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That sounds great, but what is an evalute command anyways? Good question. Language elements stored in a string are nothing, but a string. An eval command take that string and interpret its content, processing the elements accordling. That means power, the ability to define code blocks and have it processed when needed. The Interpreter Pattern allows you to define your own grammar, and process it in order to do some useful task.Iniciado por Eslopes
The Interpreter Pattern allows much more than simple arithmetic expressions evaluation, thought this is exactly what we are going to show here. The classes in this solutions are:
- IExpression: Interface that declares an operation
- Context: Global information used by the expression
- TerminalExpression: Represent elements in the grammar that do no to get replace, such as symbols
- NonTerminalExpression: Represents elements that will be replaced during the evaluation such as variables or even rules
- ComputeFormula: Client class that also contains the parser used to break expression's elements.
The ComputeFormula class deals with a lot of concepts not covered in this article, but I recommend you to understand that class in order to get yourself used to things like stacks, collections, string handling and much more.
OO Cobol code
Everything starts with an Interface definition. IExpression declares the contract to which classes must adhere in order to participate in this solution.
COBOL Código:
INTERFACE-ID. IExpression as "InterpreterPattern.IExpression". environment division. configuration section. repository. class ClassContext as "InterpreterPattern.Context". procedure division. method-id. EvaluateExpression as "Evaluate". data division. linkage section. 01 inContext object reference ClassContext. 01 outValue usage comp-2. procedure division using inContext returning outValue. end method EvaluateExpression. END INTERFACE IExpression.
The following classes implements the IExpression interface: AddExpression.cob, SubtractExpression.cob, , MultiplyExpression.cob, DivideExpression.cob, and PowerExpression.cob. These are also the classes that contains the actual calculation performed over operands.
Below the code for PowerExpression code:
COBOL Código:
CLASS-ID. PowerExpression AS "InterpreterPattern.PowerExpression" inherits ClassNonTerminalExpression. environment division. configuration section. repository. class ClassContext as "InterpreterPattern.Context" class SystemString as "System.String" class ClassNonTerminalExpression as "InterpreterPattern.NonTerminalExpression" interface IExpression as "InterpreterPattern.IExpression". *> Instance's data and methods object. data division. working-storage section. copy "types.book". 01 variable object reference SystemString private. procedure division. method-id. NEW. data division. linkage section. 01 inLeftExpression object reference IExpression. 01 inRightExpression object reference IExpression. procedure division using inLeftExpression, inRightExpression. invoke super "NEW" using inLeftExpression, inRightExpression. end method NEW. method-id. EvaluateExpresion as "Evaluate" override. data division. working-storage section. 01 anExpression object reference IExpression. linkage section. 01 inContext object reference ClassContext. 01 outValue type SystemDouble. procedure division using inContext returning outValue. invoke super "GetLeftNode" returning anExpression move anExpression::"Evaluate"(inContext) to outValue invoke super "GetRightNode" returning anExpression *> This is the real "PowerExpression" compute outValue = outValue ** anExpression::"Evaluate"(inContext) end method EvaluateExpresion. end object. END CLASS PowerExpression.
Other operations' classes such as AddExpression or MultiplyExpression have pretty much basically the same code structure.
The TerminalExpression class that defines symbols behavior:
COBOL Código:
CLASS-ID. TerminalExpression AS "InterpreterPattern.TerminalExpression". environment division. configuration section. repository. class ClassContext as "InterpreterPattern.Context" class SystemCollectionHashTable as "System.Collections.Hashtable" class SystemString as "System.String" interface IExpression as "InterpreterPattern.IExpression". *> Instance's data and methods object. implements IExpression. data division. working-storage section. copy "types.book". 01 variable object reference SystemString private. procedure division. method-id. NEW. data division. linkage section. 01 inName object reference SystemString. procedure division using inName. set variable to inName end method NEW. method-id. EvaluateExpression as "Evaluate". data division. linkage section. 01 inContext object reference ClassContext. 01 outValue type SystemDouble. procedure division using inContext returning outValue. move inContext::"GetValue"(variable) to outValue end method EvaluateExpression. end object. END CLASS TerminalExpression.
