boolean expressions

and, or, not and De Morgan's law

Boolean: True or False

• Boolean expressions are those that evaluate to either True or False
• Examples include:
  • 3 < 4, which evaluates to True
  • 1 + 2 == 7 which evaluates to False

Boolean Variables

Boolean values can be assigned to variables. For example:

x = 3 < 4

If we then examine type(x) we see that it is of type bool, as this Python shell session shows:

>>> x = 3 < 4
>>> x   
True
>>> type(x)
<class 'bool'>
>>> 

Combining booleans with and and or

If we combine two boolean expressions with and, they both must be true for the entire expression to be true. If either or both is False, then the whole expression is False

Examples:

• 3>8 and 5>8 evaluates to False because both 3>8 and 5>8 are False
• 3<5 and 5>8 evaluates to False because 5>8 is False
• 3>8 and 5<8 evaluates to False because 3>8 is False
• 3<5 and 5<8 evalutes to True because both sides of the and are True

If we combine two boolean expressions with or, they both must be true for the entire expression to be true:

Examples:

• 3>8 or 5>8 evaluates to False because both 3>8 and 5>8 are False
• 3<5 or 5>8 evaluates to True because 3<5 is True
• 3>8 or 5<8 evaluates to True because 5<8 is True
• 3<5 or 5<8 evalutes to True because both sides of the or are True

The six relational operators

There are six relational operators, each of which returns a boolean value:

The not operator

The not operator is placed in front of a boolean expression, and changes True to False and False to True

>>> x = 3 < 4
>>> x
True
>>> not x
False
>>> not 4 == 2 + 2
False
>>>     

Applying not to boolean expressions with a single relational operator

Suppose we have a boolean expression with a single relational operator. Placing a not in front of the expression is probably not the best way to change that expression to its opposite. Here we show six relational expressions with not in front of them, each followed by a more concise expression that means the same thing:

There may be isolated cases where the version with not might be easier for people reading your code to understand. However, in most cases, the more concise alternative without the not is preferred.

Note the opposite pairs:

• < and >= are opposites
• > and <= are opposites
• == and != are opposites

De Morgan’s laws

When applying not to an entire expression involving and or or, it is necessary to remember De Morgan’s laws.

These are two rules of logic that are credited to Augustus De Morgan, a 19th-century British mathematian.

The concise statement of the laws are these:

An even more concise way to say this: when “distributing” not inside the parentheses:

• and changes to or
• or changes to and

It may seem at first as if this transformation makes things more complicated. For example:

However, if we combine this with the rule that applying not to a relational expression simply changes the relational operator to it’s opposite, then we can simplify further:

For the homework and exams, you should be prepared to convert an expression involving not into one that does not use not by applying these rules.