1. Intro to Computer Science CS1510, Section 2 Dr. Sarah Diesburg
DeMorgan’s Laws
Intro to Computer Science
CS1510, Section 2
Dr. Sarah Diesburg

2. Opposites Sometimes we need to find the opposites of conditions
An example is error checking to prompt the user for the correct value
We know what kind of value we want
But we have to loop on a condition for the values we don’t want…
Other examples, too

3. Thought Puzzle What is the opposite of:
if small<middle and middle<large :
A: if small<middle or middle<large :
B: if small>=middle and middle>=large :
C: if small>=middle or middle>=large :

4. Thought Puzzle What is the opposite of:
if small<middle and middle<large :
For that whole statement to be true, both
(small < middle) == true
and
(middle < large) == true

5. Thought Puzzle What is the opposite of:
if small<middle and middle<large :
For that whole statement to be false, at least
(first statement) == false or
(second statement) == false

6. Thought Puzzle What is the opposite of:
if small<middle and middle<large :
For that whole statement to be false, at least
not(small<middle) or
not(middle<large)

7. Thought Puzzle What is the opposite of:
if small<middle and middle<large :
For that whole statement to be false, at least
(small>=middle) == false or
(middle>=large) == false

8. DeMorgan’s Laws
The opposite of: A and B Is opposite (A and B) = opposite(A) or opposite(B)

9. So how do you go the other way?
The opposite of: A or B Is opposite (A or B) = opposite(A) and opposite(B)

10. In-Class Activities
Say you only accept a positive integer between 1 and 10 (inclusive)
What is the opposite of:
If val >=1 and val <=10

11. In-Class Activities
Say you only accept the input strings “yes” and “no”
What is the opposite of
If val==“yes” or val==“no”

12. In-Class Activities Remember the Latin Squares?
order number must be positive
startingNumber between 1 and order (inclusive)
What is the opposite of:
if order > 0 and (0 < startingNumber >= order)