Python: Weird for loop behavior

Question

I'm trying to solve this: CodeEval .

The problem requires me to go through a list of possible candidates of points in a XY-coordinates. Then if they fulfill the requirements I add them to a "confirmed" list and then add the surrounding points to a "tosearch" list. However this does not behave at all the way I expect it to behave.

Example code:

Starting point
tosearch=[[0,0]] 

for point in tosearch:

    if conditions filled:
        confirmed.append(point)
        #Basically Im trying to add (x,y-1) etc. to the tosearct list
        tosearch.append([point[0],point[1]-1])  #1 
        tosearch.append([point[0]+1,point[1]])  #2
        tosearch.append([point[0]-1,point[1]-1])#3
        tosearch.append([point[0],point[1]+1])  #4
        tosearch.remove(point)
else:
     tosearch.remove(point)

This seems to result in always ignoring half of the appends. So in this case #1 and #3 are being ignored. If I left only 1&2 then only 2 would execute. I dont get it...

Maybe the problem is else where so here is the whole code: Pastebin

Answer 1

You're modifying the collection while iterating over it.
2 options:

1. copy the list, iterate the copy, and alter the original.
2. keep track of what changes need to be made, and make them all at after iterating.

Answer 2

The problem is you are modifying tosearch in the body of the loop that iterates tosearch . Since tosearch is changing, it can't be iterated reliably.

You probably don't need to iterate at all. Just use a while loop:

searched = set() # if you need to keep track of searched items
tosearch = [(0,0)] #use tuples so you can put them in a set
confirmed = []

while tosearch:
    point = tosearch.pop()
    searched.add(point) # if you need to keep track
    if CONDITIONS_MET:
        confirmed.append(point)
        # tosearch.append() ....