Boolean Logic

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# Boolean Logic - PowerPoint PPT Presentation

Boolean Logic. CS/APMA 202, Spring 2005 Rosen, section 1.1 Aaron Bloomfield. Quick survey. How familiar are you with Boolean logic? A lot Some A little Boolean what?. Boolean propositions. A proposition is a statement that can be either true or false “The sky is blue”

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## Boolean Logic

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### Boolean Logic

CS/APMA 202, Spring 2005

Rosen, section 1.1

Aaron Bloomfield

Quick survey
• How familiar are you with Boolean logic?
• A lot
• Some
• A little
• Boolean what?
Boolean propositions
• A proposition is a statement that can be either true or false
• “The sky is blue”
• “I is a English major”
• “x == y”
• Not propositions:
• “Are you Bob?”
• “x = 7”
Boolean variables
• We use Boolean variables to refer to propositions
• Usually are lower case letters starting with p (i.e. p, q, r, s, etc.)
• A Boolean variable can have one of two values true (T) or false (F)
• A proposition can be…
• A single variable: p
• An operation of multiple variables: p(qr)
Introduction to Logical Operators
• About a dozen logical operators
• Similar to algebraic operators + * - /
• In the following examples,
• p = “Today is Friday”
• q = “Today is my birthday”
Logical operators: Not
• A not operation switches (negates) the truth value
• Symbol:  or ~
• In C++ and Java, the operand is !
• p = “Today is not Friday”
Logical operators: And
• An and operation is true if both operands are true
• Symbol: 
• It’s like the ‘A’ in And
• In C++ and Java, the operand is &&
• pq = “Today is Friday and today is my birthday”
Logical operators: Or
• An or operation is true if either operands are true
• Symbol: 
• In C++ and Java, the operand is ||
• pq = “Today is Friday or today is my birthday (or possibly both)”
Logical operators: Exclusive Or
• An exclusive or operation is true if one of the operands are true, but false if both are true
• Symbol: 
• Often called XOR
• pq  (p  q)  ¬(p  q)
• In Java, the operand is ^(but not in C++)
• pq = “Today is Friday or todayis my birthday, but not both”
Inclusive Or versus Exclusive Or
• Do these sentences mean inclusive or exclusive or?
• Experience with C++ or Java is required
• Lunch includes soup or salad
• To enter the country, you need a passport or a driver’s license
• Publish or perish
Logical operators: Nand and Nor
• The negation of And and Or, respectively
• Symbols: | and ↓, respectively
• Nand: p|q ¬(pq)
• Nor: p↓q ¬(pq)
Logical operators: Conditional 1
• A conditional means “if p then q”
• Symbol: 
• pq = “If today is Friday, then today is my birthday”
• p→q=¬pq

the

antecedent

the

consequence

Logical operators: Conditional 2
• Let p = “I am elected” and q = “I will lower taxes”
• I state: p  q = “If I am elected, then I will lower taxes”
• Consider all possibilities
• Note that if p is false, then the conditional is true regardless of whether q is true or false
Quick survey
• Does truth table of the conditional make sense?
• Mostly
• Not really
• Huh? I was sleeping
Logical operators: Conditional 4
• Alternate ways of stating a conditional:
• p implies q
• If p, q
• p only if q
• p is sufficient for q
• q if p
• q whenever p
• q is necessary for p
• p only if q
• See Rosen, p. 6, for a full list

I don’t like this one

Logical operators: Bi-conditional 1
• A bi-conditional means “p if and only if q”
• Symbol: 
• Alternatively, it means “(if p then q) and (if q then p)”
• Note that a bi-conditional has the opposite truth values of the exclusive or
Logical operators: Bi-conditional 2
• Let p = “You take this class” and q = “You get a grade”
• Then pq means “You take this class if and only if you get a grade”
• Alternatively, it means “If you take this class, then you get a grade and if you get a grade then you take (took) this class”
Boolean operators summary
• Learn what they mean, don’t just memorize the table!
Quick survey
• I understand all the Boolean operators
• Absolutely!
• With a little review after class, I’ll have it down
• Can we go over it again?
• 17
Precedence of operators
• Just as in algebra, operators have precedence
• 4+3*2 = 4+(3*2), not (4+3)*2
• Precedence order (from highest to lowest): ¬   → ↔
• The first three are the most important
• This means that p  q  ¬r→s↔tyields: (p  (q  (¬r))) ↔ (s→t)
• Not is always performed before any other operation
Translating English Sentences
• Question 7 from Rosen, p. 16
• p = “It is below freezing”
• q = “It is snowing”
• It is below freezing and it is snowing
• It is below freezing but not snowing
• It is not below freezing and it is not snowing
• It is either snowing or below freezing (or both)
• If it is below freezing, it is also snowing
• It is either below freezing or it is snowing, but it is not snowing if it is below freezing
• That it is below freezing is necessary and sufficient for it to be snowing

