Dictionary returning same output Python

Question

I have written the code for testing the input string if it satisfies the Non Deterministic finite state machine/automata. The logic seems fine to me but the code crashes because of stackoverflow because the recursive function is not returning.

The reason is dictionary is always returning same result for an input even though for the same input the dictionary has two different outputs.

def hello(curExist, curTemp, string, dict, accept):

    print "curExist: ", curExist, " curTemp: ", curTemp, " Letter: ", string;

    if(string == ""):
            return curExist in accept;

    elif((curTemp,string[0]) in dict):
        curExist = curTemp = dict[(curTemp,string[0])];

        return hello(curExist, curTemp, string[1:], dict, accept);

    elif((curTemp,'') in dict):
        curTemp = dict[(curTemp, '')];

        return hello(curExist, curTemp, string, dict, accept);

    elif((curExist, '') in dict):
        curTemp = dict[(curExist, '')];

        return hello(curExist, curTemp, string, dict, accept);

    else:
        return False;


dict={(1,'a'):2, (2,''):6, (6,''):4, (2,''):3, (3,'b'):4, (4,'c'):5};

mString = "ac";
accept=[5];

print hello(1,1,mString,dict,accept);

Console Output:

curExist:  1  curTemp:  1  Letter:  ac
curExist:  2  curTemp:  2  Letter:  c
curExist:  2  curTemp:  3  Letter:  c
curExist:  2  curTemp:  3  Letter:  c
curExist:  2  curTemp:  3  Letter:  c // printing this untill crash
.
.
.

How can I solve this problem?

Ian Clark · Accepted Answer · 2014-03-08T13:49:23.673

2

Keys in dictionaries need to be unique and immutable (because they're hashed), you can't have 2 identical keys:

>>> foo = { (2, ''): "bar", (2, ''): "zulu" }
>>> foo
{(2, ''): 'zulu'}

Also:

Your conditions don't need to be wrapped in parenthesis
You don't use semi-colons to delimit statement endings, that's what the whitespace is for.
Don't name a variable dict. It's the constructor for dictionaries

edited Mar 08 '14 at 13:49

answered Mar 08 '14 at 13:28

Ian Clark

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So what can be the solution as Non deterministic automata has different outputs for same input. – Umer Farooq Mar 08 '14 at 13:38
What do you want to do when one input has multiple outputs? – Ian Clark Mar 08 '14 at 13:46
To be clear: I don't know what a *'Non Deterministic finite state machine/automata'* is, but if you tell me what you want your inputs and outputs to be, then I can help. – Ian Clark Mar 08 '14 at 13:52
Is there an alternative to dictionaries which doesn't need unique keys and can have tuple as key? – Umer Farooq Mar 08 '14 at 14:45
Not really, your best solution would be to make the values a list of integers rather than simply integers. – Ian Clark Mar 08 '14 at 14:53

score 0 · Answer 2 · edited May 23 '17 at 10:33

@IanClark is right, the keys have to be unique. One simple solution will be to turn your dictionary signature to pair -> list. This signature naturally captures Non-Deterministic Automata, (Current State, Symbol) -> (List of Possible Next States). So, your example dict becomes

dictionary={(1,'a'):[2], (2,''):[6, 3], (6,''):[4], (3,'b'):[4], (4,'c'):[5]};

Finally, in your elif branch, loop over all elements in the dictionary entry and recursively call hello. Something like:

elif((curTemp,'') in dict):
    curTempList = dictionary[(curTemp, '')];

    tempBool = False
    for newCurrTemp in currTempList: 
        tempBool = tempBool or hello(curExist, curTemp, string, dictionary, accept);
    return tempBool

Your recursive call returns True or False. THe string is accepted if at least one of those calls is True. Hence the or statement with tempBool.

This approach might still make wasteful recursive calls (Think of calling hello using same inputs but from different branches of if). A better solution would be memoization, e.g. this: What is memoization and how can I use it in Python?

manually grouping the different next states takes away the purpose of my algo. I have already tried the manual grouping method but I wanted to do it without human interventation. — Umer Farooq, Mar 08 '14 at 14:37

Dictionary returning same output Python

2 Answers2