Binary Search

Section 1: Search in python, binary search

Binary Search can only be applied to a sorted list. If the list is not sorted, add the time complexity of sorting to that of binary sort to determine the total complexity.

  • Divide the list into two halves (left, center element, right)
  • If element we are searching for is center element, stop
  • If element is greater than center element, divide right half of list and repeat the steps
  • If element is less than center element, divide left half of list and repeat the steps

Eg: Array 1 2 3 4 5 6 7 8 9 Find 1

Scan 1: Middle:1; 1<5 > Array: 1,2,3,4

Scan 2: Middle:2; 1<2 > Array: 1

Scan 3: Middle:1; 1=1 > Stop

Recurrence T(n) = T(n/2) + c T(n/2) = T(n/4)+c

  • T(n) = O(logn)
Implementation in python
#Note: Binary search only works when the list is sorted

def bsearch (seq, v, l, r):
    if (r-l == 0):
        return(False) #element not present

    mid = (l+r)//2 #finding the middle element

    if(v == seq[mid]): #if sequence equal to the middle element return true
        return (True)

    if (v < seq[mid]):
        return (bsearch(seq,v,l,mid)) #search left half

        return (bsearch(seq,v,mid+1,r)) #search right half

If you are stuck at any point or have any doubts, drop a message here

Published by in search and tagged algorithms, data structures, python and search using 209 words.

comments powered by Disqus