Encyclopedia of Computer Science and Technology

optical character recognition 355the value of the SubTotal Tax is multiplied by the value ofthe variable Tax_Rate, the result is added to the value ofSubTotal, and the result of the entire expression is storedin the variable Total. Compilers generally parse arithmeticexpressions by converting them from an “infix” form (as inA + B) to a “postfix” form (as in + A B), resolving them intoa simple form that is ready for conversion to machine code.Operator PrecedenceThe preceding example raises an important question. Howdoes one know that the subtotal is to be multiplied by thetax rate and then the result added to the subtotal, as opposedto adding the subtotal and tax and multiplying the result bythe tax rate? The former procedure is intuitively correct tohuman observers, but since computers lack intuition, specificrules of precedence are defined for operators. Theserules, which are similar for all computer languages, tell thecompiler that when code is generated for arithmetic operations,multiplications and divisions are carried out first(moving from left to right), and then additions and subtractionsare resolved in the same way. The rules of precedencedo become more complex when the relational, logical, andassignment operators are included. Finally, expressions canbe enclosed in parentheses to overrule precedence and forcethem to be evaluated. Thus in the expression (A + B) * C theaddition will be carried out before the multiplication.Generally speaking, the levels of precedence for mostlanguages are as follows:1. scope resolution operators (specify local v. globalversions of a variable)2. invoking a method from a class, array subscript,function call, increment or decrement3. size of (gets number of bytes in an object), addressand pointer dereference, other unary operators(such as “not” and complement); creation and deallocationfunctions; type castsA compiler or interpreter processes a program statement by applyingits operators in order of precedence. Here the multiplication isdone first, and then its result is used in the addition. The assignment(=) operator has the lowest precedence and is applied last,assigning the value of the entire expression to the variable Total.4. class member selection through a pointer5. multiplication, division, and modulus6. addition and subtraction7. left and right shift operators8. less than and greater than9. equal and not equal operators10. bitwise operators (AND, then exclusive OR, inclusiveOR)11. logical operators (AND, then OR)12. assignment statementsThe basic arithmetic operators are built into each programminglanguage, but many of the newer object-orientedlanguages such as C++ allow for programmer-defined operatorsand a process called overloading in which the sameoperator can be defined to work with several different kindsof data. Thus the + operator can be extended so that if it isgiven character strings instead of numbers, it will “add” thestrings by combining (concatenating) them.Further Reading“Operators in C and C++.” Wikipedia. Available online. URL:http://en.wikipedia.org/wiki/Operators_in_C_and_C++.Accessed August 16, 2007.Sebesta, Robert W. Concepts of Programming Languages. 8th ed.Upper Saddle River, N.J.: Addison-Wesley, 2007.Stroustrup, Bjarne C. The C++ Programming Language. 3rd ed.Reading, Mass.: Addison-Wesley, 1997. See chap. 6 “Expressionsand Statements” and chap. 11 “Operator Overloading.”“Summary of Operators.” Java Tutorials. Available online. URL:http://java.sun.com/docs/books/tutorial/java/nutsandbolts/opsummary.html. Accessed August 16, 2007.“Summary of Operators in Java.” Available online. URL: http://sunsite.ccu.edu.tw/java/tutorial/java/nutsandbolts/opsummary.html.optical character recognition (OCR)Today it is easy to optically scan text or graphics printed onpages and convert it into a graphical representation for storagein the computer (see scanner). However, a shape suchas a letter c doesn’t mean anything in particular as a graphic.Optical character recognition (OCR) is the process of identifyingthe letter or other document element that correspondsto a given part of the scanned image and converting it tothe appropriate character (see characters and strings). Ifthe process is successful, the result is a text document thatcan be manipulated in a word processor, database, or otherprogram that handles text. Raymond Kurzweil (1948– )marketed the first commercially practicable general-purposeoptical character recognition system in 1978.Once the document page has been scanned into an imageformat, there are various ways to identify the characters.One method is to use stored templates that indicate the patternof pixels that should correspond to each character. Generally,a threshold of similarity is defined so that an exactmatch is not necessary to classify a character: The templatemost similar to the character is chosen. Some systems storea set of templates for each of the fonts most commonly foundin printed text. (Recognizing cursive writing is a much morecomplex process: See handwriting recognition.)