Returning a boolean value

Suppose you want to complete a function:

def isBetween(val, end1, end2):
   ''' True if the number val is between
   the numbers end1 and end2,
   allowing equality on either end.
   False otherwise.'''

We want to test if end1 <= val <= end2. And since the order of end1 and end2 were not specified, it might be that end2 <= val <= end1

A naive, newbie way to do this is to think of it as a decision and use an if statement:

if end1 <= val <= end2 or end2 <= val <= end1:
    return True
else:
    return False

This certainly works, but we have a boolean test condition AND we are also returning a boolean value. Compare:

If the condition is True, the result is True.

If the condition is False, the result is False.

In other words, we are just returning the condition value! There is a much shorter way to do that than with an if statement:

def isBetween(val, end1, end2):
   ''' True if the number val is between
   the numbers end1 and end2,
   allowing equality on either end.
   False otherwise.'''

   return end1 <= val <= end2 or end2 <= val <= end1

If we only wanted to do such a calculation once, we might eliminate the separate function, and assign directly to a boolean variable, say between. Again this works:

if end1 <= val <= end2 or end2 <= val <= end1:
    between = True
else:
    between = False

And again that is verbose. Instead:

between = end1 <= val <= end2 or end2 <= val <= end1

Though not required (because of the low precedence of =), you might find this easier for humans to read with parentheses:

between = (end1 <= val <= end2 or end2 <= val <= end1)

A variation also works if you want the opposite boolean result, by using not:

outside = not (end1 <= val <= end2 or end2 <= val <= end1)

So remember: If you want to generate a boolean value, it is rarely efficient or concise to involve an ``if`` statement.

Exercise: Write efficient code to set the variable negVal to True only when the number x is negative, and set it to False otherwise.