How to use string tokenizer when reading in from a file?

问题

I am implementing a RPN calculator in Java and need help creating a class to parse the equations into separate tokens.

My input file will have an unknown number of equations similar to the ones shown below:

49+62*61-36
4/64
(53+26)
0*72
21-85+75-85
90*76-50+67
46*89-15
34/83-38
20/76/14+92-15

I have already implemented my own generic stack class to be used in the program, but I am now trying to figure out how to read data from the input file. Any help appreciated.

I've posted the source code for my stack class at PasteBin, in case it may help.

I have also uploaded the Calculator with no filereading to PasteBin to show what I have done already.

I have now managed to get the file read in and the tokens broken up thanks for the help. I am getting an error when it reaches the end of the file and was wondering how to solve that?

Here is the code:

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.StringTokenizer;

 public class TestClass {    
  static public void main(String[] args) throws IOException {
 File file = new File("testEquations.txt");
  String[] lines = new String[10];
  try {
    FileReader reader = new FileReader(file);
    BufferedReader buffReader = new BufferedReader(reader);
    int x = 0;
    String s;
    while((s = buffReader.readLine()) != null){
        lines[x] = s;
        x++;
    }
 }
  catch(IOException e){
    System.exit(0);
}
String OPERATORS = "+-*/()";

for (String st : lines) {
    StringTokenizer tokens = new StringTokenizer(st, OPERATORS, true);
    while (tokens.hasMoreTokens()) {
        String token = tokens.nextToken();
        if (OPERATORS.contains(token))
            handleOperator(token);
        else
            handleNumber(token);
    }
     }
   }

private static void handleNumber(String token) {
System.out.println(""+token);

   }

 private static void handleOperator(String token) {
System.out.println(""+token);

  }
  }

Also How would I make sure the RPN works line by line? I am getting quite confused by the algorithms I am trying to follow.

回答1:

As your question has now changed completely from it's original version - this is in response to your original one, which was how to use FileReader to get the values from your file.

This will put each line into a separate element of a String array. You should probably use an ArrayList instead, as it's far more flexible, but I have just done this as a quick demo - you can clean it up as you wish, although I notice the code you are using expects a String array as it's input. Perhaps you could read the values initially into an ArrayList, then copy that to an array once you have all the lines - that way you can put as many lines in as you wish and keep your code flexible for changes in the number of lines in your input file.

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class TestClass {    
  static public void main(String[] args) {
    File file = new File("myfile.txt");
    String[] lines = new String[10];
    try {
        FileReader reader = new FileReader(file);
        BufferedReader buffReader = new BufferedReader(reader);
        int x = 0;
        String s;
        while((s = buffReader.readLine()) != null){
            lines[x] = s;
            x++;
        }
    }
    catch(IOException e){
        //handle exception
    }
    // And just to prove we have the lines right where we want them..
    for(String st: lines)
    System.out.println(st);
  }
}

You mentioned before that you were using the code on this link:

http://www.technical-recipes.com/2011/a-mathematical-expression-parser-in-java/#more-1658

This appears to already deal with operator precedence doesn't it? And with parsing each String from the array and sorting them into numbers or operators? From my quick look it at least it appears to do that.

So it looks like all you need is for your lines to be in a String array, which you then pass to the code you already have. From what I can see anyway.

Obviously this doesn't address the issue of numbers greater than 9, but hopefully it helps with the first half.

:-)

回答2:

Because all of the operators are single characters, you can instruct StringTokenizer to return them along with the numeric tokens.

String OPERATORS = "+-*/()";
String[] lines = ...

for (String line : lines) {
    StringTokenizer tokens = new StringTokenizer(line, OPERATORS, true);
    while (tokens.hasMoreTOkens()) {
        String token = tokens.nextToken();
        if (OPERATORS.contains(token))
            handleOperator(token);
        else
            handleNumber(token);
    }
}

来源：https://stackoverflow.com/questions/13432094/how-to-use-string-tokenizer-when-reading-in-from-a-file

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java

calculator