CODEWITHSUNDEEP
c++vectorelementdifference
Marilynchua
Marilynchua

Reputation: 41

C++ calculate difference between two elements in a vector

I have a 2 vectors of size 4 to store coordinates of a shape (square/rectangle). 1st vector is for x and 2nd for y. To find out the area of the shape, I need the difference in their length. How do I find the difference between 2 elements within the same vector? Using square as an example:

vector<int> x(4);
vector<int> y(4);

double Square::computeArea()
{
    int length;
    double area;

    if (x[0] == x[1]) //x coordinates are the same
    {
        length = y[0] - y[1]; //difference in y coordinates to find the length, need help here
    }
    else if (x[1] == x[2]
    {
        length = y[1] - y[2];
    }
    else if ... //repeat

    area = length * length;
    if (area < 0) { area = -area; }
    setArea(area)
    return area;
}

Upvotes: 2

Views: 2897

Answers (2)

ARG
ARG

Reputation: 241

The beauty of C++ and OOP is that you can think more in terms of the problem than how to program it.

If I were in your place I would use std::pair to save the coordinates.

And have a class representing the rectangle.

I am using the distance between point 1 and 2 as length, and point 1 and 4 as width. It may not be the correct approach in all cases but it should show you have to go about programming your function.

using namespace std;

class Rectangle // Class Rectangle
{
public:
    Rectangle(vector<pair<double, double>> neWcoordinates);
    double computeArea();

private:
    vector<pair<double, double>> coordinates; 
};

double Rectangle::computeArea()
{
    double length = sqrt(pow(coordinates[0].first-coordinates[1].first,2)+pow(coordinates[0].second-coordinates[1].second,2)
        );
    double width = sqrt(pow(coordinates[0].first-coordinates[3].first,2)+pow(coordinates[0].second-coordinates[3].second,2));
    return length*width;
}

Upvotes: 0

Bob__
Bob__

Reputation: 12799

If your rectangle has edges which are parallel to the axis, and the points are ordered clockwise (or counterclockwise), you can simply use the first and third element of the arrays:

int yedge, xedge;

xedge = abs(x[0] - x[2]);

if ( xedge == 0 ) //beware, this check works well only for ints!
     return area = 0.0;
else yedge = abs(y[0] - y[2]);

return area = xedge * yedge;

If you have more general convex quadrilaterals use something like this:

int dx20 = x[2] - x[0];
int dy10 = y[1] - y[0];
int dy20 = y[2] - y[0];
int dx10 = x[1] - x[0];
int dy30 = y[3] - y[0];
int dx30 = x[3] - x[0];

area = 0.5*abs(dx20*dy10-dy20*dx10);
area += 0.5*abs(dx20*dy30-dy20*dx30);

Upvotes: 1

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