Comments on: Manipulating Strings in C# – Finding all occurrences of a string within another string

By: Mudasir Khan

Mudasir Khan — Wed, 18 May 2011 16:03:52 +0000

///
/// Returns the Index of All Occurances of a String
///
/// Original String
/// String to Find
///
public int[] IndexOfAll(String oString, String sString)
{
//List of Occurances
List sOccurances = new List();
int pos = 0;
for (int i = 0; i < oString.Length; i++)
{
pos = oString.IndexOf(sString, i);
if (pos != -1)
{
sOccurances.Add(pos);
i = pos;
}
}
return (int[])sOccurances.ToArray();
}

By: Hoyb

Hoyb — Mon, 21 Feb 2011 18:37:37 +0000

Wow!

Thanks a ton to Kotelni for finding the bug! I just spent 25 minutes trying to figure out why the code above wouldn’t work right (thanks for the effort though, original poster), but I just pasted Kotelni’s code in, and boom, working indexofall function. Thanks again!! You rock!

By: Lasse Espeholt

Lasse Espeholt — Sat, 15 May 2010 12:05:02 +0000

I don’t think these algorithms is very efficient. Instead I think you should look at some algorithms at http://en.wikipedia.org/wiki/List_of_algorithms#Substrings which is designed find multiple substrings instead of using a naive IndexOf loop.

Best regards, Lasse Espeholt

By: Guy Ellis

Guy Ellis — Wed, 16 Sep 2009 14:42:24 +0000

Yes – I can confirm that the comparison must be >= and not > otherwise this will fail in cases where the occurrence is at the start of the string.

By: kotelni

kotelni — Fri, 12 Jun 2009 15:54:59 +0000

I found IndexOfAll metod call very useful. I like the idea of using yield return.
This is slightly better version of IndexOfAll, I thought it is worth sharing.
It does not do Substring in a loop which can give a big performance improvement for large strings and (I think) there was a bug in original version that in ‘while’ comparison must be for >=0 not just >0.

protected static IEnumerable IndexOfAll(string haystack, string needle)
{
int pos;
int offset = 0;
int length = needle.Length;
while ((pos = haystack.IndexOf(needle, offset)) != -1)
{
yield return pos;
offset = pos + length;
}
}