Find missing number in sorted array

Question

Whats wrong with this code ? Not able able to search missing number in a consecutive array using binary search.

a = [1,2,3,4,5,7,8]

lent = len(a)
beg =0
end = lent-1

while beg < end:
    mid = (beg + end) / 2
    if (a[mid]-a[beg])==(mid - beg):
        beg = mid + 1
    else:
        end = mid -1
    if(beg == end):
        mid = (beg + end) / 2
        print "missing"
        print a[0]+ beg

Answer 1

Update #1: Yes, there was another one mistake. You're right. Here's updated version

Try this variant:

a = [1,2,3,4,5,7,8]

lent = len(a)
beg =0
end = lent-1

while beg < end:
    mid = (beg + end) / 2
    if (a[mid]-a[beg])==(mid - beg):
        beg = mid
    else:
        end = mid
    if abs(beg-end) <= 1:
        print "missing: %s" % (a[0] + max(beg, mid),)

Result:

missing: 6

Also, try use functions, so you could easily test and debug your code on different lists:

def find_missing(a):
    lent = len(a)
    beg =0
    end = lent-1

    while beg < end:
        mid = (beg + end) / 2
        if (a[mid]-a[beg])==(mid - beg):
            beg = mid
        else:
            end = mid
        if abs(beg-end) <= 1:
            return a[0] + max(beg, mid)

a = [1,2,3,4,5,7,8]
print find_missing(a)

a = [1,3,4,5,6]
print find_missing(a)

a = [1,2,3,4,5,7,8,9,10]
print find_missing(a)

Result:

6
2
6