The NonTerminalExpression class that defines rules:
COBOL Código:
CLASS-ID. NonTerminalExpression AS "InterpreterPattern.NonTerminalExpression". environment division. configuration section. repository. class ClassContext as "InterpreterPattern.Context" class SystemCollectionHashTable as "System.Collections.Hashtable" class SystemString as "System.String" interface IExpression as "InterpreterPattern.IExpression". *> Instance's data and methods object. implements IExpression. data division. working-storage section. copy "types.book". 01 leftNode object reference IExpression private. 01 rightNode object reference IExpression private. procedure division. method-id. NEW. data division. linkage section. 01 inLeftExpression object reference IExpression. 01 inRightExpression object reference IExpression. procedure division using inLeftExpression inRightExpression. invoke self "SetLeftNode" using inLeftExpression invoke self "SetRightNode" using inRightExpression end method NEW. method-id. SetLeftNode. data division. linkage section. 01 inNode object reference IExpression. procedure division using inNode. set leftNode to inNode end method SetLeftNode. method-id. SetRightNode. data division. linkage section. 01 inNode object reference IExpression. procedure division using inNode. set rightNode to inNode end method SetRightNode. method-id. GetLeftNode. data division. linkage section. 01 outNode object reference IExpression. procedure division returning outNode. set outNode to leftNode end method GetLeftNode. method-id. GetRightNode. data division. linkage section. 01 outNode object reference IExpression. procedure division returning outNode. set outNode to rightNode end method GetRightNode. method-id. EvaluateExpression as "Evaluate". data division. linkage section. 01 inContext object reference ClassContext. 01 outValue type SystemDouble. procedure division using inContext returning outValue. *> Implemented because NetCobol.Net does not supports Abstract classes end method EvaluateExpression. end object. END CLASS NonTerminalExpression.
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The ComputeFormula class is the "client" class of this example. It contains a parser, the expression tree builder and, of course, uses Interpreter pattern to arrange things.
COBOL Código:
CLASS-ID. ComputeFormula AS "InterpreterPattern.ComputeFormula". environment division. configuration section. repository. class SystemCollectionHashTable as "System.Collections.Hashtable" class SystemString as "System.String" class SystemCollectionStack as "System.Collections.Stack" class SystemDoubleWrapper as "System.Double" class SystemObject as "System.Object" class ClassAddExpression as "InterpreterPattern.AddExpression" class ClassSubtractExpression as "InterpreterPattern.SubtractExpression" class ClassMultiplyExpression as "InterpreterPattern.MultiplyExpression" class ClassDivideExpression as "InterpreterPattern.DivideExpression" class ClassPowerExpression as "InterpreterPattern.PowerExpression" class ClassTerminalExpression as "InterpreterPattern.TerminalExpression" class ClassNonTerminalExpression as "InterpreterPattern.NonTerminalExpression" class ClassContext as "InterpreterPattern.Context" interface IExpression as "InterpreterPattern.IExpression" property StringNull as "Empty" property StringLength as "Length" property StackCount as "Count". *> Instance's data and methods object. data division. working-storage section. copy "types.book". 01 booleanTrue pic 1 value B"1". 01 booleanFalse pic 1 value B"0". 01 expression object reference SystemString. 01 ctx object reference ClassContext. 01 operators object reference SystemCollectionHashTable private. 01 tempObject object reference SystemObject. procedure division. method-id. NEW. procedure division. invoke SystemCollectionHashTable "NEW" returning operators invoke operators "Add" using by value "+" by value "1" invoke operators "Add" using by value "-" by value "1" invoke operators "Add" using by value "/" by value "2" invoke operators "Add" using by value "*" by value "2" invoke operators "Add" using by value "^" by value "2" invoke operators "Add" using by value "(" by value "0" end method NEW. method-id. SetContext as "SetContext". data division. linkage section. 