Question

I want that user write what ever string he wish (one word or whole sentence) and then I want to find each “in” and replace it with “ter” but before replacing it, the position of each “in” should be printed.

  Console.Write("Write some string: ");
  string s1 = Console.ReadLine();

  s1 = s1.Replace("in", "ter");
  Console.WriteLine("After replacement we got new string {0}!", s1);
  Console.ReadKey(true);

Answer 1

You are looking for IndexOf.

bool done = false;
int startIndex = 0;
while (!done)
{
    var index = s1.IndexOf("in", startIndex);
    if (index < 0)
    {
        done = true;
    }
    else
    {
        Console.WriteLine("Found at position {0}", index);
        startIndex = index + 2;
    }
    if (startIndex >= s1.Length)
    {
        done = true;
    }
}