Counting occurrences of substring in string

Question

I need to know how many times a substring occurs in a given string. I figured that I might as well create an extension method:

public static int Occurences(this string str, string val)
{
    string copy = str;

    int instancesOf = 0;
    int indexOfVal;

    while ((indexOfVal = copy.IndexOf(val)) != -1)
    {
        copy = copy.Remove(indexOfVal, val.Count());
        instancesOf++;
    }

    return instancesOf;
}

I'm not sure that the assignment in the while loop is good practice. Should I change this to compute the value once outside the loop, and once in the loop, like this?

int indexOfVal = copy.IndexOf(val);

while (indexOfVal != -1)
{
    copy = copy.Remove(indexOfVal, val.Count());
    instancesOf++;
    indexOfVal = copy.IndexOf(val);
}

Any and all comments appreciated, the more the better.

Answer 1

Your current implementation could be more efficient. Copying the passed in string and chopping it up is probably pretty expensive (I'm not a master of .NET internals though).

val.Count()

Why not val.Length? Count() is very inefficient compared to Length.

copy = copy.Remove(indexOfVal, val.Count());

We don't need to remove part of the string, just search again starting at indexOfVal + val.Length.

Here is an example using an overload of String.IndexOf:

public static int Occurences(this string str, string val)
{  
    int occurrences = 0;
    int startingIndex = 0;

    while ((startingIndex = str.IndexOf(val, startingIndex)) >= 0) 
    {
        ++occurrences;
        ++startingIndex;
    }

    return occurrences;
}

This implementation will count overlapping occurrences (so "bbb".Occurences("bb") will return 2.

If you don't want to count overlapping occurences, you can replace ++startingIndex; with:

startingIndex += val.Length

---

On why an exception isn't thrown in the case of "foo".Occurrences("o"), from MSDN:

If startIndex equals the length of the string instance, the method returns -1.