7.2. Logical operators¶
There are three logical operators:
semantics (meaning) of these operators is similar to their meaning in English.
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 would you describe this in words? You would 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 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.
not operator negates a boolean expression, so
not x > y
is true if
x > y is false, that is, if
x is less than or equal to
There is a very common mistake that occurs when programmers try to write boolean expressions. For example, what if we have a variable
number and we want to check to see if its value is 5,6, or 7. In words we might say: “number equal to 5 or 6 or 7”. However, if we translate this into Python,
number == 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
number == 5 or number == 6 or number == 7. This may seem like a lot of typing but it is absolutely necessary. You cannot take a shortcut.
Check your understanding
- 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. Although most other programming languages do not allow this mathematical syntax, in Python, you could also write 0 < x < 5.
select-2-1: What is a correct Python expression for checking to see if a number stored in a variable x is between 0 and 5?