Task: Parsing_RPN to infix conversion




Go: package main

import (
    "fmt"
    "strings"
)

var tests = []string{
    "3 4 2 * 1 5 - 2 3 ^ ^ / +",
    "1 2 + 3 4 + ^ 5 6 + ^",
}

var opa = map[string]struct {
    prec   int
    rAssoc bool
}{
    "^": {4, true},
    "*": {3, false},
    "/": {3, false},
    "+": {2, false},
    "-": {2, false},
}

const nPrec = 9

func main() {
    for _, t := range tests {
        parseRPN(t)
    }
}

func parseRPN(e string) {
    fmt.Println("\npostfix:", e)
    type sf struct {
        prec int
        expr string
    }
    var stack []sf
    for _, tok := range strings.Fields(e) {
        fmt.Println("token:", tok)
        if op, isOp := opa[tok]; isOp {
            rhs := &stack[len(stack)-1]
            stack = stack[:len(stack)-1]
            lhs := &stack[len(stack)-1]
            if lhs.prec < op.prec || (lhs.prec == op.prec && op.rAssoc) {
                lhs.expr = "(" + lhs.expr + ")"
            }
            lhs.expr += " " + tok + " "
            if rhs.prec < op.prec || (rhs.prec == op.prec && !op.rAssoc) {
                lhs.expr += "(" + rhs.expr + ")"
            } else {
                lhs.expr += rhs.expr
            }
            lhs.prec = op.prec
        } else {
            stack = append(stack, sf{nPrec, tok})
        }
        for _, f := range stack {
            fmt.Printf("    %d %q\n", f.prec, f.expr)
        }
    }
    fmt.Println("infix:", stack[0].expr)
}


Java: import java.util.Stack;

public class PostfixToInfix {

    public static void main(String[] args) {
        for (String e : new String[]{"3 4 2 * 1 5 - 2 3 ^ ^ / +",
            "1 2 + 3 4 + ^ 5 6 + ^"}) {
            System.out.printf("Postfix : %s%n", e);
            System.out.printf("Infix : %s%n", postfixToInfix(e));
            System.out.println();
        }
    }

    static String postfixToInfix(final String postfix) {

        class Expression {
            final static String ops = "-+/*^";

            String op, ex;
            int prec = 3;

            Expression(String e) {
                ex = e;
            }

            Expression(String e1, String e2, String o) {
                ex = String.format("%s %s %s", e1, o, e2);
                op = o;
                prec = ops.indexOf(o) / 2;
            }

            @Override
            public String toString() {
                return ex;
            }
        }

        Stack<Expression> expr = new Stack<>();

        for (String token : postfix.split("\\s+")) {
            char c = token.charAt(0);
            int idx = Expression.ops.indexOf(c);
            if (idx != -1 && token.length() == 1) {

                Expression r = expr.pop();
                Expression l = expr.pop();

                int opPrec = idx / 2;

                if (l.prec < opPrec || (l.prec == opPrec && c == '^'))
                    l.ex = '(' + l.ex + ')';

                if (r.prec < opPrec || (r.prec == opPrec && c != '^'))
                    r.ex = '(' + r.ex + ')';

                expr.push(new Expression(l.ex, r.ex, token));
            } else {
                expr.push(new Expression(token));
            }
            System.out.printf("%s -> %s%n", token, expr);
        }
        return expr.peek().ex;
    }
}