- Boolean logic
**Boolean logic**is a complete system forlogic al operations. It was named afterGeorge Boole , who first defined an algebraic system of logic in the mid 19th century. Boolean logic has many applications in electronics, computer hardware and software, and is the base ofdigital electronics . In 1938, Claude Shannon showed how electric circuits with relays were a model for Boolean logic. This fact soon proved enormously consequential with the emergence of the electroniccomputer ."Using the

algebra of sets , this article contains a basic introduction to sets, Boolean operations,Venn diagram s,truth tables , and Boolean applications. TheBoolean algebra article discusses a type ofalgebraic structure that satisfies the axioms of Boolean logic. Thebinary arithmetic article discusses the use of binary numbers incomputer systems."**et logic versus Boolean logic**Sets can contain any elements. We will first start out by discussing general set logic, then restrict ourselves to Boolean logic, where elements (or "bits") each contain only two possible values, called various names, such as "true" and "false", "yes" and "no", "on" and "off", or "1" and "0".

**Terms**Let "X" be a set:

* An**element**is one member of a set. This is denoted by $in$. If it's not an element of the set, this is denoted by $otin$.* The

**universe**is the set "X", sometimes denoted by 1. Note that this use of the word universe means "all elements being considered", which are not necessarily the same as "all elements there are".* The

**empty set**or**null set**is the set of no elements, denoted by $varnothing$ and sometimes 0.* A

**unary operator**applies to a single set. There is one unary operator, called logical**NOT**. It works by taking the complement.* A

**binary operator**applies to two sets. The basic binary operators are logical**OR**and logical**AND**. They perform the union and intersection of sets. There are also other derived binary operators, such as**XOR**(exclusive OR).* A

**subset**is denoted by $A\; subseteq\; B$ and means every element in set A is also in set B.* A

**proper subset**is denoted by $A\; subset\; B$ and means every element in set A is also in set B and the two sets are not equal.* A

**superset**is denoted by $A\; supseteq\; B$ and means every element in set B is also in set A.* A

**proper superset**is denoted by $A\; supset\; B$ and means every element in set B is also in set A and the two sets are not equal.**Example**Imagine that set A contains all even numbers (multiples of two) in "the universe" (defined in the example to the right as all integers between 0 and 30 inclusive) and set B contains all multiples of three in "the universe". Then the

**intersection**of the two sets (all elements in sets A AND B) would be all multiples of six in "the universe". The complement of set A (all elements NOT in set A) would be all odd numbers in "the universe".**Chaining operations together**While at most two sets are joined in any Boolean operation, the new set formed by that operation can then be joined with other sets utilizing additional Boolean operations. Using the previous example, we can define a new set C as the set of all multiples of five in "the universe". Thus "sets A AND B AND C" would be all multiples of 30 in "the universe". If more convenient, we may consider set AB to be the intersection of sets A and B, or the set of all multiples of six in "the universe". Then we can say "sets AB AND C" are the set of all multiples of 30 in "the universe". We could then take it a step further, and call this result set ABC.

**Use of parentheses**While any number of logical ANDs (or any number of logical ORs) may be chained together without ambiguity, the combination of ANDs and ORs and NOTs can lead to ambiguous cases. In such cases, parentheses may be used to clarify the order of operations. As always, the operations within the innermost pair is performed first, followed by the next pair out, etc., until all operations within parentheses have been completed. Then any operations outside the parentheses are performed.

