Here is an example of using a boolean expression:

list = ["Life","The Universe","Everything","Jack","Jill","Life","Jill"]

# Make a copy of the list.
copy = list[:]
# Sort the copy
copy.sort()
prev = copy[0]
del copy[0]

count = 0

# Go through the list searching for a match
while count < len(copy) and copy[count] != prev:
    prev = copy[count]
    count = count + 1

# If a match was not found then count can't be < len
# since the while loop continues while count is < len
# and no match is found
if count < len(copy):
    print ("First Match:",prev)

See the Lists chapter for an explanation of the slice operator, [:], occurring in copy = list[:].

Here is the output:

First Match: Jill

This program works by continuing to check for a match while count < len(copy) and copy[count] != prev. When either count is greater than the last index of copy or a match has been found the and is no longer true so the loop exits. The if simply checks to make sure that the while exited because a match was found.

The other 'trick' of and is used in this example. If you look at the table for and notice that the third entry is "false and won't check". If count >= len(copy) (in other words count < len(copy) is false) then copy[count] is never looked at. This is because Python knows that if the first is false then they both can't be true. This is known as a short circuit and is useful if the second half of the and will cause an error if something is wrong. I used the first expression ( count < len(copy)) to check and see if count was a valid index for copy. (If you don't believe me remove the matches `Jill' and `Life', check that it still works and then reverse the order of count < len(copy) and copy[count] != prev to copy[count] != prev and count < len(copy).)

Boolean expressions can be used when you need to check two or more different things at once.

Examples

password1.py

## This programs asks a user for a name and a password.
# It then checks them to make sure that the user is allowed in.
# Note that this is a simple and insecure example,
# real password code should never be implemented this way.

name = raw_input("What is your name? ")
password = raw_input("What is the password? ")
if name == "Josh" and password == "Friday":
    print ("Welcome Josh")
elif name == "Fred" and password == "Rock":
    print ("Welcome Fred")
else:
    print ("I don't know you.")

Sample runs

What is your name? Josh
What is the password? Friday
Welcome Josh

What is your name? Bill
What is the password? Saturday
I don't know you.

Exercises

1. Write a program that has a user guess your name, but they only get 3 chances to do so until the program quits.

Solutions