Controlling distance of shuffling

Question

I have tried to ask this question before, but have never been able to word it correctly. I hope I have it right this time:

I have a list of unique elements. I want t

Answer 1

Here's an adaptation of @גלעד ברקן's code that takes only one pass through the list (in random order) and swaps only once (using a random choice of possible positions):

from random import choice, shuffle

def magicFunction(L, d):
  n = len(L)
  swapped = [0] * n     # 0: position not swapped, 1: position was swapped
  positions = list(xrange(0,n))         # list of positions: 0..n-1
  shuffle(positions)                    # randomize positions
  for x in positions:
    if swapped[x]:      # only swap an item once
      continue
    # find all possible positions to swap
    possible = [i for i in xrange(max(0, x - d), min(n, x + d)) if not swapped[i]]
    if not possible:
      continue
    y = choice(possible)        # choose another possible position at random
    if x != y:
      L[y], L[x] = L[x], L[y]   # swap with that position
      swapped[x] = swapped[y] = 1       # mark both positions as swapped
  return L

Here is a refinement of the above code that simply finds all possible adjacent positions and chooses one:

from random import choice

def magicFunction(L, d):
  n = len(L)
  positions = list(xrange(0, n))        # list of positions: 0..n-1
  for x in xrange(0, n):
    # find all possible positions to swap
    possible = [i for i in xrange(max(0, x - d), min(n, x + d)) if abs(positions[i] - x) <= d]
    if not possible:
      continue
    y = choice(possible)        # choose another possible position at random
    if x != y:
      L[y], L[x] = L[x], L[y]   # swap with that position
      positions[x] = y
      positions[y] = x
  return L