01 inContext object reference ClassContext. procedure division using inContext. set ctx to inContext end method SetContext. method-id. SetExpression as "SetExpression". data division. linkage section. 01 inExpression object reference SystemString. procedure division using inExpression. set expression to inExpression end method SetExpression. method-id. EvaluateExpression as "Evaluate". data division. working-storage section. 01 postFixExp object reference SystemString. 01 rootNode object reference IExpression. linkage section. 01 outValue type SystemDouble. procedure division returning outValue. set postFixExp to self::"infixToPostFix"(expression) set rootNode to self::"BuildTree"(postFixExp) set outValue to rootNode::"Evaluate"(ctx) end method EvaluateExpression. method-id. GetNonTerminalExpression as "GetNonTerminalExpression" private. data division. working-storage section. 01 postFixExp object reference SystemString. 01 rootNode object reference IExpression. 01 singleChar pic x. linkage section. 01 operation object reference SystemString. 01 leftExpression object reference IExpression. 01 rightExpression object reference IExpression. 01 outValue object reference ClassNonTerminalExpression. procedure division using operation leftExpression rightExpression returning outValue. set outValue to null set singleChar to operation::"Trim"() evaluate singleChar when "+" invoke ClassAddExpression "NEW" using leftExpression, rightExpression returning outValue when "-" invoke ClassSubtractExpression "NEW" using leftExpression, rightExpression returning outValue when "*" invoke ClassMultiplyExpression "NEW" using leftExpression, rightExpression returning outValue when "/" invoke ClassDivideExpression "NEW" using leftExpression, rightExpression returning outValue when "^" invoke ClassPowerExpression "NEW" using leftExpression, rightExpression returning outValue end-evaluate end method GetNonTerminalExpression. method-id. BuildTree as "BuildTree" private. data division. working-storage section. 01 currentChar object reference SystemString. 01 leftOperand object reference IExpression. 01 rightOperand object reference IExpression. 01 exp object reference IExpression. 01 stack object reference SystemCollectionStack. 01 rootNode object reference IExpression. 01 indexer pic s9(09) value zeros. linkage section. 01 inFormula object reference SystemString. 01 outExpression object reference IExpression. procedure division using inFormula returning outExpression. invoke SystemCollectionStack "NEW" returning stack move zeros to indexer perform until indexer = StringLength of inFormula set currentChar to inFormula::"Substring"(indexer, 1) if (self::"IsOperator"(currentChar) = booleanFalse) set exp to ClassTerminalExpression::"NEW"(currentChar) invoke stack "Push" using by value exp else set tempObject to stack::"Pop"() set rightOperand to tempObject as IExpression set tempObject to stack::"Pop"() set leftOperand to tempObject as IExpression set exp to self::"GetNonTerminalExpression" (currentChar, leftOperand, rightOperand) invoke stack "Push" using by value exp end-if add 1 to indexer end-perform set tempObject to stack::"Pop"() set outExpression to tempObject as IExpression end method BuildTree. method-id. IsOperator as "IsOperator" private. data division. working-storage section. 01 singleChar pic x. linkage section. 01 inCharacter object reference SystemString. 01 outBool pic 1. procedure division using inCharacter returning outBool. set singleChar to inCharacter if (singleChar = "+" or "-" or "*" or "/" or "^") move booleanTrue to outBool else move booleanFalse to outBool end-if end method IsOperator. method-id. infixToPostFix as "InFixToPostFix". data division. working-storage section. 01 stack object reference SystemCollectionStack. 01 pfExpr object reference SystemString. 01 tempString object reference SystemString. 01 expr object reference SystemString. 01 indexer type SystemInt32. 01 stringVal1 object reference SystemString. 01 stringVal2 object reference SystemString. 