问题
I have a java class that converts infix expression to postfix. I have managed to make the class run without errors but it is not giving me the right output. The output is suppose to be in form :
InToPost: converting expressions from infix to postfix...
[A, +, B]
[A, +, B, *, C]
[A, +, B, *, C, +, D]
[(, A, +, B, ), *, C]
[(, A, +, B, ), /, (, C, -, D, )]
[(, (, A, +, B, ), *, C, -, (, D, -, E, ), )]
InToPost: emitting postfix expressions...
A B +
A B C * +
A B C * + D +
A B + C *
A B + C D - /
A B + C * D E - -
But my output is:
InToPost: converting expressions from infix to postfix... [A, +, B]
[A, +, B, *, C]
[A, +, B, *, C, +, D]
[(, A, +, B, ), *, C]
[(, A, +, B, ), /, (, C, -, D, )]
[(, (, A, +, B, ), , C, -, (, D, -, E, ), )]
InToPost: emitting postfix expressions... A+B
A+BC
A+B*C+D
(A+B)*C
(A+B)/(C-D)
((A+B)*C-(D-E))
The convert method is as follows:
private List<String> convert(List<String> tokens) {
// YOUR IMPLEMENTATION HERE
StackNode opstack = new StackNode();
//List<Object> postfix = new ArrayList<Object>();
//int sizeOfList=tokens.size();
String postfixString = " ";
for (int index = 0; index < tokens.size(); ++index) {
String chValue = tokens.get(index);
if (chValue == "(") {
opstack.push("(");
} else if (chValue == ")") {
String oper = String.valueOf(opstack.top());
while (!(oper.equals("(")) && !(opstack.empty())) {
postfixString += oper;
opstack.pop();
oper = String.valueOf(opstack.top());
}
opstack.pop();
} else if (chValue == "+" || chValue == "-") {
//Stack is empty
if (opstack.empty()) {
opstack.push(chValue);
//current Stack is not empty
} else {
String oper = String.valueOf(opstack.top());
while (!(opstack.empty() || oper.equals(new String("(")) || oper.equals(new String(")")))) {
opstack.pop();
postfixString += oper;
}
opstack.push(chValue);
}
} else if (chValue == "*" || chValue == "/") {
if (opstack.empty()) {
opstack.push(chValue);
} else {
String oper = String.valueOf(opstack.top());
while (!oper.equals(new String("+")) && !oper.equals(new String("-")) && !opstack.empty()) {
opstack.pop();
postfixString += oper;
}
opstack.push(chValue);
}
} else {
postfixString += chValue;
}
}
while (!opstack.empty()) {
String oper = String.valueOf(opstack.top());
if (!oper.equals(new String("("))) {
opstack.pop();
postfixString += oper;
}
}
String[] strValues = postfixString.split(",");
ArrayList<String> postfix = new ArrayList<String>(Arrays.asList(strValues));
return postfix;
}
The whole class code is:
package sr.stu;
import sr.cs.Stack;
import sr.stu.StackNode;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Scanner;
/**
* A program that converts an infix expression to postfix.<br>
* <br>
* Usage: java InToPost filename<br>
* <br>
* For example, prog1.in, containing infix expressions (one per line):<br>
* <br>
* A + B<br>
* A + B * C<br>
* A + B * C + D<br>
* ( A + B ) * C<br>
* ( A + B ) / ( C - D )<br>
* ( ( A + B ) * C - ( D - E ) )<br>
* <br>
* When run: java InToPost prog1.in:<br>
* <br>
* InToPost: converting expressions from infix to postfix...<br>
* [A, +, B]<br>
* [A, +, B, *, C]<br>
* [A, +, B, *, C, +, D]<br>
* [(, A, +, B, ), *, C]<br>
* [(, A, +, B, ), /, (, C, -, D, )]<br>
* [(, (, A, +, B, ), *, C, -, (, D, -, E, ), )]<br>
* InToPost: emitting postfix expressions...<br>
* A B +<br>
* A B C * +<br>
* A B C * + D +<br>
* A B + C *<br>
* A B + C D - /<br>
* A B + C * D E - -<br>
* <br>
*/
public class InToPost{
/** The add operator */
public final static String ADD = "+";
/** The subtract operator */
public final static String SUBTRACT = "-";
/** The multiply operator */
public final static String MULTIPLY = "*";
/** The divide operator */
public final static String DIVIDE = "/";
/** The open parentheses operator */
public final static String OPEN_PAREN = "(";
/** The close parentheses operator */
public final static String CLOSE_PAREN = ")";
/** The list of converted postfix expressions */
private List<List<String>> expressions;
/** The name of the infix source file */
private String srcFile;
/** A dictionary that maps operators (strings) to their precedence level
* (integer) from highest (3) to lowest (1):
* *, /: 3
* +, -: 2
* (: 1
*/
private Map<String, Integer> precedence;
/**
* Create a new IntoPost object.
* @param filename The name of the infix source file
*/
public InToPost(String filename) {
this.expressions = new LinkedList<>();
this.srcFile = filename;
/** populate the precedence map */
this.precedence = new HashMap<>();
this.precedence.put(MULTIPLY, 3);
this.precedence.put(DIVIDE, 3);
this.precedence.put(ADD, 2);
this.precedence.put(SUBTRACT, 2);
this.precedence.put(OPEN_PAREN, 1);
}
/**
* A private utility method that can be used by the private convert
* method when dealing with the precedence of math operators on
* the stack. Indicates whether the top token on stack has greater
* than or equal precedence than the current token. For example:
* greaterEqualPrecedence("+", "*") -> false
* greaterEqualPrecedence("*", "+") -> true
* greaterEqualPrecedence("+", "-") -> true
* greaterEqualPrecedence("(", "/") -> false
*
* @param token the current math token, e.g. "+"
* @param top the top token to compare to, e.g. "*"
* @return true if top has greater than or equal precedence than
* token, false otherwise
*/
private boolean greaterEqualPrecedence(String top, String token) {
return this.precedence.get(top) >= this.precedence.get(token);
}
/**
* A private helper conversion function that takes a list of tokens
* (strings) in infix form and converts them to a list of tokens
* (strings) in postfix form.
*
* For example:
* tokens = ["A", "+", "B"]
* postfix = ["A", "B", "+"]
*
* Note, this method must use either StackList<T> or StackNode<T>
* to represent the stack structure in the conversion algorithm.
*
* @param tokens the list of tokens (strings) in infix form
* @return a new list of tokens (strings) in postfix form
*/
private List<String> convert(List<String> tokens) {
// YOUR IMPLEMENTATION HERE
StackNode opstack = new StackNode();
//List<Object> postfix = new ArrayList<Object>();
//int sizeOfList=tokens.size();
String postfixString = " ";
for (int index = 0; index < tokens.size(); ++index) {
String chValue = tokens.get(index);
if (chValue == "(") {
opstack.push("(");
} else if (chValue == ")") {
String oper = String.valueOf(opstack.top());
while (!(oper.equals("(")) && !(opstack.empty())) {
postfixString += oper;
opstack.pop();
oper = String.valueOf(opstack.top());
}
opstack.pop();
} else if (chValue == "+" || chValue == "-") {
//Stack is empty
if (opstack.empty()) {
opstack.push(chValue);
//current Stack is not empty
} else {
String oper = String.valueOf(opstack.top());
while (!(opstack.empty() || oper.equals(new String("(")) || oper.equals(new String(")")))) {
opstack.pop();
postfixString += oper;
}
opstack.push(chValue);
}
} else if (chValue == "*" || chValue == "/") {
if (opstack.empty()) {
opstack.push(chValue);
} else {
String oper = String.valueOf(opstack.top());
while (!oper.equals(new String("+")) && !oper.equals(new String("-")) && !opstack.empty()) {
opstack.pop();
postfixString += oper;
}
opstack.push(chValue);
}
} else {
postfixString += chValue;
}
}
while (!opstack.empty()) {
String oper = String.valueOf(opstack.top());
if (!oper.equals(new String("("))) {
opstack.pop();
postfixString += oper;
}
}
String[] strValues = postfixString.split(",");
ArrayList<String> postfix = new ArrayList<String>(Arrays.asList(strValues));
return postfix;
}
/**
* The public conversion function that converts all infix expressions
* in the source file (one per line), into postfix expressions.
