Question

我正在尝试实现一个函数来计算多边形的面积。这段代码允许你绘制一个多边形但是当它显示该区域时，我不知道该怎么做。我试过几个方面，但我仍然是编程的初学者，所以我很感激任何帮助。这是代码：

import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.*;
import javax.swing.*;
import java.awt.Color;
import java.awt.Polygon;

public class DrawPolygons extends JApplet implements ActionListener, MouseListener
{
private static final int NUMPOINTS = 500; //Up to 500 points can be chosen

private JButton finish;                   //Button to indicate user is done entering     points
private Polygon shape;                    //polygon object to be drawn
private boolean isDrawn;                  //boolean flag for when the user is finished drawing
private int count;                        //how many points the user has clicked
private Color color;                      //color of the polygon after user finalizes points
private int[] x;                          //x coordinates of each point user picks
private int[] y;                          //y coordinates of each point user picks
private float sum;
private double area;



public void init()                        //set up GUI
{
    setLayout(new FlowLayout());

    addMouseListener(this);               //adds MouseListener for mouse clicks

    isDrawn = false;                      //isDrawn is initially false
    count = 0;                            //count starts as 0
    x = new int[NUMPOINTS];               //allows for up to 500 points to be chosen
    y = new int[NUMPOINTS];               //allows for up to 500 points to be chosen

    finish = new JButton("Finalize points");    //creates finish button
    finish.addActionListener(this);             //adds finish button to ActionListener
    add(finish);                                //adds finish button to GUI

    color = Color.BLACK;                //color is intially black, and will remain 
                                        //  so if user cancels the color chooser

    //JOptionPane.showMessageDialog(null, "Click points that will make up the polygon. After each" +    
    //                                    "point is entered, press the Finalize Points button");
    shape = new Polygon();              //creates the Polygon shape



}



 public void paint(Graphics g)          //draws the Polylines, Polygons, and sets the color
{            
    super.paint(g); 

    g.drawPolyline(x, y, count);        //draws the polyline specified by user
                                        //   mouseclick
    g.setColor(color);                  //sets the color to the user chosen color

    if(isDrawn)                         //if finalize button is clicked
    {

        g.fillPolygon(x, y, count);     //finalizes polylines into


    }                                   //   polygon and fills shape
}

  public void actionPerformed(ActionEvent a)  //decides what to do when finalize button is pressed
{
    if(a.getSource() == finish)             //if the finalize button is pressed
    {
        isDrawn = true;                     //isDrawn is set to true, ending the users ability
                                            //  to add more points, and fills the polygon
        color = Color.red; 


        //JColorChooser.showDialog(this, "Choose a color", color); //color is set to users choice
        repaint();    

      JOptionPane.showMessageDialog(null, "The area is ");
    }
 }

 public void mouseClicked(MouseEvent e)        //save coordinates of clicks
 { 


    if(isDrawn == false && count < NUMPOINTS) //if the finalize button is not pressed
    {                                         //  the user can add additional points
        x[count] = e.getX();                  //adds the x point at the current mouse x coordinate
        y[count] = e.getY();                  //adds the y point at the current mouse y coordinate
        count++;                              //count increases with each mouse click
        repaint();

    }

    else if (e.isShiftDown()) {
             // Clear the applet. (This only requires a repaint.)
             // Also, set count to zero to start a new polygon.
          count = 0;
          isDrawn = false;
          repaint();
                    } 


    }

     private float getPolygonArea(int[] x, int[] y, int count)
    {
      float sum_but_no_result=0;

     for(int i=0;i<(count-1);i++)      // count is point number of polygon
      {
        sum_but_no_result+=x[i]*y[i+1] + y[i]*x[i+1];
        }
       sum_but_no_result+=x[count-1]*y[0] + y[count-1]*x[0];

       float sum = (float)Math.abs(sum_but_no_result) / 2.0f;
     return sum;
     }


//Empty Implementation provided here so we can implement MouseListener (needed because we must
//  provide concrete forms of all methods of an interface to implement it

  public void mousePressed(MouseEvent e){};
  public void mouseReleased(MouseEvent e){};
  public void mouseEntered(MouseEvent e){};
  public void mouseExited(MouseEvent e){};    

  }

Answer 1

float x[N],y[N]; // point coordinates as x1,y1   x2,y2 ....
...
  ...
  ...
...
float sum_but_no_result=0;

for(int i=0;i<(N-1);i++) // N is point number of polygon
{
      sum_but_no_result+=x[i]*y[i+1] + y[i]*x[i+1];
}
sum_but_no_result+=x[N-1]*y[0] + y[N-1]*x[0];

float sum= (float)Math.abs(sum_but_no_result) / 2.0f;

http://www.mathopenref.com/coordpolygonarea.html

对于自相交多边形，您可以添加交叉坐标查找器和多边形加法器算法，以查找通过交叉递归生成的所有子多边形。

Answer 2

在Kotlin中，使用java.awt.Polygon上的扩展名：

df %>%
  rowwise() %>%
  do( (.) %>% as.data.frame %>% 
  mutate(mean = mean(.[. != 4], na.rm = TRUE)))

在Java中绘制多边形并计算面积

2 个答案: