Linear search

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 i in array
        return true if i == 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;
}

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.