4-5: Boolean Operators
Exercise 4-3: The Check It Button Event code (continued)
Turning now to the first block of the If statement; it is executed when the textbox does contain a string that can be converted to a date value. Having obtained the date value we must extract the year from that value and then determine if the year is a leap year. To do this we must first determine if the year is divisible (with remainder being zero) by 4 but not by 100. Essentially we want to see if there is any remainder when we divide the year by 4 or by 100. This introduces a new arithmetic operator - the Mod operator.
The Mod
operator divides its first operand by the second and creates a result
that is the value of the remainder. Thus 12 mod 5 has
the result 2, for example. If the result from the mod operation is 0 then the
first operand is divisible (without remainder) by the second. For example, 16 mod 8 has the result 0.
In this exercise we need to write the boolean expressions
Year Mod 4 = 0
meaning that
the value of year is divisible by 4, and
Year Mod 100 <> 0
meaning that the value of
year is not divisible by 100, and combine these with a boolean operator. But
which operator should it be?
If this first expression is false (i.e. divisible by 4 but not by 100 is false) it is still possible for the year to be a leap year if it is divisible by 400. There are a number of ways in which this logic maybe implemented.