Java Reading Lines and Doing Math Equations

Question

So I have this project to do, that I need to read a text file named Input, and I'm doing it like this:

    public static void textParser() {
    File inputFile = new File("Input.txt");
    try {
        BufferedReader br = new BufferedReader(new FileReader(inputFile));
        String inputsText;
        while ((inputsText = br.readLine()) != null) {
            System.out.println(inputsText);
        }
        br.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

and it works. Inside of Input.txt, it shows:

The first line (6) will always be the amount of equations to-do, can be different than 6. Then I have to do how many equations the first line says to, how would I go on doing that? Thanks!

First thing you should change is to read your file into an ArrayList so that you can store the String values somewhere less temporary than the standard output. — Kon, Jul 19 '13 at 17:52
@Kon no justification is provided to use the ArrayList, why would you suggest that? — Woot4Moo, Jul 19 '13 at 17:55
He intends to perform operations on the data inside the file. He's reading the file but printing it directly to sdout. — Kon, Jul 19 '13 at 17:58
@Kon sure but i dont think arraylist is the correct structure. — Woot4Moo, Jul 19 '13 at 18:07
For the calculations, I'd use the `ScriptEngine`, [e.g.](http://stackoverflow.com/questions/7441625/how-to-find-a-button-source-in-awt-calculator-homework/7441804#7441804) — Andrew Thompson, Jul 19 '13 at 18:26

score 3 · Answer 1 · answered Jul 19 '13 at 17:54

3

You need to write a parser. Without doing your homework for you this is the pseudo-code that should be sufficient:

for line in ReadFile()  
{  
  for token in split(line,expression)  
  {  
      if token is digit  
         digits.enqueue(token) 
      if token is symbol  
         symbols.enqueue(token)    
  }  
     for element in digits,symbols:   
         applySymbol(firstDigit,secondDigit,symbol)
}

answered Jul 19 '13 at 17:54

Woot4Moo

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Good answer, however you don't account for order of precedence. As this wasn't explicity asked... +1! – William Morrison Jul 19 '13 at 17:58
Hmm, this is confusing.. but thanks for the pseudo, I'll try and figure it out – user2230236 Jul 19 '13 at 18:47
@user2230236 what is confusing about it? – Woot4Moo Jul 19 '13 at 18:49
I just wouldn't know how to do it.. I'm still a beginner, but I want to do this, but just don't know how. I can read the text file, but then I'm just stumped after. – user2230236 Jul 19 '13 at 18:53

William Morrison · Answer 2 · 2013-07-19T19:20:39.400

1

I've solved this problem a couple times in different languages. Look into the Shunting-yard algorithm

Basically you push and pop operators and operands onto a priority queue. You're basically converting infix to post-fix. Once your equation is in post-fix notation its much easier to solve.

If you don't have order of precedence to worry about the problem is much simpler but can still be solved by the same approach.

Edit:

We humans use in fix notation: 3 + 5 - 1 The operators are between the operands.

In Post fix notation looks like this: 3 5 + 1 -

The operators appear after the operands. Equations written this way are easy to evaluate. You just push operands onto a stack, then evaluate the last 2 using the next operator. So here, you'd push 3, and 5 onto a stack. Then you encounter + operator, so you add 3 and 5, get 8. Push 8 onto stack. now you read 1. Push 1 onto stack. Now you read -. Subtract 8 from 1. You get an answer of 7.

The shunting yard algorithm tells you how to convert between infix to post fix.

Good luck!

edited Jul 19 '13 at 19:20

answered Jul 19 '13 at 17:51

William Morrison

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Thanks for the link sir, quite helpful. But could you provide an example in Java? Thanks! – user2230236 Jul 19 '13 at 17:54
1

Yes I could from my PC at home. But I'd rather not solve this assignment for you. It will teach you quite a bit! – William Morrison Jul 19 '13 at 17:55
Hmm I'm still confused and don't know where to start – user2230236 Jul 19 '13 at 18:47
What precisely are you confused about? Do you know the difference between post-fix and in-fix? Did you read about the Shunting yard algorithm? Woot4Moo posted some good pseudocode as a guide. Do you understand data structures, and that you need to store operators and operands in them? – William Morrison Jul 19 '13 at 18:58
I read about Shunting algorithm, and I still don't know the difference between post-fix and in-fix. :\ – user2230236 Jul 19 '13 at 19:10
I explained the difference in the question. – William Morrison Jul 19 '13 at 19:20

score 0 · Answer 3 · answered Jul 19 '13 at 17:57

0

An option would be using ANTLR to generate a parser, this tutorial pretty much covers what you're trying to do

answered Jul 19 '13 at 17:57

Guillaume

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score 0 · Answer 4 · answered Jul 19 '13 at 18:00

First you need to store them in a array of strings

Then get the first element in the array and convert it to an integer.

Based on the integer value the loop has to be iterated. so loop is formed. now you need to start reading the string array from the next index.

For doing the arithmetic operations first you need to have an array of 4 chars '+','-','*','%'

split the string based on the char array. this you can do it as a separate function. since for everytime it needs to get called. for performance i am saying.

Then you will get the two values parsed and their operator which splits them.

now you can perform the arithmetic operations.

thats it you got the required.

score 0 · Answer 5 · answered Jul 20 '13 at 20:34

I have finally figured it out a different way that works, here is how I'm doing it:

    public static void textParser() {
    File inputFile = new File("Input.txt");
    try {
        Scanner scanner = new Scanner(inputFile);
        int numberOfQuestions = Integer.parseInt(scanner.next());
        for (int i = 1; i <= numberOfQuestions; i++) {
            int firstInt = Integer.parseInt(scanner.next());
            String operationSign = scanner.next();
            int secondInt = Integer.parseInt(scanner.next());
            if (operationSign.contains("+")) {
                int answer = firstInt + secondInt;
                System.out.println("Equation " + i + " : " + firstInt
                        + " + " + secondInt + " = " + answer);
            } else if (operationSign.contains("-")) {
                int answer = firstInt - secondInt;
                System.out.println("Equation " + i + " : " + firstInt
                        + " - " + secondInt + " = " + answer);
            } else if (operationSign.contains("/")) {
                int answer = firstInt / secondInt;
                System.out.println("Equation " + i + " : " + firstInt
                        + " / " + secondInt + " = " + answer);
            } else if (operationSign.contains("*")) {
                int answer = firstInt * secondInt;
                System.out.println("Equation " + i + " : " + firstInt
                        + " * " + secondInt + " = " + answer);
            } else if (operationSign.contains("%")) {
                int answer = firstInt % secondInt;
                System.out.println("Equation " + i + " : " + firstInt
                        + " % " + secondInt + " = " + answer);
            }
        }

    } catch (Exception e) {
        e.printStackTrace();
    }
}

Thank you to everyone for helping!

Java Reading Lines and Doing Math Equations

5 Answers5