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5.5. DeMorgan’s Laws

DeMorgan’s laws were developed by Augustus De Morgan in the 1800s. They show how to handle the negation of a complex conditional, which is a conditional statement with more than one condition joined by an and (&&) or or (||), such as (x < 3) && (y > 2).

• not (a and b) is the same as (not a) or (not b). In Java this is written as !(a && b) == !a || !b

• not (a or b) is the same as (not a) and (not b). In Java this is written as !(a || b) == !a && !b

Applying DeMorgan’s Laws to !(x < 3 && y > 2) yields !(x < 3) || !(y > 2) which means that this complex conditional will be true when (x >= 3 || y <= 2).

Note

Notice that the negation is distributed to all parts of a complex conditional. It negates each part of the conditional and changes and (&&) to or (||) and or (||) to and (&&).

The negation modifies each conditional as shown below.

• < becomes >=

• > becomes <=

• == becomes !=

• <= becomes >

• >= becomes <

• != becomes ==

See the example below. For what values of x and y will it print true? Try out different values of x and y to check your answer.

public class Test1 { public static void main(String[] args) { int x = 2; int y = 3; System.out.println(!(x < 3 && y > 2)); } }

For more information about DeMorgan’s laws see http://en.wikipedia.org/wiki/De_Morgan’s_laws.

5-5-2: What is printed when the following code executes and x equals 4 and y equals 3?

if (!(x < 3 || y > 2)) System.out.println("first case");
else System.out.println("second case");

• first case
• This will be printed if x is greater or equal to 3 and y is less than or equal to 2. The first part is true but the second is false. Since the statements are joined by an and the complex conditional is false.
• second case
• This will be printed if x is less than 3 or y is greater than 2. In this case the first will be false, but the second true so since the statements are joined with an or the complex conditional is true.

5-5-3: What is printed when the following code executes and x equals 4 and y equals 3?

if (!(x < 3 && y > 2)) System.out.println("first case");
else System.out.println("second case");

• first case
• This will be printed if x is greater than or equal to 3 or y is less than or equal to 2. In this case x is greater than 3 so the first condition is true.
• second case
• This will be printed if x is less than 3 and y is greater than 2.

5-5-4: Which of the following is the same as the code below?

!(x > 2 && y < 4)

• (x < 2) || (y > 4)
• The negation of x > 2 is x <= 2
• (x < 2) && (y > 4)
• Don't forget that the and is changed to an or
• (x <= 2) || (y >= 4)
• The x > 2 becomes x <= 2, the y < 4 becomes y >= 4 and the and changes to or
• (x <= 2) && (y >= 4)
• Don't forget that the and is changed to an or

5-5-5: Which of the following is the same as the code below?

!(x == 2 && y > 4)

• (x != 2) || (y < 4)
• The negation of y > 4 is y <= 4
• (x != 2) && (y < 4)
• Don't forget that the and is changed to an or
• (x != 2) && (y <= 4)
• Don't forget that the and is changed to an or
• (x != 2) || (y <= 4)
• The and is changed to an or, the (x == 2) becomes (x != 2) and (y > 4) becomes (y <= 4)

5-5-6: Which of the following is the same as the code below?

!(x!=5 && y!=7)

• (x == 5) || (y == 7)
• The negation of && is || and the negation of != is ==
• (x == 5) && (y == 7)
• The negation of && is ||
• (x != 5) || (y != 7)
• The negation of x != 5 is x == 5. The negation of y != 7 is y == 7.
• (x < 5) || (x > 5) || (y > 7) || (y < 7)
• The negation of == is != which is the same as < or >. The negation of != is ==.

5-5-7: Which of the following is the same as the code below?

!(x<= 5 && y > 7)

• (x > 5) && (y < 7)
• The negation of && is || and the negation of y > 7 is y <= 7.
• (x > 5) || (y < 7)
• The negation of y > 7 is y <= 7.
• (x > 5) && (y <= 7)
• The negation of && is ||.
• (x > 5) || (y <= 7)
• The negation of (x <= 5) is (x > 5). The negation of && is ||. The negation of (y > 7) is (y <= 7).

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