Principles of Programming: Java Level 1 by Jonathan Frank

Author:Jonathan Frank
Language: eng
Format: azw3, epub
ISBN: 9781514430378
Publisher: Xlibris US
Published: 2016-01-16T16:00:00+00:00

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The Boolean expression, A OR B, has two input values: the values of Boolean expressions A and B. A OR B only yields false if BOTH A AND B are false. Otherwise, it yields true.

Checking to see whether a user’s input matches at least one of several choices is a common use of the logical OR operator. The program checks whether the user’s integer is evenly divisible by two or three, or whether it is divisible by neither. If X is evenly divisible by Y, then when X is divided by Y, the remainder is zero. Also, remember from Lesson 5: an expression, such as i % 2, which yields a result of type int, can be used anywhere that a literal value or the value of a variable, of type int, can be used. The Boolean expression, i % 2 == 0 || i % 3 == 0, gets evaluated.

Let’s try this with the integer, 8. Substitute 8 for i, in the Boolean expression, to make 8 % 2 == 0 || 8 % 3 == 0. The result of 8 mod 2 is equal to 0, so 8 % 2 == 0 yields true. In order for A || B || … to yield false, ALL of its input values must be false. If ANY of them are true, the expression will yield true. Therefore, there is no need to evaluate 8 % 3 == 0 here. The logical OR operator uses lazy evaluation, just like logical AND. If the user enters the number 8, the message, 8 is divisible by 2 or 3., will be printed.

Let’s try this with the integer, 5. Substitute 5 for i, in the Boolean expression, to make 5 % 2 == 0 || 5 % 3 == 0. The result of 5 mod 2 is NOT equal to 0, so 5 % 2 == 0 yields false. The result of 5 mod 3 is NOT equal to 0, so 5 % 3 == 0 also yields false. Substitute false for 5 % 2 == 0 and false for 5 % 3 == 0 to make false || false, which yields false. Since the first Boolean expression, 5 % 2 == 0, yielded false, the second one, 5 % 3 == 0, gets evaluated. This is because it’s still possible for the A || B || … expression to yield false, since a value of true has not been encountered yet (when only 5 % 2 == 0 has been evaluated). If the user enters the number 5, the message, 5 is not divisible by 2 or 3., will be printed.

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