Question

To implement depth-first search in a grid, I wrote a function like this

(define (expand dim node visited obs)
  (not-in (neighbor dim obs node) visited))

The function dfs is

(dfs dim obs visited fringe path goal)

The only appearance of expand in dfs is

(expand dim (car fringe) visited obs)

And I called dfs using

(dfs '(5 5) '((2 2) (3 2)) '((2 1)) '((1 1) (3 1)) '((2 1)) '(4 5))

dim is the dimension of the grid, obs is a list of the blocked positions in the grid.

When I call expand by hand it works perfectly. But when it’s called in dfs, it says “expand: expects 1 argument, given 4: ‘(5 5) ‘(1 1) ‘((2 1)) ‘((2 2) (3 2) (3 4) (4 4) (5 4))“

However, if I change the position of arguments of expand, e.g. put obs as the second argument,

(define (expand dim obs node visited)
  (not-in (neighbor dim obs node) visited))

and modify dfs and my callings correspondingly, then the error won’t appear and everything works well. Then next time I launch DrRacket and paste the modified code into it, the same error reappears. If I swap obs back, it works well again…
What on earth is happening here?! Thanks!

Answer 1

expand is an awfully generic name that might have a definition somewhere else. Rename your funciton to dfs-expand and see if this fixes it.