Binary search Algorithm

• Usama Ashafq
• Zaigham Abbas
• Fazal ur Rehman
• Umer Sarwar

Binary search is a fast search algorithm with run-time
complexity of Ο(log n). This search algorithm works on
the principle of divide and conquer. For this algorithm
to work properly, the data collection should be in the
sorted form.
Binary search looks for a particular item by
comparing the middle most item of the collection. If a
match occurs, then the index of item is returned. If
the middle item is greater than the item, then the
item is searched in the sub-array to the left of the
middle item.

Otherwise the item is searched for in the sub-array
to the right of the middle item. This process
continues on the sub-array as well until the size of
the sub-arrays reduces to zero.
Binary search works only on a sorted set of
elements. To use binary search on a
collection, the collection must first be sorted.
When binary search is used to perform
operations on a sorted set, the number of
iterations can always be reduced on the basis
of the value that is being searched.

Algorithm
Algorithm is quite simple. It can be done either recursively
or iteratively:
1. get the middle element;
2. if the middle element equals to the searched value,
the algorithm stops;
3. otherwise, two cases are possible:
*searched value is less, than the middle element. In
this case, go to the step 1 for the part of the array,
before middle element.
*searched value is greater, than the middle element. In
this case, go to the step 1 for the part of the array, after
middle element.

How Binary Search Works?
For a binary search to work, it is mandatory for the target array to be sorted. We shall
learn the process of binary search with a pictorial example. The following is our
sorted array and let us assume that we need to search the location of value 31
using binary search.

First, we shall determine half of the array by using this formula
−
mid = low + (high - low) / 2
Here it is, 0 + (9 - 0 ) / 2 = 4 (integer value of 4.5). So, 4 is the
mid of the array.

Now we compare the value stored at location 4, with the value being
searched, i.e. 31. We find that the value at location 4 is 27, which is not a
match. As the value is greater than 27 and we have a sorted array, so we also
know that the target value must be in the upper portion of the array.
We change our low to mid + 1 and find the new mid value
again.
low = mid + 1
mid = low + (high - low) / 2
Our new mid is 7 now. We compare the value stored at
location 7 with our target value 31.

We compare the value stored at location 5 with
our target value. We find that it is a match.
We conclude that the target value 31 is stored at
location 5.
Binary search halves the searchable items and thus
reduces the count of comparisons to be made to very
less numbers.

The value stored at location 7 is not a match, rather it is more
than what we are looking for. So, the value must be in the lower
part from this location.
Hence, we calculate the mid again. This time it
is 5.

Examples
Example 1. Find 6 in {-1, 5, 6, 18, 19, 25, 46,
78, 102, 114}.
Step 1 (middle element is 19 > 6): -
1 5 6 18 19 25 46 78 102 114
Step 2 (middle element is 5 < 6): -
1 5 6 18 19 25 46 78 102 114
Step 3 (middle element is 6 == 6): -
1 5 6 18 19 25 46 78 102 114

• #include<iostream>
• Using namespace std;
• Int main()
• {
• Int a[100];
• Int n, I, beg, ed, mid;
• Cout<<“enter no of elements:”;
• Cin>>n;
• Cout<<“enter sorted elements:n”;
• for(i=1;i<=n;i++)
• {
• Cin>>a[i];
• }

cout<<“enter search element:’;
cin>>item;
Beg=1;
Ed=n;
Mid=(beg+ed)/2;
While(beg<=ed &&
a[mid]!=item)
{
If(a[mid]<item)
Beg=mid+1;
else

• Ed=mid-1;
• mid=(beg+end)/2;
• }
• If(a[mid]==item)
• {
• cout<<“item found at location:n”;
• }
• Else
• cout<<“data not found”;
• cout<<“nnn****************end of
program***************nnn”;
• }

 Binary search is much faster then linear search.
 Binary search takes an average log2(n) , whereas linear
search is n/2.
 Binary search on average of 20 , whereas linear search
is 500,000 comparisons.

 It works only on sorted list, otherwise it is not applicable.
 There is a great loss of efficiency if the list does not
support random access.
 It is more complicated then linear search.

Binary search Algorithm

More Related Content

What's hot (20)

Similar to Binary search Algorithm (20)

Recently uploaded (20)

Binary search Algorithm