finding common prefix of array of strings

Question

I have an array like this:

$sports = array(
\'Softball - Counties\',
\'Softball - Eastern\',
\'Softball - North Harbour\',
\'Softball - South\',
\'Softball -         


        

Answer 1

Probably there is some terribly well-regarded algorithm for this, but just off the top of my head, if you know your commonality is going to be on the left-hand side like in your example, you could do way better than your posted methodology by first finding the commonality of the first two strings, and then iterating down the rest of the list, trimming the common string as necessary to achieve commonality or terminating with failure if you trim all the way to nothing.

Answer 2

I think you're on the right way. But instead of incrementing i when all of the string passes, you could do this:

1) Compare the first 2 strings in the array and find out how many common characters they have. Save the common characters in a separate string called maxCommon, for example.

2) Compare the third string w/ maxCommon. If the number of common characters is smaller, trim maxCommon to the characters that match.

3) Repeat and rinse for the rest of the array. At the end of the process, maxCommon will have the string that is common to all of the array elements.

This will add some overhead because you'll need to compare each string w/ maxCommon, but will drastically reduce the number of iterations you'll need to get your results.

Answer 3

1. not that I know of

2. yes: instead of comparing the substring from 0 to length i, you can simply check the ith character (you already know that characters 0 to i-1 match).

Answer 4

I assume that by "common part" you mean "longest common prefix". That is a much simpler to compute than any common substring.

This cannot be done without reading (n+1) * m characters in the worst case and n * m + 1 in the best case, where n is the length of the longest common prefix and m is the number of strings.

Comparing one letter at a time achieves that efficiency (Big Theta (n * m)).

Your proposed algorithm runs in Big Theta(n^2 * m), which is much, much slower for large inputs.

The third proposed algorithm of finding the longest prefix of the first two strings, then comparing that with the third, fourth, etc. also has a running time in Big Theta(n * m), but with a higher constant factor. It will probably only be slightly slower in practice.

Overall, I would recommend just rolling your own function, since the first algorithm is too slow and the two others will be about equally complicated to write anyway.

Check out WikiPedia for a description of Big Theta notation.

Answer 5

The top answer seemed a bit long, so here's a concise solution with a runtime of O(n²).

function findLongestPrefix($arr) {
  return array_reduce($arr, function($prefix, $item) {
    $length = min(strlen($prefix), strlen($item));
    while (substr($prefix, 0, $length) !== substr($item, 0, $length)) {
      $length--;
    }
    return substr($prefix, 0, $length);
  }, $arr[0]);
}

print findLongestPrefix($sports); // Softball -

Answer 6

The solutions here work only for finding commonalities at the beginning of strings. Here is a function that looks for the longest common substring anywhere in an array of strings.

http://www.christopherbloom.com/2011/02/24/find-the-longest-common-substring-using-php/