* @throws FileNotFoundException if the file is not found
*/
public void convert() throws FileNotFoundException {
Scanner in = new Scanner(new File(this.srcFile));
while (in.hasNext()) {
// convert the line into a list of strings, e.g.
// line = "A B +"
// tokens = ["A", "B", "+"]
List<String> tokens = Arrays.asList(in.nextLine().split(" "));
// if the line isn't empty, convert it to postfix and add it to
// the list of expressions
if (tokens.size() > 0) {
System.out.println(tokens);
this.expressions.add(convert(tokens));
}
}
}
/**
* Display the converted postfix expressions.
*/
public void emit() {
for (List<String> expression : this.expressions) {
for (String token : expression) {
System.out.print(token + " ");
}
System.out.println();
}
}
/**
* The main program takes the source file of infix expressions, converts
* them to postfix, and displays them.
* @param args command line arguments
* @throws FileNotFoundException if the source file is not found
*/
public static void main(String[] args) throws FileNotFoundException {
if (args.length != 1) {
System.out.println("Usage: java InToPost.py source-file.in");
return;
}
InToPost inToPost = new InToPost(args[0]);
System.out.println("InToPost: converting expressions from infix to postfix...");
inToPost.convert();
System.out.println("InToPost: emitting postfix expressions...");
inToPost.emit();
}
}
回答1:
Here is a possibility. You could analyze this code and find the error(s) in yours.
public class InToPost {
private Stack theStack;
private String input;
private String output = "";
public InToPost(String in) {
input = in;
int stackSize = input.length();
theStack = new Stack(stackSize);
}
public String doTrans() {
for (int j = 0; j < input.length(); j++) {
char ch = input.charAt(j);
switch (ch) {
case '+':
case '-':
gotOper(ch, 1);
break;
case '*':
case '/':
gotOper(ch, 2);
break;
case '(':
theStack.push(ch);
break;
case ')':
gotParen(ch);
break;
default:
output = output + ch;
break;
}
}
while (!theStack.isEmpty()) {
output = output + theStack.pop();
}
System.out.println(output);
return output;
}
public void gotOper(char opThis, int prec1) {
while (!theStack.isEmpty()) {
char opTop = theStack.pop();
if (opTop == '(') {
theStack.push(opTop);
break;
}
else {
int prec2;
if (opTop == '+' || opTop == '-')
prec2 = 1;
else
prec2 = 2;
if (prec2 < prec1) {
theStack.push(opTop);
break;
}
else
output = output + opTop;
}
}
theStack.push(opThis);
}
public void gotParen(char ch){
while (!theStack.isEmpty()) {
char chx = theStack.pop();
if (chx == '(')
break;
else
output = output + chx;
}
}
public static void main(String[] args)
throws IOException {
String input = "1+2*4/5-7+3/6";
String output;
InToPost theTrans = new InToPost(input);
output = theTrans.doTrans();
System.out.println("Postfix is " + output + '\n');
}
class Stack {
private int maxSize;
private char[] stackArray;
private int top;
public Stack(int max) {
maxSize = max;
stackArray = new char[maxSize];
top = -1;
}
public void push(char j) {
stackArray[++top] = j;
}
public char pop() {
return stackArray[top--];
}
public char peek() {
return stackArray[top];
}
public boolean isEmpty() {
return (top == -1);
}
}
}
Source: http://www.tutorialspoint.com/javaexamples/data_intopost.htm
来源：https://stackoverflow.com/questions/35914209/infix-to-postfix-conversion-in-java