java - How to find value of prefix expression using a recursive method? -


i have following pseudocode , need write java method evaluate prefix expression:

algorithm valueofprefixexpression(prefixexpression)
input: valid positive-integer arithmetic expression in prefix form
return value: value of prefix expression

if next token integer
return integer
else
read operator, op
firstoperand gets valueofprefixexpression(remainingexpression)
secondoperand gets valueofprefixexpression(remainingexpression)
return firstoperand op secondoperand
endif

how can write method? tried , think right getting "missing return statement" error can't compile. assume method called if args has 1 or more elements. (no empty arrays)

public static int prefixalgorithm(string[] args) {     (int = 0; < args.length; i++) {         if (!args[i].equals("+") && !args[i].equals("-")                 && !args[i].equals("*") && !args[i].equals("/")) {             int operand = parseint(args[i]);             return operand;         } else {             int firstoperand = prefixalgorithm(arrays.copyofrange(args, i, (args.length - 1)));             int secondoperand = prefixalgorithm(arrays.copyofrange(args, i, (args.length - 1)));             if (args[i].equals("+")) {                 return firstoperand + secondoperand;             } else if (args[i].equals("-")) {                 return firstoperand - secondoperand;             } else if (args[i].equals("*")) {                 return firstoperand * secondoperand;             } else if (args[i].equals("/")) {                 return firstoperand / secondoperand;             }         }     } }

prefixevaluator input , scanner:

import java.util.*;  public class prefixevaluator {     public static void main(string[] args) {         scanner console = new scanner(system.in);         system.out.println("this program evaluates prefix expressions");         system.out.println("for operators +, -, *, / , %");         system.out.print("expression? ");         system.out.println("value = " + evaluate(console));     }      // pre : input contains legal prefix expression     // post: expression consumed , result returned     public static double evaluate(scanner input) {         if (input.hasnextdouble()) {             return input.nextdouble();         } else {             string operator = input.next();             double operand1 = evaluate(input);             double operand2 = evaluate(input);             return evaluate(operator, operand1, operand2);         }     }      // pre : operator 1 of +, -, *, / or %     // post: returns result of applying given operator     //       given operands     public static double evaluate(string operator, double operand1,                                   double operand2) {         if (operator.equals("+")) {             return operand1 + operand2;         } else if (operator.equals("-")) {             return operand1 - operand2;         } else if (operator.equals("*")) {             return operand1 * operand2;         } else if (operator.equals("/")) {             return operand1 / operand2;         } else if (operator.equals("%")) {             return operand1 % operand2;         } else {             throw new runtimeexception("illegal operator " + operator);         }     } }

prefixevaluator no input , queue:

import java.util.*;  public class prefixevaluator {     public static void main(string[] args) {         string input = "- * + 4 3 2 5";         string[] expression = input.split ( " " );          queue<string> expressionqueue = new linkedlist<string>();          (string element : expression)         {             expressionqueue.add ( element );         }          system.out.println("value = " + evaluate(expressionqueue));     }      // pre : input contains legal prefix expression     // post: expression consumed , result returned     public static double evaluate(queue <string> input) {         if(input.peek ( ) != null && input.peek ( ).matches ( "^(-?)\\d+$" ))         {             return long.parselong ( input.poll ( ) );         }         else          {             string operator = input.poll();             double operand1 = evaluate(input);             double operand2 = evaluate(input);             return evaluate(operator, operand1, operand2);         }     }      // pre : operator 1 of +, -, *, / or %     // post: returns result of applying given operator     //       given operands     public static double evaluate(string operator, double operand1,                                   double operand2) {         if (operator.equals("+")) {             return operand1 + operand2;         } else if (operator.equals("-")) {             return operand1 - operand2;         } else if (operator.equals("*")) {             return operand1 * operand2;         } else if (operator.equals("/")) {             return operand1 / operand2;         } else if (operator.equals("%")) {             return operand1 % operand2;         } else {             throw new runtimeexception("illegal operator " + operator);         }     } }

i/o example:

this program evaluates prefix expressions

for operators +, -, *, / , %

expression? - * + 4 3 2 5

value = 9.0

documentation:

queues: queue (java platform se 7 )

patterns: pattern (java platform se 7 )


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