Question

Let’s say you have a two dimensional plane with 2 points (called a and b) on it represented by an x integer and a y integer for each point.

How can you determine if another point c is on the line segment defined by a and b?

I use python most, but examples in any language would be helpful.

Answer 1

Check if the cross product of (b-a) and (c-a) is 0, as tells Darius Bacon, tells you if the points a, b and c are aligned.

But, as you want to know if c is between a and b, you also have to check that the dot product of (b-a) and (c-a) is positive and is less than the square of the distance between a and b.

In non-optimized pseudocode:

def isBetween(a, b, c):     crossproduct = (c.y - a.y) * (b.x - a.x) - (c.x - a.x) * (b.y - a.y)      # compare versus epsilon for floating point values, or != 0 if using integers     if abs(crossproduct) > epsilon:         return False      dotproduct = (c.x - a.x) * (b.x - a.x) + (c.y - a.y)*(b.y - a.y)     if dotproduct < 0:         return False      squaredlengthba = (b.x - a.x)*(b.x - a.x) + (b.y - a.y)*(b.y - a.y)     if dotproduct > squaredlengthba:         return False      return True