Replace fifth space with new line

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);

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);

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);