01 val1 type SystemDouble. 01 val2 type SystemDouble. 01 currentChar object reference SystemString. linkage section. 01 inString object reference SystemString. 01 outString object reference SystemString. procedure division using inString returning outString. invoke SystemCollectionStack "NEW" returning stack set pfExpr to StringNull of SystemString set tempString to StringNull of SystemString set expr to inString::"Trim"() move zeros to indexer perform until indexer = StringLength of inString set currentChar to inString::"Substring"(indexer, 1) if (currentChar::"Equals"(" ") = booleanTrue) add 1 to indexer exit to test of perform end-if if (self::"IsOperator"(currentChar) = booleanFalse) and (currentChar::"Equals"("(") = booleanFalse) and (currentChar::"Equals"(")") = booleanFalse) set pfExpr to SystemString::"Concat"(pfExpr, currentChar) end-if if (currentChar::"Equals"("(") = booleanTrue) invoke stack "Push" using currentChar end-if if (currentChar::"Equals"(")") = booleanTrue) set tempObject to stack::"Pop"() set tempString to tempObject as SystemString perform until tempString::"Equals"("(") = booleanTrue set pfExpr to SystemString::"Concat"(pfExpr, tempString) set tempObject to stack::"Pop"() set tempString to tempObject as SystemString end-perform set tempString to StringNull of SystemString end-if if (self::"IsOperator"(currentChar) = booleanTrue) if (StackCount of stack >; 0) set tempObject to stack::"Pop"() set tempString to tempObject as SystemString set tempObject to operators::"Get_Item"(tempString) set stringVal1 to tempObject as SystemString set tempObject to operators::"Get_Item"(currentChar) set stringVal2 to tempObject as SystemString move SystemDoubleWrapper::"Parse"(stringVal1) to val1 move SystemDoubleWrapper::"Parse"(stringVal2) to val2 perform until val1 <; val2 set pfExpr to SystemString::"Concat"(pfExpr, tempString) move -100 to val1 if (StackCount of stack >; 0) set tempObject to stack::"Pop"() set tempString to tempObject as SystemString set tempObject to operators::"Get_Item"(tempString) set stringVal1 to tempObject as SystemString set val1 to SystemDoubleWrapper::"Parse"(stringVal1) end-if end-perform if (val1 <; val2) and (val1 > -100) invoke stack "Push" using tempString end-if end-if invoke stack "Push" using currentChar end-if add 1 to indexer end-perform perform until StackCount of stack = 0 set tempObject to stack::"Pop"() set tempString to tempObject as SystemString set pfExpr to SystemString::"Concat"(pfExpr, tempString) end-perform set outString to pfExpr end method infixToPostFix. end object. END CLASS ComputeFormula.
Testing our expression evaluator
The little application below shows how to use our expression evaluator:
COBOL Código:
IDENTIFICATION DIVISION. PROGRAM-ID. MAIN AS "ConsoleApplication1.Main". ENVIRONMENT DIVISION. CONFIGURATION SECTION. SPECIAL-NAMES. REPOSITORY. class ClassComputeFormula as "InterpreterPattern.ComputeFormula" class ClassContext as "InterpreterPattern.Context". DATA DIVISION. WORKING-STORAGE SECTION. 01 computeFormula object reference ClassComputeFormula. 01 context object reference ClassContext. 01 result comp-2. 01 displayResult pic s9(09)v99. PROCEDURE DIVISION. invoke ClassComputeFormula "NEW" returning computeFormula invoke ClassContext "NEW" returning context invoke computeFormula "SetExpression" using "((a ^ b) + c) * d" invoke context "CreateVariable" using "a", 10 invoke context "CreateVariable" using "b", 2 invoke context "CreateVariable" using "c", 2 invoke context "CreateVariable" using "d", 8.5 invoke computeFormula "SetContext" using context invoke computeFormula "Evaluate" returning result move result to displayResult display displayResult END PROGRAM MAIN.
What else can I do with that Pattern?
You name it! Maybe a script language to provide some automation support in your application. Maybe you could create basic Cobol evaluators. That would be something really cool! There are no limits!
Download the source
Cool things with OO Cobol - Source Code
Some references in the web
Interpreter .NET Design Pattern in C# and VB - dofactory.com
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