Getting a substring of a delimited string

Question

I have some underscore-delimited strings like abc_def_ghi. I would like to get a substring out of the string made of one or more delimited substrings, so that if I call:

getUnderscoreSubstring("abc_def_ghi",2)

then I get:

abc_def

This is the C# code I'm using:

public string getUnderscoreSubstring(string fullStr,int substringCount)
{
    string[] splitArray = fullStr.Split('_');
    if (substringCount>splitArray.Count())
    {
        return null;
    }
    else
    {
        string output = "";
        for(int c=0;c<substringCount;c++)
        {
            output += splitArray[c];
            if (c<substringCount-1)
            {
                output += "_";
            }
        }
        return output;
    }
}

I'm wondering if there's a simpler way to rewrite this, possibly using extension methods.

Answer 1

Yes, you can definitely simplify this.

public string GetSubstring(string input, int count, char delimiter)
{
    return string.Join(delimiter.ToString(), 
                       input.Split(delimiter).Take(count));
}

Calling it is as easy as GetSubstring(input, 2, '_')

What does it do?

• Split the input string by the delimiter
• Take the amount of substrings you want
• Glue the selected substrings together with your delimiter

Very short and sexy!

This doesn't take the substringCount > splitArray.Count() in account but you can easily add that yourself: just split up the oneliner and add the appropriate check.

---

Now, some comments about your code:

• Use .Length instead of .Count() when possible: the former will always be an O(1) operation, the latter sometimes an O(n). It won't make a difference here since an array implements ICollection<T> (and will use this optimization) but it's a good practice to observe.
• Returning null is typically avoided for good reasons, consider an empty string or exception instead (don't go for the exception in this case).
• I prefer to explicitly use string.Empty rather than an empty string to avoid confusion.
• Write your variable names in full -- nobody is helped by abbreviating them.
• Use a StringBuilder to concatenate in a loop to avoid unnecessary string object creating.
• Leave some space in your code, it will read more fluently.

Answer 2

Basically, you want to find the substringCountth occurrence of '_' in the string, and return all the characters before that.

Basing on the code from https://stackoverflow.com/a/11363213/1108056

private int IndexOfNth(string str, char c, int n)
{
    int index = -1;
    while (n-- > 0)
    {
        index = str.IndexOf(c, index + 1);
        if (index == -1) break;
    }
    return index;
}

public string GetSubstring(string input, int count, char delimiter)
{
    return input.Substring(0,IndexOfNth(input, delimiter, count));
}