java - 使用 Scanner 或 Buffer 读取 .txt 文件后在 Java 中操作表数据

Question

我正在尝试操作 Java .txt 文件中包含的波浪号分隔表中的行和列数据。我已成功扫描/读取数据，但不确定如何操作它来镜像关系代数的操作，然后将输出作为波浪号分隔表(如原始文件一样)写入新的 .txt 文件，以便可以通过其他方法(如 project())读取。例如，我想创建一个 restrict() 方法，将输出限制为仅丰田汽车，因此驱动程序的主体可能如下所示:

//restrict the cars table to toyotas producing a table named toyotas    
Algebra.Restrict("cars","MAKE='Toyota'","toyotas");  

//project just three columns from the toyotas table producing a table named answer    
Algebra.Project("Toyotas","Make,Model,Price","answer");  

//display the contents of the answer table    
Algebra.Display("answer");

输出为:

MAKE MODEL PRICE
---------------- 
Toyota Camry 18000    
Toyota Tacoma 19000    
Toyota Highlander 35000 

来自 cars.txt 的输入

MAKE~MODEL~TYPE~PRICE

Toyota~Camry~Sedan~18000

Toyota~Tacoma~Truck~19000

Ford~Mustang~Sport~21000

Chevrolet~Corvette~Sport~48000

Ford~F150~Truck~25000

Toyota~Highlander~SUV~35000

对 algebra.Restrict("Ford", "", "Truck"); 使用以下代码:

   public void Restrict(String a, String b, String c )throws FileNotFoundException, IOException{
    {
        Scanner x = null;
        try
        {
            x = new Scanner(new File("cars.txt"));
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
        ArrayList<String[]> arr = new ArrayList<String[]>();
        while (x.hasNext())
        {
            String str[] = x.next().split("~");
            arr.add(str);
        }

        for (String[] column : arr)
        {
            if (column[0].equals(a))
            {System.out.println(column[0] + " " + column[1] + " " + column[2]);

                // save table to disk     
                PrintWriter outputfile = new PrintWriter("RestrictTable.txt");
                outputfile.print(column[0] + " " + column[1] + " " + column[2]);
                outputfile.close(); 

            }


        }
        for (String[] column : arr)
        {
            if (column[1].equals(b))
            {System.out.println(column[0] + " " + column[1] + " " + column[2]);

                // save table to disk     
                PrintWriter outputfile = new PrintWriter("RestrictTable.txt");
                outputfile.print(column[0] + " " + column[1] + " " + column[2]);
                outputfile.close(); 

            }



        }
        for (String[] column : arr)
        {
            if (column[2].equals(c))
            {System.out.println(column[0] + " " + column[1] + " " + column[2]);

                // save table to disk     
                PrintWriter outputfile = new PrintWriter("RestrictTable.txt");
                outputfile.print(column[0] + " " + column[1] + " " + column[2]);
                outputfile.close(); 

            }


        }
                        System.out.println("NEW TABLE SAVE TO DISK");




    }


    }
    public void Project(String a)throws FileNotFoundException, IOException{
        {
        Scanner x = null;
        try
        {
            x = new Scanner(new File("cars.txt"));
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
        ArrayList<String[]> arr = new ArrayList<String[]>();
        while (x.hasNext())
        {
            String str[] = x.next().split("~");
            arr.add(str);
        }

        for (String[] car : arr)
        {
            if (car[0].equals(a))
            {
                System.out.println(car[0] + " " + car[1] + " " + car[2]);

                // save table to disk     
                PrintWriter outputfile = new PrintWriter("ProjectTable.txt");
                outputfile.print(car[0] + " " + car[1] + " " + car[2]);
                outputfile.close(); 

            }

        }
                        System.out.println("NEW TABLE SAVED TO DISK / ProjectTable.txt");




    }


    }

我得到的输出:

Ford Mustang Sport
Ford F150 Truck
Toyota Tacoma Truck
Ford F150 Truck
NEW TABLE SAVE TO DISK

所需输出

MAKE~MODEL~TYPE
Ford~F150~Truck
NEW TABLE SAVE TO DISK

或者至少

Ford F150 Truck
NEW TABLE SAVE TO DISK

Answer 1

第1步)将数据读入变量并创建数据对象。最简单的是数组和列表，如果您创建汽车类并创建键值对会更好，但为了简单起见，这里是一个潜在的解决方案。

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;

public class cars
{

    public static void main(String[] args)
    {
        Scanner x = null;
        try
        {
            x = new Scanner(new File("cars.txt"));
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
        ArrayList<String[]> arr = new ArrayList<String[]>();
        while (x.hasNext())
        {
            String str[] = x.next().split("~");
            arr.add(str);
        }

        for (String[] car : arr)
        {
            if (car[0].equals("Toyota"))
            {
                System.out.println(car[0] + " " + car[1] + " " + car[2] + " " + car[3]);
            }
        }
    }
}

输出:

Toyota Camry Sedan 18000
Toyota Tacoma Truck 19000
Toyota Highlander SUV 35000

我在 5 分钟内完成了此操作，但您可以轻松地将其用作模板来创建方法restrict。模板类似于。

private boolean restrictBy (String make){
  if make.equals("Totota") {
    return true;
  } else {
    return false;
  }
}

编辑:

File outFile = new File ("output.txt");
FileWriter fWriter = null;
try { fWriter = new FileWriter (outFile); }
catch (IOException e) { e.printStackTrace(); }
PrintWriter pWriter = new PrintWriter (fWriter);
for (String[] car : arr)
{
    if (restrictBy(car, "Ford" , "", "Truck"))
    {
        System.out.println(car[0] + " " + car[1] + " " + car[2] + " " + car[3]);
        pWriter.println(car[0] + " " + car[1] + " " + car[2] + " " + car[3]);
    }
}
pWriter.close();    

和方法restrictBy

private static boolean restrictBy (String[] car, String make, String model, String type) {
    boolean filtered = true;
    if (make.length() > 0 && !car[0].equals(make))
    {
        filtered = false;
    }
    if (model.length() > 0 && !car[1].equals(model))
    {
        filtered = false;
    }
    if (type.length() > 0 && !car[2].equals(type))
    {
        filtered = false;
    }
    return filtered;
}