how to write this algorthim in c++ plz its important

Question

Algorithm

• Read x1, y1, x2, y2
• Read x3, y3, x4, y4
• Compute vectors r, s, and q
• Compute cross-products qxs, qxr, and rxs
• Compute auxiliary values: q*r, r*r, q*s, and s*s
• If rxs is zero and qxr is zero, then
  • The two lines are collinear
    • If 0 <= q*r <= r*r or 0 <= q*s <= s*s
      • The two lines are overlapping
    • else
      • The two lines are disjoint
• If rxs is zero and qxr is not zero
  • The two lines are parallel and not intersecting
• If rxs is not zero , compute t and u
  • If   0 <= t <= 1 and 0 <= u <= 1
    • The two lines intersect at (x1)
  • else
    • The two line segments do not intersect

Answer 1

What does "compute vectors r, s and q" mean?  Compute is a fairly generic word.  Can you explain how

to computer the vectors?