Python binary search algorithmn returns None

Question

So i tried to do binary search in python, which i think syntactically right, but logically somewhere going wrong, despite of checking it infinite times. Here is my code.

def m(a,ele):
    a=a
    ele=ele

    length = len(a)
    if length <=1:
        return a
    mid = a[length//2]

    if ele == mid:
        return True
    elif ele > mid:
        print("if")
        if len( a[a.index(mid):]) == 0:
            return False
        elif len( a[a.index(mid):]) ==1 and ele !=  a[a.index(mid):]:
            return False
        elif len( a[a.index(mid):]) ==1 and ele ==  a[a.index(mid):]:
            return True
        else:
            m( a[a.index(mid):],ele)
    else:
        print("else")
        if len( a[:a.index(mid)]) == 0:
            return False
        elif len( a[:a.index(mid)]) ==1 and ele !=  a[:a.index(mid)]:
            return False
        elif len( a[:a.index(mid)]) ==1 and ele ==  a[:a.index(mid)]:
            return True
        else:
            m( a[:a.index(mid)],ele)


print(m([1,3,6,8],8))

SO, in this case it should print True but it is printing None. Any help regarding this code would be helpful, i know i am doing some silly mistakes.

You need to return the result of recursion. i.e: return m( a[a.index(mid):],ele) — Mark
– Mark, Commented May 12, 2020 at 16:50

Arpit Omprakash · Accepted Answer · 2020-05-12 16:53:28Z

1

You need to return the result of each step of recursion. (In all the else's)

else:
       return m( a[a.index(mid):],ele)

answered May 12, 2020 at 16:53

Arpit Omprakash

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Comments

Chidananda Nayak · Accepted Answer · 2020-05-12 17:04:09Z

THank you people for correcting me. So this is my final code

def m(a,ele):
    a=a
    ele=ele

    length = len(a)
    if length <=1:
        return a
    mid = a[length//2]

    if ele == mid:
        return True
    elif ele > mid:
        if len( a[a.index(mid):]) == 0:
            return False
        elif len( a[a.index(mid):]) ==1 and ele !=  a[a.index(mid):][0]:
            return False
        elif len( a[a.index(mid):]) ==1 and ele ==  a[a.index(mid):][0]:
            return True
        else:
            return m( a[a.index(mid):],ele)
    else:
        if len( a[:a.index(mid)]) == 0:
            return False
        elif len( a[:a.index(mid)]) ==1 and ele !=  a[:a.index(mid)][0]:
            return False
        elif len( a[:a.index(mid)]) ==1 and ele ==  a[:a.index(mid)][0]:
            return True
        else:
            return m( a[:a.index(mid)],ele)


print(m([1,3,6,8,77,777],777))
True

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