Replace fifth space with new line

Question

I want to replace the fifth space in a sentence with a new line. Sample input:

"Hi this is empty string. This should be in new line!"

Expected output:

"Hi this is empty string.

This should be in new line!"

I'm getting the expected output with this code, but I need to know, is this the optimized solution or there are any better solution than this?

    string s = "Hi this is empty string. This should be in new line!", t = "";
    int countSpaces = s.Count(Char.IsWhiteSpace);
    if (countSpaces > 3)
    {
        int count = 0;
        char n;
        foreach (char c in s)
        {
            if (c == ' ') count++;
            if (count == 5)
            {
                n = '\n';
                count++;
            }
            else n = c;
            t += n;
        }
    }
    else t = s;
    MessageBox.Show(t);

Answer 1

Using += to concatenate strings in a loop scales very badly. Each iteration creates a new string instance by copying the previous string and the new character into a new string, and that string grows for each iteration. Using a StringBuilder (as Dmitry suggests) would be a great improvment.

However, you don't need to build the string character by character. You could use a regular expression to replace the fifth space with a newline:

string t = Regex.Replace(s, "^([^ ]+(?: [^ ]+){4}) ", "$1" + Environment.NewLine);

Explanation of the pattern:

^       mathces the beginning of the string
(       starts a catching group
[^ ]+   matches the first word (a sequence of non-space characters)
(?:     start a non-catching group
space   matches a space
[^ ]+   mathces a word
)       ends the non-catching group
{4}     repeats the group four times
)       ends the catching group
space   matches the fifth space

The $1 in the replacement string gets the value caught by the group.

---

Another alternative would be to loop through the string and look for spaces, then use Substring to get the parts before and after the fifth space:

int cnt = 0, index = 0;
while (cnt < 5) {
  if (s[index++] == ' ') cnt++;
}
string t = s.Substring(0, index - 1) + Environment.NewLine + s.Substring(index);

Answer 2

You could use the StringBuilder class and the String.IndexOf method as follows:

Option 1

private static string ReplaceNthOccurrence(string input, int n, char find, char replaceWith)
{
    int index = -1;
    int count = 0;
    StringBuilder sBuilder = new StringBuilder(input);
    while ((index = input.IndexOf(find, index + 1)) != -1)
    {
        if (++count == n)
        {
            sBuilder[index] = replaceWith;
            break;
        }
    }
    return sBuilder.ToString();
}

Usage:

string input = "Hi this is empty string. This should be in new line!";
string output = ReplaceNthOccurrence(input , 5, ' ', '\n');

The StringBuilder class allows us to replace a particular char in the string by its index.
The String.IndexOf method allows us to fast search for a desired char in the string.

Option 2

private static string ReplaceNthOccurrence(string input, int n, char find, char replaceWith)
{
    int index = -1;
    // Loop for `n` occurrences:
    for (int count = 0; count < n; count++)
    {
        // Find a position of the next occurrence:
        index = input.IndexOf(find, index + 1);
        // If not found, return the `input` string:
        if (index == -1)
            return input;
    }
    // Replace the char and return a resulting string:
    StringBuilder sBuilder = new StringBuilder(input);
    sBuilder[index] = replaceWith;
    return sBuilder.ToString();
}

In the latter option, you can use any other replacement approach instead of the StringBuilder.
For instance:

return input.Substring(0, index) + replaceWith + input.Substring(index + 1);