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!

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) — inspectorG4dget
– inspectorG4dget, Commented Jan 23, 2014 at 19:15

inspectorG4dget · Accepted Answer · 2014-01-23 19:25:27Z

3

You have three problems that I can see:

"cab " has a space in it, and LETTERS does not have a space. So you'll never be able to guess a space
You don't reset guess. You simply keep adding to it
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)

edited Jan 23, 2014 at 19:25

answered Jan 23, 2014 at 19:19

inspectorG4dget

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5 Comments

user2357112 Over a year ago

Your second code snippet doesn't work. You're throwing away the shuffled list.

inspectorG4dget Over a year ago

@user2357112: False. random.shuffle is IN-PLACE. It returns None

user2357112 Over a year ago

In-place on a list you immediately throw away.

inspectorG4dget Over a year ago

@user2357112: good catch. I totally missed that. It's fixed now. Thanks

willsc123 Over a year ago

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!

Martijn Pieters · Accepted Answer · 2014-01-23 19:20:29Z

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

edited Jan 23, 2014 at 19:20

answered Jan 23, 2014 at 19:14

Martijn Pieters

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