**Application to binary values**In this example we have used

natural numbers , while in Boolean logic binary numbers are used. The universe, for example, could contain just two elements, "0" and "1" (or "true" and "false", "yes" and "no", "on" or "off", etc.). We could also combine binary values together to get binary words, such as, in the case of two digits, "00", "01", "10", and "11". Applying set logic to those values, we could have a set of all values where the first digit is "0" ("00" and "01") and the set of all values where the first and second digits are different ("01" and "10"). The intersection of the two sets would then be the single element, "01". This could be shown by the following Boolean expression, where "1st" is the first digit and "2nd" is the second digit:**(NOT 1st) AND (1st XOR 2nd)****Properties**We define symbols for the two primary binary operations as $land\; /\; cap$ (logical AND/set intersection) and $lor\; /\; cup$ (logical OR/set union), and for the single unary operation $lnot$ / ~ (logical NOT/set complement). We will also use the values 0 (logical FALSE/the empty set) and 1 (logical TRUE/the universe). The following properties apply to both Boolean logic and set logic (although only the notation for Boolean logic is displayed here):

:

* More complex truth tables involving multiple inputs, and other Boolean operations, may also be created.

* Truth tables have applications in

logic , interpreting 0 as FALSE, 1 as TRUE, $cap$ as AND, $cup$ as OR, and ¬ as NOT.**Other notations**Mathematician s andengineer s often use plus (+) for OR and a product sign ($cdot$) for AND. OR and AND are somewhat analogous to addition and multiplication in otheralgebraic structure s, and this notation makes it very easy to get sum of products form for normal algebra. NOT may be represented by a line drawn above the expression being negated ($overline\{x\}$).Programmer s will often use a pipe symbol (|) for OR, an ampersand (&) for AND, and a tilde (~) for NOT. In manyprogramming language s, these symbols stand forbitwise operation s. "||", "&&", and "!" are used for variants of these operations.Another notation uses "meet" for AND and "join" for OR. However, this can lead to confusion, as the term "join" is also commonly used for any Boolean operation which combines sets together, which includes both AND and OR.

**Basic mathematics use of Boolean terms*** In the case of simultaneous equations, they are connected with an implied logical AND:

::x + y = 2

::AND

::x - y = 2

* The same applies to simultaneous inequalities:

::x + y < 2

::AND

::x - y < 2

*The greater than or equals sign ($ge$) and less than or equals sign ($le$) may be assumed to contain a logical OR:

::X < 2

::OR

::X = 2

* The plus/minus sign ($pm$), as in the case of the solution to a square root problem, may be taken as logical OR:

::WIDTH = 3

::OR

::WIDTH = -3

**English language use of Boolean terms**Care should be taken when converting an English sentence into a formal Boolean statement. Many English sentences have imprecise meanings, e.g. "All that glitters is

**not**gold," which could mean that "nothing that glitters is gold" or "some things which glitter are not gold".AND and OR can also be used interchangeably in English, in certain cases:

* "I always carry an umbrella for when it rains

**and**snows."* "I always carry an umbrella for when it rains

**or**snows."Sometimes the English words AND and OR have the opposite meaning in Boolean logic:

* "Give me all the red

**and**blue berries" usually means "Give me all berries that are red**or**blue". An alternative phrasing for standard written English: "Give me all berries that are red as well as all berries that are blue".Also note that the word OR in English may correspond with either logical OR or logical XOR, depending on the context:

* "I start to sweat when the humidity

**or**temperature is high." (logical OR)* "You want ice cream and candy? You may have ice cream

**or**candy." (logical XOR)The combination AND/OR is sometimes used in English to specify a logical OR, when just using the word OR alone might have been mistaken as meaning logical XOR:

* "I'm having chicken

**and/or**beef for dinner." (logical OR). An alternative phrasing for standard written English: "I'm having chicken, or beef, or both, for dinner."* The use of the "

and/or "virgule is generally disfavored in formal written English. [*[*] Such usage may introduce critical imprecision in legal instruments, research findings, and*http://www.ntsc.navy.mil/Resources/Library/Acqguide/SpecWord.htm#and_or Usage Guide*] .specification s for computer programs or electronic circuits.A case where this is an issue is when specifications for a computer program or electronic circuit are supplied as an English paragraph describing their function. For example, the statement: "the program should verify that the applicant has checked the male

