Linear search

In computer science, linear search is a search algorithm, also known as sequential search, that is suitable for searching a set of data for a particular value.

It operates by checking every element of a list until a match is found. Linear search runs in O(N). If the data are distributed randomly, on average N/2 comparisons will be needed. The best case is that the value is equal to the first element tested, in which case only 1 comparison is needed. The worst case is that the value is not in the list, in which case N comparisons are needed.

Here is a sample implementation in Ruby:

def linear_search(array, value)
   for element in array
       return true if element == value
   end
   return false
end

Here is a sample implementation in PHP:

function linear_search($array, $value)
{
   foreach ($array as $current) {
       if ($current == $value) {
           return TRUE;
       }
   }
   return FALSE;
}

And here is a sample implementation in C++ using templates and STL containers:

template 
bool linear_search(const C& array, const T& value) {
   C::const_iterator iter = array.begin();
   C::const_iterator end = array.end();
   while (iter != end) {
       if (*iter == value)
           return true;
       ++iter;
   }
   return false;
}

Linear search can be used to search an unordered list. The more efficient binary search can only be used to search an ordered list.

If more than a small number of searches are needed, it is advisable to use a more efficient data structure. One approach is to sort and then use binary searches. Another common one is to build up a hash table and then do hash lookups.