Question

I have been tasked with creating a binary search over a list of words, I have come up with 2 implementations (and clearly haven’t put in a case where the word isn’t found yet but that’s not an issue yet), however when the list is narrowed down to the word I am looking for, my function does not finish, instead it keeps running until maximum recursive depth is exceeded.

I put in print and it clearly shows the word at dasList[mid], and shows this over and over again until it finally gives up.

def _bisect2(dasList, word):
    mid = int(len(dasList)/2)
    if word.lower() > dasList[mid].lower():
        return _bisect2(dasList[mid: len(dasList)], word)            
    if word.lower() < dasList[mid].lower():
        return _bisect2(dasList[0: mid], word)
    else:
        return mid

this is being called by

print(_bisect2(fileList, input('Please type a word')))

I am using the Python 3.0 interpretor. Any suggestions?

Answer 1

Your implementation (almost) works for me (and doesn’t show the behavior you described with my pre-sorted input). I assume you’ve sorted your input files? A slightly modified (working) example is posted below.

def _bisect2(dasList, word,lidx=0):
    mid = int(len(dasList)/2)
    if word.lower() > dasList[mid].lower():
        return _bisect2(dasList[mid:], word,lidx=lidx+mid)            
    elif word.lower() < dasList[mid].lower():
        return _bisect2(dasList[:mid], word,lidx=lidx)
    return lidx+mid

words=sorted(["one","two","three","four","five","twenty","foo"])
print (words)
print (_bisect2(words,'three'))

Note that you were returning the index in the last partial list (which will always be 0)…