嘗試從文本文件中讀取公式、存儲公式行、計算公式並將答案寫入新文件

Question

您好，我遇到了以下代碼的問題：它從具有以下內容的文本文件中讀取：

4 * 5
3 / 4
3 - 1
2 + 3

我特別堅持使用聲明 ArrayList of 用於存儲字符串行中的標記化公式和確定運算符並計算結果值的評論。

import java.io.*;
import java.util.*;
public class ReadFileLineByLine {
 public static void main(String[] args) throws FileNotFoundException {
 String line;
 Scanner input = null;
 PrintWriter output = null;

 try {

 //open file for reading the calculated formulas "formulas.txt"
 input = new Scanner(new File("C:\\formulas.txt"));
 //open file for storing the calculated formulas "results.txt"
 output = new PrintWriter(new File("C:\\results.txt"));

 // read one line at a time 
 while( input.hasNextLine()) {
 line = input.nextLine();

 System.out.println("read <" + line + ">"); // Display message to commandline
 // Declare ArrayList of for storing tokenized formula from String line

 double result = 0; // The variable to store result of the operation
 // Determine the operator and calculate value of the result
 System.out.println(formula.get(0) + ' ' + formula.get(1) + ' ' +
 formula.get(2) + " = " + result); // Display result to command line
 // Write result to file
 output.println("Print result of " + line + " to Results.txt");
 }
 // Need to close input and output files

 input.close();
 output.close();

 }
 catch (FileNotFoundException e) {
 // Display meaningful error message
  System.out.println("File Not Found: " + e.getMessage());
 }
 }
}

如果有人能提出決定這些評論的代碼，我將不勝感激！

Answer 1

如果您打算逐行評估表達式，那么我們可以使用ScriptEngine

您可以從這些帖子中獲得詳細信息

我修改了你的樣本如下。 請看一看。

public static void main(String[] args) {
    String line;
    Scanner input = null;
    PrintWriter output = null;
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("js");
    try {

        // open file for reading the calculated formulas "formulas.txt"
        input = new Scanner(
                new File("/Users/xxx/Downloads/formulas.txt"));
        // open file for storing the calculated formulas "results.txt"
        output = new PrintWriter(new File(
                "/Users/xxx/Downloads/results.txt"));

        // read one line at a time
        while (input.hasNextLine()) {
            line = input.nextLine();
            Object result = null;
            try {
                result = engine.eval(line);
            } catch (ScriptException e) {
                e.printStackTrace();
            }
            // Write result to file
            output.println(line + " = " + result);
        }
        // Need to close input and output files

        input.close();
        output.close();

    } catch (FileNotFoundException e) {
        // Display meaningful error message
        System.out.println("File Not Found: " + e.getMessage());
    }
}

輸出如下

4 * 5 = 20
3 / 4 = 0.75
3 - 1 = 2
2 + 3 = 5

Answer 2

這段代碼有點亂，它並沒有避免空格，但在這里。

import java.io.*;
import java.util.*;
public class ReadFromFile {
    public static void main(String[] args) throws FileNotFoundException {
        String line;
        Scanner input = null;
        PrintWriter output = null;
        try
        {
            //open file for reading the calculated formulas "formulas.txt"
            input = new Scanner(new File("C:\\formulas.txt"));
            //open file for storing the calculated formulas "results.txt"
            output = new PrintWriter(new File("C:\\results.txt"));

            // read one line at a time 
            while( input.hasNextLine())
            {
                line = input.nextLine();
                System.out.println("read <" + line + ">"); // Display message to commandline
                //toString("4 * 5".split("\\s+")
                // Declare ArrayList of for storing tokenized formula from String line
                ArrayList<String> formula = new ArrayList<String>();
                for (int i = 0; i < line.length(); i++)
                {
                    formula.add(String.valueOf(line.charAt(i)));
                }
                double result = 0; // The variable to store result of the operation
                // Determine the operator and calculate value of the result
                int firstNum = Integer.parseInt(formula.get(0));
                int secondNum = Integer.parseInt(formula.get(4));
                char operator = formula.get(2).charAt(0);
                result = (operator == '+' ? firstNum + secondNum
                        : operator == '-' ? firstNum - secondNum
                                : operator == '*' ? firstNum * secondNum
                                        : operator == '/' ? firstNum / secondNum : 0);
                System.out.println(formula.get(0) + ' ' + formula.get(2) + ' ' +
                        formula.get(4) + " = " + result); // Display result to command line
                // Write result to file
                output.println("Print result of " + line + " to Results.txt");
            }
            // Need to close input and output files
            input.close();
            output.close();
        }
        catch (FileNotFoundException e)
        {
            // Display meaningful error message
            System.out.println("File Not Found: " + e.getMessage());
        }
    }
}