2. Logical Operators¶
There are three logical operators: and, or, and not. The semantics (meaning) of these operators is similar to their meaning in English. For example, x > 0 and x < 10 is true only if x is greater than 0 and, at the same time, x is less than 10. How else might you describe this? You could say that x is between 0 and 10, not including the endpoints.
n % 2 == 0 or n % 3 == 0 is true if either of the conditions is true, that is, if the number is divisible by 2 or the number is divisible by 3. In this case, one, or the other, or both of the parts has to be true for the result to be true.
Finally, the not operator negates a boolean expression, so not i > j is true if i > j is false, that is, the statement will evaluate to true if i is less than or equal to j.
Common Mistake!
There is a very common mistake that occurs when programmers try to write boolean expressions. For example, what if we have a variable num and we want to check if its value is 5, 6, or 7. In words we might say: “num equal to 5 or 6 or 7”. However, if we translate this into Python as num == 5 or 6 or 7, it will not be correct. The or operator must join the results of three equality checks. The correct way to write this is num == 5 or num == 6 or num == 7. This may seem like a lot of typing but it is absolutely necessary. You cannot take a shortcut.
An exception is the case of chaining comparison operators. For example, in Python it is permissible to write x < y < z which means the same as its mathematical expression and is functionally equivalent to the Python expression x < y and y < z.
Truth tables can be very helpful to us in determining the boolean value of an expression that uses a logical operator. Here is an example of a truth table that looks at two statements, p and q, that are boolean expressions. It tells us which result, True or False, we will get based on whether the boolean value of each statement is True or False:
p |
q |
p and q |
|---|---|---|
| True | True | True |
| True | False | False |
| False | True | False |
| False | False | False |
You can see from this that both statements must be true in order for the expression p and q to evaluate to True. Similarly, we can do a truth table for the expression p or q and it will show us that only either p or q must be true for the whole expression to evaluate to True:
p |
q |
p or q |
|---|---|---|
| True | True | True |
| True | False | True |
| False | True | True |
| False | False | False |
Finally, we can make a truth table that will show us the value of the expression not p given the boolean value of the statement p:
p |
not p |
|---|---|
| True | False |
| False | True |
Check your understanding
-
What is the correct Python expression for checking to see if a number stored in a variable x is between 0 and 5.
- x > 0 and < 5
- Each comparison must be between exactly two values. In this case the right-hand expression < 5 lacks a value on its left.
- x > 0 or x < 5
- Although this is legal Python syntax, the expression is incorrect. It will evaluate to true for all numbers that are either greater than 0 or less than 5. Because all numbers are either greater than 0 or less than 5, this expression will always be True.
- x > 0 and x < 5
- Yes, with an and keyword both expressions must be true so the number must be greater than 0 an less than 5 for this expression to be true.