8 queen with hill climbing algorithm doesn't return anything?

Question

I know this is kinda long but do you know why my 8 queen algorithm doesn't return anything. I created an empty board, similar neighbour board, and queens (for following where they are) I put one queen for each column and put them into random rows then I calculated total collision and then I put queens at other rows (at the same column) one by one and calculated total collision again. After that, I found the position that would create the minimal collision, and until I run out of positions that I can minimise the collision, and after breaking the first(calculates neighbours collision) loop I break the second loop(resets all queens positions) if collision is 0.

import random
from array import array


board = [[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0]]
neighbour = [[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0]]
queens = [0,0,0,0,0,0,0,0]


def collision_count(column,row):
    coll = 0
   
    for j in range(8):
        if j == row:
            continue
        if board[column][j] == 1 :
            coll += 1
    
    while(column < 7 and row < 7):
        row += 1
        column +=1
        if board[column][row] == 1:
            coll += 1
  
    while(column > 0 and row > 0):
        row -= 1
        column -=1
        if board[column][row] == 1:
            coll += 1
    
    while(column > 0 and row < 7):
        row += 1
        column -=1
        if board[column][row] == 1:
            coll += 1
 
    while(column < 7 and row > 0):
        row -= 1
        column +=1
        if board[column][row] == 1:
            coll += 1
                     
    return coll     

def totalcoll():
    totcoll = 0
    for i in range(8):
       totcoll += collision_count(i,queens[i])
    return totcoll
            
while True:
 
 for i in range(8):
     queens[i] = random.randrange(0,8)
     board[i][queens[i]] = 1



 totalcollision =  totalcoll()
    
 while True:

  for i in range(8):
     
     oldqueen = queens[i]

     
     for j in range(8):
       
         queens[i] = j
       
         neighbour[i][j] = totalcoll()
     
     queens[i] = oldqueen

 
  min = neighbour[0][0]
  minqueencol = 0
  minqueenrow = 0
  for i in range(8):
      for j in range(8):
          if(neighbour[i][j]<min):
              min = neighbour[i][j]
              minqueenrow = j
              minqueencol = i

  if min<totalcollision:
     totalcollision = min
     queens[minqueencol] = minqueenrow
    

  else:
      break

 if totalcollision == 0:
        break

print("a")

for i in range(8):
    for j in range(8):
        print(board[i][j])

@NikhilKumar your suggestion works, but why is `[0]*8` not already 8 copies of the same list? — user_na, May 12 '21 at 18:55
@user_na Because lists are immutable. [Here's](https://stackoverflow.com/a/975024/13945013) an in-detail explanation. — Nikhil Kumar, May 12 '21 at 18:59
@user_na: cleaner idiom to construct n lists is `board = [[0,0,0,0,0,0,0,0] for _ in range(8)]`. That uses a list comprehension, and in Python `_` is a don't-care variable that doesn't get read. — smci, May 13 '21 at 02:00

fordev · Answer 1 · 2021-05-12T19:49:40.970

You've put infinite loop with while True. Your loops never break. So your program flow never reached to print(a) and print(board[i][j]) lines. I checked your code and your second while loop never breaks and works continuously. Please check your break conditions.
Actually I noticed a problem in your code: as far as I read the algorithm, if I understood correctly, you're miscalculating the number of collisions.

This picture is your board status.if I understood correctly the algorithm, there is 4 collision in there. (correct me if I'm wrong) But your totalcoll() function calculated it as 18. I ran the code again just to be sure. and I saw this: while there are 3 collisions, totalcoll() returns 13 collision.

There is also another sample:

This is your board with 5 collision. But this is what says your program about collision (23 collision):

You should check your totalcoll() func.

I don't get it why it does not break? Total collision never reaches zero? — Anıl Atasoy, May 12 '21 at 19:01

8 queen with hill climbing algorithm doesn't return anything?

1 Answers1