pq

p¬q

¬p¬q

pq

p→q

(pq)(p→¬q)

p↔q

Translation Example 2
• Heard on the radio:
• A study showed that there was a correlation between the more children ate dinners with their families and lower rate of substance abuse by those children
• Announcer conclusions:
• If children eat more meals with their family, they will have lower substance abuse
• If they have a higher substance abuse rate, then they did not eat more meals with their family
Translation Example 2
• Let p = “Child eats more meals with family”
• Let q = “Child has less substance abuse
• Announcer conclusions:
• If children eat more meals with their family, they will have lower substance abuse
• p q
• If they have a higher substance abuse rate, then they did not eat more meals with their family
• q  p
• Note that p q and q  p are logically equivalent
Translation Example 2
• Let p = “Child eats more meals with family”
• Let q = “Child has less substance abuse”
Translation Example 3
• “I have neither given nor received help on this exam”
• Rephrased: “I have not given nor received …”
• Let p = “I have given help on this exam”
• Let q = “I have received help on this exam”
• Translation is: pq
Translation Example 3
• What they mean is “I have not given and I have not received help on this exam”
• Or “I have not (given nor received) help on this exam”
• The problem:  has a higher precedence than  in Boolean logic, but not always in English
• Also, “neither” is vague
A bit of humor:Yale vs. Harvard
• Web references: http://www.harvardsucks.org/, http://www.yaledailynews.com/article.asp?AID=27506

equal

System Specifications 1
• Just like translating English sentences
• In order for a system specification to be valid, there must be at least ONE truth assignment that allows the propositions to be true
• Question 43 from Rosen, p. 10
• p = “the message is scanned for virii”
• q = “the message was sent from and unknown system”
• The message is scanned for virii whenever themessage was sent from an unknown system
• The message was sent from an unknownsystem but it was not scanned for virii
• It is necessary to scan the message for viriiwhenever it was sent from an unknown system
• When a message is not sent from an unknownsystem, it is not scanned for virii

q→p

q¬p

q→p

¬q→¬p

System Specifications 2
• Is this a valid system?
• p = “the message is scanned for virii”
• q = “the message was sent from and unknown system”
• Requirements: (q→p)  (q¬p)  (¬q→¬p)
Quick survey
• Is this a valid system?
• Yes
• No
• I have no idea
System Specifications 2
• The system as specified is not valid
• There is no assignment of truth values that makes all the system specs true
System Specifications 2
• Let’s remove one of the specifciations:
• q¬p = The message was sent from an unknown system but it was not scanned for virii
• Is this a valid system?
• Requirements: (q→p)  (¬q→¬p)
Boolean Searches

(101 OR 202) AND bloomfield AND “computer science”

• Note that Google requires you to capitalize Boolean operators
• Google defaults to AND; many others do not
Bit Operations 1
• Boolean values can be represented as 1 (true) and 0 (false)
• A bit string is a series of Boolean values
• 10110100 is eight Boolean values in one string
• We can then do operations on these Boolean strings
• Each column is its ownBoolean operation

01011010

10110100

11101110

Bit Operations 2
• Evaluate the following

11000  (01011  11011)

= 11000  (11011)

= 11000

01011

11011

11011

11000

11011

11000

Quick survey
• What programming languages do you know?
• Java only
• C / C++ only
• Java and C/C++
• not (Java or C/C++)
&& vs. & in C/C++

Consider the following:

In C/C++, any value other than 0 is true

int p = 11;

int q = 20;

if ( p && q ) {

}

if ( p & q ) {

}

The binary for the integer 11 is 01011

The binary for the integer 20 is 10100

Notice the double ampersand – this is a Boolean operation

As p and q are both true, this is true

Notice the single ampersand – this is a bitwise operation

01011

10100

00000

This evaluates to zero (false)!

Bitwise Boolean And operation:

&& vs. & in C/C++
• Note that Java does not have this “feature”
• If p and q are int:
• p & q is bitwise
• p && q will not compile
• If p and q are boolean:
• Both p & q and p && q will be a Boolean operation
• The same holds true for the or operators (| and ||) in both Java and C/C++
Quick survey
• I felt I understood the material in this slide set…
• Very well
• Pretty much
• Not really
• Not at all
Quick survey
• The pace of the lecture for this slide set was…
• Too slow
• A little slow