ICS Boolean Logical Operators
What is a Boolean Expression?
A Boolean expression is an expression that evaluates to TRUE or FALSE.
Examples in English:
“It is not snowing”
TRUE
FALSE
“I am human”
TRUE
FALSE
“I am human and I have 12 fingers.”
TRUE
FALSE
Examples in C#:
x != 57
TRUE when ______________________________________________
x == 28
TRUE when ______________________________________________
Review: The basic comparison operators:
== means ______________________
!= means ______________________
>= means ______________________
<= means ______________________
< means ______________________
> means ______________________
The && Operator (AND)
The && (and) operator returns TRUE when both the things on either side of it are true, and false
otherwise…
I am human
I am a dog
I am human
&&
&&
&&
I have 10 fingers
I have 12 toes
I have 12 toes
TRUE
FALSE
FALSE
(both true)
(both false)
(one of them is false)
When will these be true?
x == 4 && y < 7
________________________________________
a >= 5 && b <= 4 && name == “bob” ________________________________________
x == 4 && x == 2
________________________________________
ICS Boolean Logical Operators
The || Operator (OR)
The | | (or) operator returns TRUE when one or both of the things on either side of it are true, and
false otherwise. You find this symbol to the right of the curly brackets on most keyboards.
I am human
I am a dog
I am human
||
||
||
I have 10 fingers
I have 12 toes
I have 12 toes
TRUE
FALSE
TRUE
(both true)
(both false)
(one of them is true)
When will these be true?
x == 4 || y < 7
_________________________________________
a >= 5 || b <= 4 || s.Equals("bob")
_________________________________________
x == 4 || x == 2
_________________________________________
NOTE: Logical OR is different from regular English OR
Usually if I say I’m going to bed or watching TV, we interpret that to mean I will do one or the
other but not both. But to a computer, the statement that I am going to bed or watching TV could
also mean that I am going to go to bed AND watch TV…
A Common Mistake
The following will not work …
if (x < 5 && > 1) …
if (x == 5 || 6) …
This will give you a syntax error, even though it is clear to us humans what it is supposed to
mean. The problem is that the & and | operators need to have fully formed Boolean expressions
to either side of them.
This is the correct way to write it…
if (x < 5 && x > 1) …
if (x == 5 || x == 6)
Evaluate the following expressions
Say whether each evaluates to TRUE or FALSE
1. 1+2 > 4-2 && 12 < 23
___
2. 1+2 > 4-2 || 12 < 23
___
3. 1+2 > 4-2 && 12 > 23
___
4. 1+2 > 4-2 || 12 > 23
___
ICS Boolean Logical Operators
The ! Operator (NOT)
The ! (not) operator returns TRUE when the thing beside it is FALSE, and FALSE when the
thing beside it is TRUE.
!(I am human)
!(I am a dog)
!(I am human || I have 12 toes)
FALSE
TRUE
FALSE
When will these be true?
!(age == 2)
_________________________________________
a >= 5 || !(b <= 4)
_________________________________________
!(x == 4 || x == 2)
_________________________________________
Evaluate the Following
mood>=3 && mood<8
TRUE for which values of mood?
1 2 3 4 5 6 7 8 9 10 11
mood == 5 || mood > 10
TRUE for which values of mood?
1 2 3 4 5 6 7 8 9 10 11
!(mood>=3 && mood<8)
TRUE for which values of mood?
1 2 3 4 5 6 7 8 9 10 11
!(mood == 5) || mood > 10
TRUE for which values of mood?
1 2 3 4 5 6 7 8 9 10 11
Order of Operations for the Boolean Operators
Just like arithmetic expressions, Boolean expressions have an order of operations. The order is…
1. Brackets
3. Arithmetic Operators (+, -, / *, %)
2. Boolean Operators (==, !=, <, etc.)
3. Not
4. And
5. Or
Instead of BEDMAS,
it’s BABNAO!
ICS Boolean Logical Operators
Evaluate the Following
For each of these expressions, state whether it does what it is supposed to do. If it’s wrong, give
an example where it fails and show how to fix it.
An expression that’s true if both x and y are equal to either 8 or 9.
x == 8 || x == 9 && y == 8 || y == 9
An expression that’s true when x is in the range 0 to 5 or 10 to 15 inclusive.
x > 0 && x < 5 || x > 10 && x < 15
What will this code print in the Message Box?
int x = 5, y = -4;
double z = 7.8;
if (x == 5)
MessageBox.Show("duck");
else
MessageBox.Show ("goose");
if (x !=5 && z < 8)
MessageBox.Show ("duck");
else
MessageBox.Show ("goose");
OUTPUT:
if (y > 0 || z < 8 )
MessageBox.Show ("goose");
else
MessageBox.Show ("duck");
____________
____________
____________
Write the following expressions.
1. This expression is TRUE if the variable mark is in the range 70 to 80. This range includes
70 but does not include 80.
2. This expression is TRUE if the variable userInput is 5 or 0.
3. This expression is TRUE if the variable number is greater than 0 but not equal to 3.
4. This expression is TRUE if the variable age is 16, 18, 19, or 25.
5. This expression is TRUE if a string is "Yes".