**or**female box", should be taken as an XOR, and a check added to ensure that one, and only one, box is selected. In other cases, the interpretation of English may be less certain, and the author of the specification may need to be consulted to determine their true intent.**Applications****Digital electronic circuit design**Boolean logic is also used for circuit design in

electrical engineering ; here 0 and 1 may represent the two different states of onebit in adigital circuit , typically high and lowvoltage . Circuits are described by expressions containing variables, and two such expressions are equal for all values of the variables if, and only if, the corresponding circuits have the same input-output behavior. Furthermore, every possible input-output behavior can be modeled by a suitable Boolean expression.Basic

logic gates such as AND, OR, and NOT gates may be used alone, or in conjunction with NAND, NOR, and XOR gates, to control digital electronics and circuitry. Whether these gates are wired in series or parallel controls the precedence of the operations.**Database applications**Relational databases useSQL , or other database-specific languages, to perform queries, which may contain Boolean logic. For this application, each record in a table may be considered to be an "element" of a "set". For example, in SQL, theseSELECT statements are used to retrieve data from tables in the database:SELECT * FROM EMPLOYEES WHERE LAST_NAME = 'Smith' AND FIRST_NAME = 'John' ;

SELECT * FROM EMPLOYEES WHERE LAST_NAME = 'Smith' OR FIRST_NAME = 'John' ;

SELECT * FROM EMPLOYEES WHERE NOT LAST_NAME = 'Smith' ;

Parentheses may be used to explicitly specify the order in which Boolean operations occur, when multiple operations are present:

SELECT * FROM EMPLOYEES WHERE (NOT LAST_NAME = 'Smith') AND (FIRST_NAME = 'John' OR FIRST_NAME = 'Mary') ;

Multiple sets of nested parentheses may also be used, where needed.

Any Boolean operation (or operations) which combines two (or more) tables together is referred to as a

**join**, in relational database terminology.In the field of

Electronic Medical Records , some software applications use Boolean logic to query their patient databases, in what has been namedConcept Processing technology.**earch engine queries**Search engine queries also employ Boolean logic. For this application, each web page on the Internet may be considered to be an "element" of a "set". The following examples use a syntax supported by

Google . [*Not all search engines support the same query syntax. Additionally, some organizations (such as Google) provide "specialized" search engines that support alternate or extended syntax. (See e.g., [*]*http://www.google.com/help/cheatsheet.html Syntax cheatsheet*] , [*http://www.google.com/intl/en/help/faq_codesearch.html#regexp Google codesearch supports regular expressions*] ).* Doublequotes are used to combine whitespace-separated words into a single search term. [

*Doublequote-delimited search terms are called "exact phrase" searches in the Google documentation.*]

* Whitespace is used to specify logical AND, as it is the default operator for joining search terms:"Search term 1" "Search term 2"

*The OR keyword is used for logical OR:

"Search term 1" OR "Search term 2"

*The minus sign is used for logical NOT (AND NOT):

"Search term 1" -"Search term 2"

**ee also***

Boolean algebra topics

*Boolean domain

*Boolean function

*Boolean-valued function

* "Laws of Form "

*Logic minimization

*Logic gate

*Logical graph

*Venn diagram

*Ternary logic **Notes and references****External links*** [

*http://www.maths.tcd.ie/pub/HistMath/People/Boole/CalcLogic/CalcLogic.html The Calculus of Logic*] , by George Boole, Cambridge and Dublin Mathematical Journal Vol. III (1848), pp. 183-98.

* [*http://sourceforge.net/projects/logicaleval/ Logical Formula Evaluator*] (for Windows), a software which calculates all possible values of a logical formula

* [*http://computer.howstuffworks.com/boolean.htm How Stuff Works - Boolean Logic*]

* Maiki & Boaz [*http://www.bdd-project.com BDD-PROJECT*] , a Web Application for BDD reduction and visualization.

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