Finding longest common matching substring from given List in a string

Question

I have a list of strings, which I want to find the start and end indices of their occurrence in a given string.

I want to find the longest common substring that is present in the original string and only print it.

Here is my code:

public static void Main(string[] args)
{
    //This is my original string where I want to find the occurance of the longest common substring
    string str = "will you consider the lic premium of my in-laws for tax exemption";

    //Here are the substrings which I want to compare   
    List<string> subStringsToCompare = new List<string>
    {
        "Life Insurance Premium",
        "lic",
        "life insurance",
        "life insurance policy",
        "lic premium",
        "insurance premium",
        "insurance premium",
        "premium"
    };

    foreach(var item in subStringsToCompare)
    {
        int start = str.IndexOf(item);

        if(start != -1)
        {
            Console.WriteLine("Match found: '{0}' at {1} till {2} character position", item, start, start + item.Length);
        }
    }
}

The problem is I am getting 3 occurrences instead of one. I can't seem to figure out the condition where it gets the longest common matched substring from all substring to compare.

Output I am getting:

Match found: 'lic' at 22 till 25 character position

Match found: 'lic premium' at 22 till 33 character position

Match found: 'premium' at 26 till 33 character position

Output expected:

Match found: 'lic premium' at 22 till 33 character position

.NET Fiddle

Can you not sort `subStringsToCompare` by length (descending) and bail out at the first match found? That way "lic premium" will be found first and displayed before "lic" and "premium" get in the foreach loop. — vc 74, Jun 21 '18 at 19:18
@vc74 I mean it stills looks for `lic` and `premium` later in the loop. How to bail out if I get the match? Here's the [updated fiddle](http://rextester.com/ELAV2246) — Kunal Mukherjee, Jun 22 '18 at 07:11

vc 74 · Answer 1 · 2018-06-22T10:18:28.320

Here's what I suggested in my comment

public static void Main(string[] args)
{
    //This is my original string where I want to find the occurance of the longest common substring
    string str = "will you consider the lic premium of my in-laws for tax exemption";

    // Here are the substrings which I want to compare
    // (Sorted by length descending) 
    List<string> subStringsToCompare = new List<string>
    {
        "Life Insurance Premium",
        "life insurance policy",
        "insurance premium",
        "life insurance",
        "lic premium",
        "premium",
        "lic"
    };

    foreach(var item in subStringsToCompare)
    {
        int start = str.IndexOf(item);

        if(start != -1)
        {
            Console.WriteLine("Match found: '{0}' at {1} till {2} character position", item, start, start + item.Length);

            break; // Stop at the first match found
        }
    }
}

score 2 · Answer 2 · answered Jun 21 '18 at 19:18

if you only needs exact match from the list of strings (not substring of the strings within the list) then you are pretty close

string longest = null;
int longestStart = 0;
foreach(var item in subStringsToCompare)
{
    int start = str.IndexOf(item);

    if(start != -1 && (longest == null || item.Length > longest.Length))
    {
        longest = item;
        longestStart = start
    }
}

if (longest != null)
{
    Console.WriteLine("Match found: '{0}' at {1} till {2} character position", longest, longestStart, longestStart + longest.Length);
}

score 0 · Answer 3 · answered Jun 21 '18 at 19:18

0

I haven't tried but try the following:

List<string> matches = new List<string>();
for( int i = 0; i < str.Length; i++ )
{
    foreach ( string toCompare in subStringsToCompare )
    {
        if ( str.SubString( i, toCompare.Length ) == toCompare )
            matches.Add( toCompare );
    }
}

string longest = "";
foreach ( string match in matches )
{
    if ( match.Length > longest.Length )
        longest = match;
}

answered Jun 21 '18 at 19:18

Niklas Fehde

47
8

This would further increase the complexity resulting in unnecessary computations. – Kunal Mukherjee Jun 21 '18 at 19:22
Could you perhaps explain why you think your changes are better? As it is we are left guessing why you think this is an improvement. For example I can't see what advantage there is in going character by character through the string, finding a substring and comparing them rather than just using .indexof as the OP did originally. – Chris Jun 22 '18 at 10:23
"Changes"? My answer was no change of some code there is not existing... – Niklas Fehde Jun 22 '18 at 12:51

Finding longest common matching substring from given List in a string

3 Answers3