Checking if points are diagonal

Question

Hello I'm having a little trouble on this one aspect in my code that I'm working on. It deals with points and returns true or not if it's diagonal to other points in my arrayList of Points. Here my code so far:

private List<Point> point;

 public void check()
        {
            for (int i = 0; i < point.Count; i++)
            {
               validSpot(i);
            }
        }

Answer 1

One note, your graph has your y upside down from the way an array stores it.

You could treat your indexes as points on a graph. To find what falls on the diagonal use slope intercept form. y = m x + b. Since you only want diagonals, valid slopes are limited to 1 and -1. Then you just have to define the line that passes through the point of interest and test if the point in question satisfies one of the equations for m = -1 or m = 1. Since the slope is known you only need one point to define the line.

public bool func(int[] knownPt,int xTest, int yTest)
{
      //knownPt is int[]{x,y}
      // y = m*x + (yi - xi)
      return yTest== xTest + knownPt[1] - KnownPt[0] || yTest == -xTest +knownPt[1] + KnownPt[0];

 }

As a walk through here is m = -1

 yi = -xi + b

 yi + xi  = b 

 since b = yi + xi

 y = -x + (yi + xi)