Cannot get simple while loop working in Python

Question

LETTERS = "abc"
correct = "cab "
guess = ""
while guess != correct:
 for i in LETTERS:
 position = random.randrange(len(LETTERS))
 guess += LETTERS[position]
 LETTERS = LETTERS[:position] + LETTERS[(position + 1):]
 print(guess)

I'm new in Python and I want to make this simple program:

• With the letters "abc", jumble them and create a new three-lettter word randomly.
• Print that jumble
• Continue doing this loop until the computer jumbles "cab".
• Then stop.

I came up with this code, and it gives me an infinite loop. I can't figure out why is doing it. I'm sure it's something easy but I can't see it. Need some help! Thanks!

• Martijn Pieters: Did you mean to end your correct value with a space?
• user2357112: And did you mean to reset your guesses after they fail?
• inspectorG4dget: OP is also altering LETTERS on every iteration of the for-loop. Needs to alter a copy of LETTERS instead with a myLetters = LETTERS[:] immediately inside the while-loop (and reset guess too)

Answer 1

You have three problems that I can see:

1. "cab " has a space in it, and LETTERS does not have a space. So you'll never be able to guess a space
2. You don't reset guess. You simply keep adding to it
3. You change LETTERS in your for-loop, so in the second iteration of your while-loop, it will be completely empty.

This is how I would go about doing what you're trying to do (with minimal modification):

_LETTERS = "abc"
correct = "cab"
guess = ""
while guess != correct:
 LETTERS = _LETTERS[:]
 guess = ""
 for i in LETTERS:
 position = random.randrange(len(LETTERS))
 guess += LETTERS[position]
 LETTERS = LETTERS[:position] + LETTERS[(position + 1):]
 print(guess)

Here's how I would do a random search (which is what you're trying to do):

guess = "abc"
correct = "cab"
while guess != correct:
 guess = list(guess)
 random.shuffle(guess)
 guess = ''.join(guess)
 print(guess)
print(guess)

Of course, there are better techniques to correctly guess "cab". If you really want to try an exhaustive search, then you could use a backtracking DFS:

def DFS(letters, correct, sofar=None)
 if sofar is None:
 sofar = ''
 if not letters:
 if sofar == correct:
 print("Yay! I found it")
 else:
 print("Oops! I found %s instead" %sofar)
 else:
 for i,char in enumerate(letters):
 DFS(letters[:i]+letters[i+1:], correct, sofar+char)

• user2357112: Your second code snippet doesn't work. You're throwing away the shuffled list.
• inspectorG4dget: @user2357112: False. random.shuffle is IN-PLACE. It returns None
• user2357112: In-place on a list you immediately throw away.
• inspectorG4dget: @user2357112: good catch. I totally missed that. It's fixed now. Thanks
• willsc123: Thank you for all your answers! That space at "cab" was a typo. It wasn't meant to be there! I see now what was the problem haha, what a silly thing. Now it works fine. Also thank you for the random.shuffle thing. I didn't know that one!

Answer 2

Your correct value contains a space, but your loop never generates spaces:

correct = "cab "

Remove that space:

correct = "cab"

Next, your loop reduces LETTERS to an empty string, so only once does your loop produce a random guess, but afterwards, you forever are stuck with LETTERS = '', so no for loop is run.

You'd be better off using random.shuffle to produce guesses:

LETTERS = list("abc")
correct = "cab"
while True:
 random.shuffle(LETTERS)
 guess = ''.join(LETTERS)
 if guess == correct:
 print(guess)
 break