RuntimeError: maximum recursion depth exceeded in Python

Question

#Recursive BinaryChop
def recursiveBinaryChop( value, elementList, min, max ):
    if len( elementList ) == 0:
        return -1
    if max <= min:
        if ( max == min and elementList[min] == value ):
            return min
        else:
            return -1
    else:
        midPointOfList = ( min + max ) / 2

        if elementList[midPointOfList] > value:
            max = --midPointOfList
            return recursiveBinaryChop( value, elementList, min, max )
        elif elementList[midPointOfList] < value:
            min = ++midPointOfList
            return recursiveBinaryChop( value, elementList, min, max )
        else:
            return midPointOfList

#Recursive BinaryChop Test Cases
assert recursiveBinaryChop(3, [], 0, 0) == -1
assert recursiveBinaryChop(3, [1], 0, 0) == -1
assert recursiveBinaryChop(1, [1], 0, 0) == 0
assert recursiveBinaryChop(1, [1, 3, 5], 0, 2) == 0
assert recursiveBinaryChop(3, [1, 3, 5], 0, 2) == 1
assert recursiveBinaryChop(5, [1, 3, 5], 0, 2) == 2
assert recursiveBinaryChop(0, [1, 3, 5], 0, 2) == -1
assert recursiveBinaryChop(2, [1, 3, 5], 0, 2) == -1
assert recursiveBinaryChop(4, [1, 3, 5], 0, 2) == -1
assert recursiveBinaryChop(6, [1, 3, 5], 0, 2) == -1
assert recursiveBinaryChop(1, [1, 3, 5, 7], 0, 3) == 0
assert recursiveBinaryChop(3, [1, 3, 5, 7], 0, 3) == 1
assert recursiveBinaryChop(5, [1, 3, 5, 7], 0, 3) == 2
assert recursiveBinaryChop(7, [1, 3, 5, 7], 0, 3) == 3
assert recursiveBinaryChop(0, [1, 3, 5, 7], 0, 3) == -1
assert recursiveBinaryChop(2, [1, 3, 5, 7], 0, 3) == -1
assert recursiveBinaryChop(4, [1, 3, 5, 7], 0, 3) == -1
assert recursiveBinaryChop(6, [1, 3, 5, 7], 0, 3) == -1
assert recursiveBinaryChop(8, [1, 3, 5, 7], 0, 3) == -1

I am getting the run time error for this simple code, I have tried searching but all answer seems to suggest setting the recursion limit but I don't see that happening with my test input. I am not sure whether my algorithm is wrong or has some logical error. I have the same algorithm working for me in C++.

Please help.

Same with ++. +x is just +x unchanged, so ++x = +(+x) = x. — Andrew Janke
– Andrew Janke, Commented May 4, 2015 at 14:59
@Kevin unfortunately it didn't throw any error and hence a possible source of logical errors such as this since I come from C/C++ background. — Krishna
– Krishna, Commented May 4, 2015 at 15:01
Incidentally, great job providing all those test case asserts. I wish every SO post was that thorough :-) — Kevin
– Kevin, Commented May 4, 2015 at 15:02
@Kevin How can we edit the question title to reflect the real problem the OP is encountering and help others find it? Something like "Why don't ++ and -- work in Python?" — kdopen
– kdopen, Commented May 4, 2015 at 15:09
@kdopen there are questions with similar title, it was just the ignorance on my part to not know this. — Krishna
– Krishna, Commented May 4, 2015 at 15:12

kdopen · Accepted Answer · 2015-05-04 14:59:03Z

6

These two lines don't do what you think:

max = --midPointOfList
min = ++midPointOfList

Python doesn't have this type of increment operators, but they do parse and execute successfully.

++i parses as +(+i) and --i as -(-i). Both leave i unchanged and are effectively

max = midPointOfList
min = midPointOfList

answered May 4, 2015 at 14:59

kdopen

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2 Comments

Krishna Over a year ago

I don't understand the real use case of this operators for int like variables.

kdopen Over a year ago

The point is that ++ and -- aren't operators in Python. Therefore, Python parses them according to its actual grammar. The +(+i) form is the only way it can make sense of ++. Personally, I think it should be a syntax error.

Kevin · Accepted Answer · 2015-05-04 15:06:11Z

    if elementList[midPointOfList] > value:
        max = --midPointOfList
        return recursiveBinaryChop( value, elementList, min, max )
    elif elementList[midPointOfList] < value:
        min = ++midPointOfList
        return recursiveBinaryChop( value, elementList, min, max )

Python doesn't have -- or ++ operators. If you're trying to decrement and increment, try "-1" and "+1".

    if elementList[midPointOfList] > value:
        max = midPointOfList - 1
        return recursiveBinaryChop( value, elementList, min, max )
    elif elementList[midPointOfList] < value:
        min = midPointOfList + 1
        return recursiveBinaryChop( value, elementList, min, max )

(This isn't exactly the same behavior as C++'s -- and ++, since midPointOfList's value remains unchanged, but that doesn't seem to matter in this particular circumstance; midPointOfList doesn't get referred to after those lines anyway)

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