Binary Search

Binary search is a searching algorithm to find the given value within an array. This algorithm only works if the given array is already sorted. So that it can figure out whether the given element exists or not.

It works by dividing a list into two halves and compare the middle element with the desired value.

If the value doesn’t match, the half in which the desired value can’t be present gets eliminated and again the remaining one is split down from the middle and that middle term again compared. It is repeated until a match is not found.

It requires O(log n) running time to search an element in the worst case scenario. It is also known as half-interval search.

Pseudocode

first=0
last=Array.Length -1 
WHILE first<= last
mid= (first+last)/2
IF arr[mid]== element 
return mid
ELSE IF arr[mid]> element
last=mid-1
ELSE
first=mid+1
END IF
END IF
END WHILE

C program to search an element using binary search

#include<stdio.h>
#include<math.h>
int Binary_Search(int arr[], int element,int size)
{
int pos=-1,mid,first,last;
first=0;
last=size-1;
while(first<= last)
{
mid= (first+last)/2;
if(arr[mid]== element)
{
pos = mid;
break;
}
else if(arr[mid]>element)
last=mid-1;
else
first=mid+1;
}
return pos; 
}

int main()
{
int arr[]={1,2,3,4,5}; 
int i,pos,element;
printf("\n Please enter an element to search in list: ");
scanf("%d",&element);
pos =Binary_Search(arr,element,5);
if(pos!=-1)
printf("\n your element found at : %d position and the number was %d ", pos+1,arr[pos]);
else
printf("element is not found");
return 0;
}

Output:

How does Binary search work ?