Java Community | Help. Code. Learn.•3y ago

Polygon Matrix task

34 Replies

Unknown UserOP•3y ago

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public class CalculateArea {

    public static void main(String[] args) {
        // bi-dimensional array: coord[4][2];
        float[][] coord = {{0f, 0f}, {2f, 0f}, {2f, 2f}, {0f, 2f}};
        
        float area = calculateArea(coord);
        
        showArea(area);
    }
    
    public static float calculateArea (float[][] coord) {
        float area = 0;
        //(x0+x1)(y1-y0) + (x1+x2)(y2-y1) + (x2+x3)(y3-y2) + (x3+x0)(y0-y3);
        area = ((coord[0][0]+coord[1][0])*(coord[1][1]-coord[0][1]) +
                (coord[1][0]+coord[2][0])*(coord[2][1]-coord[1][1]) +
                (coord[2][0]+coord[3][0])*(coord[3][1]-coord[2][1]) +
                (coord[3][0]+coord[0][0])*(coord[0][1]-coord[3][1])
               )/2;
        return area;
    }
    
    public static void showArea (float area) {
        System.out.print("The area is: "+area);
    }
}

public class CalculateArea {

    public static void main(String[] args) {
        // bi-dimensional array: coord[4][2];
        float[][] coord = {{0f, 0f}, {2f, 0f}, {2f, 2f}, {0f, 2f}};
        
        float area = calculateArea(coord);
        
        showArea(area);
    }
    
    public static float calculateArea (float[][] coord) {
        float area = 0;
        //(x0+x1)(y1-y0) + (x1+x2)(y2-y1) + (x2+x3)(y3-y2) + (x3+x0)(y0-y3);
        area = ((coord[0][0]+coord[1][0])*(coord[1][1]-coord[0][1]) +
                (coord[1][0]+coord[2][0])*(coord[2][1]-coord[1][1]) +
                (coord[2][0]+coord[3][0])*(coord[3][1]-coord[2][1]) +
                (coord[3][0]+coord[0][0])*(coord[0][1]-coord[3][1])
               )/2;
        return area;
    }
    
    public static void showArea (float area) {
        System.out.print("The area is: "+area);
    }
}

Unknown UserOP•3y ago

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Tomasm21•3y ago

To calculate not only square, but any polygon area, the function is:

public static float calculateAnyPolygonArea (float[][] coord) {
    int side_num = coord.length;
    float area = 0;
    int k = 0;
    
    for(int i = 0; i < side_num; i++) {
        k = i+1;
        if(k >= side_num) k=0;
        area = area + (coord[i][0]+coord[k][0])*(coord[k][1]-coord[i][1]);
    }    
    return area/2;
}

public static float calculateAnyPolygonArea (float[][] coord) {
    int side_num = coord.length;
    float area = 0;
    int k = 0;
    
    for(int i = 0; i < side_num; i++) {
        k = i+1;
        if(k >= side_num) k=0;
        area = area + (coord[i][0]+coord[k][0])*(coord[k][1]-coord[i][1]);
    }    
    return area/2;
}

It works only if you assign adjacent points coordinates into the array one after the other consistently. Do not mix points coordinates chaotically.

Unknown UserOP•3y ago

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Tomasm21•3y ago

You're welcome

Unknown UserOP•3y ago

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Tomasm21•3y ago

instead of 0 and 2? or why 0 and 2? f because type is float

Tomasm21•3y ago

https://www.javatpoint.com/float-keyword-in-java#:~:text=The%20Java%20float%20keyword%20is%20a%20primitive%20data,range%20from%201.40129846432481707e-45%20to%203.40282346638528860e%2B38%20%28positive%20or%20negative%29.

www.javatpoint.com

Java Float Keyword - Javatpoint

Java Float Keyword with java keywords, java tutorial, features, history, variables, object, class, programs, operators, oops concept, inheritance, array, string, methods, examples etc.

Unknown UserOP•3y ago

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Tomasm21•3y ago

just an example you can write any numbers you like but be sure that in the end it will be polygon

Unknown UserOP•3y ago

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Tomasm21•3y ago

yes area will be 1 if 4 points square but you can use any polygon coordinates.

Tomasm21•3y ago

for example from this picture:

Tomasm21•3y ago

float[][] coord = {{2f, 3f}, {4f, -4f}, {-3f, -2f}, {-4f, 4f}}; in this example the area will be -40.0 minus but this could be fixed by: if (area < 0) area = -area;

public static float calculateAnyPolygonArea (float[][] coord) {
        int side_num = coord.length;
        float area = 0;
        int k = 0;
        
        for(int i = 0; i < side_num; i++) {
            k = i+1;
            if(k >= side_num) k=0;
            area = area + (coord[i][0]+coord[k][0])*(coord[k][1]-coord[i][1]);
        }
        if (area < 0) area = -area;
        return area/2;
    }

public static float calculateAnyPolygonArea (float[][] coord) {
        int side_num = coord.length;
        float area = 0;
        int k = 0;
        
        for(int i = 0; i < side_num; i++) {
            k = i+1;
            if(k >= side_num) k=0;
            area = area + (coord[i][0]+coord[k][0])*(coord[k][1]-coord[i][1]);
        }
        if (area < 0) area = -area;
        return area/2;
    }

Unknown UserOP•3y ago

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Tomasm21•3y ago

@YavuzSultanEnes An example from website: https://www.mathopenref.com/coordpolygonarea.html float[][] coord = {{4f, 10f}, {9f, 7f}, {11f, 2f}, {2f, 2f}}; area will be 45.5

Unknown UserOP•3y ago

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Tomasm21•3y ago

yes

Unknown UserOP•3y ago

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Tomasm21•3y ago

from our example about square yes x would be rows y columns

Tomasm21•3y ago

Here is another example:

Tomasm21•3y ago

float[][] coord = {{3f, 3f}, {4f, 2f}, {3f, 1f}, {2f, -1f}, {1f, 1f}, {-1f, 2f}}; area will be 8.0 On the Internet there is another solution similar, but a bit fifferent:

Tomasm21•3y ago

https://www.geeksforgeeks.org/area-of-a-polygon-with-given-n-ordered-vertices/

GeeksforGeeks

Area of a polygon with given n ordered vertices - GeeksforGeeks

A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions.

Unknown UserOP•3y ago

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Tomasm21•3y ago

with two arrays but not bidimensional array when we say point 2 out of 4 coord 2 it's index is 1 because if you have 4 length array, it's indexes are 0, 1, 2, 3

Unknown UserOP•3y ago

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Tomasm21•3y ago

1st is 0, 2nd - 1, 3rd - 2, 4th - 3 3 is Fourth point 4th vertex or point

Tomasm21•3y ago

@YavuzSultanEnes

Tomasm21•3y ago

coord[n][0] - X coordinate, coord[n][1] - Y coordinate 0 0 2 0 2 2 0 2 square matrix float[][] coord = {{0f, 0f}, {2f, 0f}, {2f, 2f}, {0f, 2f}};

Unknown UserOP•3y ago

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Tomasm21•3y ago

A square as a polygon has 4 vertices. Each vertex has two coordinates - X coordinate and Y coordinate.

Unknown UserOP•3y ago

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JavaBot•3y ago

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