Infinite loop in binary search

Question

I am trying to implement binary search with the following function:

def buggy_binary_search(input, key):
    low = 0
    high = len(input)-1
    mid = (low + high)/2
    while low <= high:
        if input[mid] == key:
            return mid
        if input[mid] > key:
            high = mid - 1
        else:
            low = mid
    return -1

The above function when run, gets into a infinite loop. How can I correct that?

Chatterton

You should update mid within the while loop – Kamila

should I update the low value??@Dmitry Bychenko – Chatterton

you should put "mid = (low + high)/2" into the while loop; you should update low and high values (that you do) as well – Kamila

check this out, #9918848 – Lintwhite

let me tell you something, print low, high inside while loop and see, whether the values are updated or not, after you can possibly think where to change it. – Lintwhite

It should be low = mid + 1 also in the else condition, apart from updating mid in the while loop. – Verdi

Note that python has a bisect module that implements binary search. I suggest you to read its source code. The module implements both "right" binary search and "left" binary search, i.e. searches that always return the rightmost or leftmost occurrence of the element. (it's under Lib/bisect.py in the python sources). – Zonnya

You can accept my answer as it is solving your problem. It is considered a bad practice on SO to not accept answers that are correct and provide solution to your problem – Verdi

Answer 1

Since, you are not updating the value of mid the while loop keeps on checking the same element and runs into an infinite loop, to correct that as many people have pointed out, update mid in the while loop.
Also, you should do low = mid+1 and not low = mid.

The full code is given below:-

    def binary_search(input, key):
       low = 0
       high = len(input)-1
       mid = (low + high)/2
       while low <= high:
          mid = (low + high)/2
          if input[mid] == key:
             return mid
          if input[mid] > key:
             high = mid - 1
          else:
             low = mid + 1
       return -1

Make sure the input is sorted!

Verdi

In python 2 what if high == sys.maxint? – Addendum

Answer 2

def binary_search(input, key):
    low = 0
    high = len(input)-1
    mid = (low + high)/2
    while low <= high:
       mid = (low + high)/2
       if input[mid] == key:
           return mid
       if input[mid] > key:
           high = mid - 1
       else:
           low = mid + 1
    return -1

as Dmitry Bychenko said, you should put mid = (low + high)/2 in the loop.

Sass

what's the data in input? – Sass

@Chatterton is your input sorted in ascending or descending order. – Verdi

Since the item isn't in input[mid], the range should be adjusted so low = mid + 1, shouldn't it? – Houdon

@JonathanLeffler Right, low = mid+1 is correct, should be adjusted. – Verdi

Answer 3

"""don't use "input" as a variable name. its a python keyword.
make sure your array is sorted
use integer division when computing midpoint
"""

def bsearch(input_array,target):
    lo,hi=0,len(input_array)-1
    while lo<=hi:
        mid=(lo+hi)//2
        if input_array[mid]==target:
            print "found at ",mid
            return mid
        if input_array[mid]>target:
            print "look left"
            hi=mid-1
        if input_array[mid]<target:
            print "look right"
            lo=mid+1
    return -1

a=[2,4,7,8,12,88,99,101]
target=7

assert bsearch(a,1134)==-1,"value 1134 isn't in array but says it is"
assert bsearch(a,2)==0,"value 2 is in the zero element of array.begenning"
assert bsearch(a,101)==7,"value 101 is in the 7th element of array. end"
assert bsearch(a,12)==4,"value 12 is in the 4th element of array. midpoint"

Restrained