"Null"s are being added randomly to a list

Question

I am having troubles trying to run the following code. It is meant to calculate a series of rays reflections inside a closed room (2D).
Collision[] is a function that returns the intersection point (ray hit) with the closest object/wall.
p = {10,0} is the source location.
line is a list contains {a,b} where y=ax+b is the ray's line equation.

Code:

path[line_, k_] :=  ( 
  p = {10, 0};
   l = {p};
  line1 = line;
  For[i = 1, i <= k, i = i + 1, 
    p = collision[line1, p];
    AppendTo[l, p];
    If[9.5 <= p[[1]] <= 10.5 && p[[2]] == 15 , Break[], 
     line1 = {-1/line1[[1]], p[[2]] - -1/line1[[1]]*p[[1]]}]
     ]
   l
  )

Calling this function with the needed parameters:
path[{1, -10}, 2]
Returns the output:
{{10 Null, 0}, {20 Null, 10 Null}, {0, 30 Null}}
While the expected output is supposed to be:
{{10, 0}, {20, 10}, {0, 30}}
Where did I go wrong?
Any help will be appreciated!

In p[[2]] - -1/line1[[1]] probably you mean a single - ? — rhermans, May 15 '17 at 16:58
No, I wrote it this way just for the sake of (my own) convenience (look more clear to me in the Traditional form) — kobibo, May 15 '17 at 17:01
Your For loop returns Null that you then multiply with the list l. Did you mean ; l after the For? — rhermans, May 15 '17 at 17:07
Your question may be put on-hold as it seems to be off-topic, i.e it arises from a simple mistake (syntax error, incorrect capitalization, spelling mistake) and is unlikely to help any future visitors, or else it is easily found in the documentation. Don't be discouraged by that cleaning-up policy. Your future good questions are welcome. Learn about common pitfalls here. — rhermans, May 15 '17 at 17:15

score 1 · Accepted Answer · answered May 15 '17 at 17:11

1

rhermans has it right: You forgot a semicolon. What's happening is that the For[...] evaluates to Null. Since you left the ; out before the last l, the result you're getting is the product of Null and l.

answered May 15 '17 at 17:11

garrett mitchener

86
5

"Null"s are being added randomly to a list

1 Answers1