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?

you should put "mid = (low + high)/2" into the while loop; you should update low and high values (that you do) as well — Dmitry Bychenko, Jan 30 '14 at 06:34
check this out, http://stackoverflow.com/questions/9918348/binary-search-in-python-weird-behavior?rq=1 — James Sapam, Jan 30 '14 at 06:38
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. — James Sapam, Jan 30 '14 at 06:40
It should be `low = mid + 1` also in the `else` condition, apart from updating `mid` in the while loop. — Pratik Singhal, Jan 30 '14 at 06:57
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). — Bakuriu, Jan 30 '14 at 07:59
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 — Pratik Singhal, Jan 30 '14 at 14:36

Pratik Singhal · Accepted Answer · 2014-01-30T07:03:33.270

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!

In python 2 what if `high == sys.maxint`? – Peter Wood Sep 10 '18 at 14:06 — Peter Wood, Sep 10 '18 at 14:06

Daniel Qiu · Answer 2 · 2014-01-30T06:54:36.807

0

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.

edited Jan 30 '14 at 06:54

answered Jan 30 '14 at 06:31

Daniel Qiu

617
5
14

what's the data in input? – Daniel Qiu Jan 30 '14 at 06:46
@Ramya is your `input` sorted in ascending or descending order. – Pratik Singhal Jan 30 '14 at 06:48
Since the item isn't in `input[mid]`, the range should be adjusted so `low = mid + 1`, shouldn't it? – Jonathan Leffler Jan 30 '14 at 06:51
@JonathanLeffler Right, `low = mid+1` is correct, should be adjusted. – Pratik Singhal Jan 30 '14 at 06:52

score 0 · Answer 3 · answered Mar 27 '19 at 13:16

"""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"

Infinite loop in binary search

3 Answers3