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Python Python Collections (Retired) Lists Redux Pop

Steven Wheeler
Steven Wheeler
2,330 Points
Posted by Steven Wheeler
Steven Wheeler
2,330 Points

Moving items in a list?

def show_list():
  count = 1
  for item in shopping_list:
    print("{}: {}".format(count, item))
    count += 1

def move_item():
  show_list()
  print("Give me the number of the item you want to move")
  userNumber = input(">>> ")
  print("Now give me the number where you want this item to be put")
  destNumber = input(">>> ")
  idx = int(userNumber) - 1
  item = shopping_list.pop(idx)
  finalDestinationNumber = int(destNumber) - 1
  shopping_list.insert(finalDestinationNumber, item)
  print("Item moved!")
  show_list()

def add_list():
  while True:
    newItem = input(">>> What would you like to add to the list?  ")
    if (newItem == "DONE"):
      show_list()
      break
    elif (newItem == "SHOW"):
      show_list()
      continue
    elif (newItem == "CLEAR"):
      del shopping_list[:]
      print("Shopping list cleared")
      continue    
    elif (newItem == "MOVE"):
      move_item()
      continue
    else:
      shopping_list.append(newItem)
      continue



show_help()
add_list()

Hey there, I'm really not sure why my move_item() function isn't working as expected. I'm sure its something quite simple but... I just don't get why it's doing what it's doing. I've been trying for about 30 minutes and my head hurts haha. The problem has been making the number they give into an integer and minus 1 from it so that it will align with the item's actual index. There must be a simple gap in my logic but I can't see what it is right now.

I've only pasted the relevant code.

EDIT: I've updated the code but it still has the same problem :)

2 Answers

Nathan Smutz
Nathan Smutz
9,492 Points

First problem: Both inputs assign to the same variable.

You use userNumber for both the item and where you want it to go. Right now idx and newNumber both will equal the "where you want this" number.

A quick fix would be to move `idx = int(userNumber) - 1 so that it's before the second input ststement.

Second problem: Your insert index is a moving target

When you pop the item, you change (lower) the index of every item after it.

Moving an item higher in the list:
Once you've popped it, everything higher has shifted down. When you went to insert it at the index you originally intended, that original index points at one item to the right.

Loking at a list like this:

 0, 1, 2, 3
[a, b, c, d]

If you try to move a to after b: Your program would pop from 0 and insert at 2

 0, 1, 2, 3
[a, b, c, d]
pop(0)
 0, 1, 2
[b, c, d]
insert(2, a)
 0, 1, 2, 3
[b, c, a, d]

So we meant to insert after b; but, since the items changed indices, we inserted after c.

So, you'll want to check whether you're inserting after the original item and, in that case, subtract 1 from the index you use when you insert.

if(idx < newNumber):
    shopping_list.insert(newNumber-1 , item)
else:
    shopping_list.insert(newNumber, item)

It seems like python should have a built in function for this; but a search hasn't turned anything up yet.

Steven Wheeler
Steven Wheeler
2,330 Points
Steven Wheeler
Steven Wheeler
2,330 Points

Thanks for the help, I'm not sure why I changed both inputs to assign the same variable (it hadn't always been like that).

It does seem to be very awkward with lists. I am guessing in reality you would maybe use a dictionary or something else to do this more efficiently.

Nathan Smutz
Nathan Smutz
9,492 Points
Nathan Smutz
Nathan Smutz
9,492 Points

I saw your edit. What is the behavior that you're seeing? Are items not moving at all still? If it's just going to the wrong place (or erroring when moving to the end): I see you're not yet compensating for changed indices when moving higher.