title: string in c#. replace certain number in a loop
body: <p>I have string. "12341234115151_log_1.txt"  (this string length is not fixed. but "<em>log</em>" pattern always same)
  I have a for loop. 
  each iteration, I want to set the number after "log" of i.</p>
  
  <p>like "12341234115151_log_2.txt"
  "12341234115151_log_3.txt"
  ....</p>
  
  <p>to
  "12341234115151_log_123.txt"</p>
  
  <p>in c#, what is a good way to do so?
  thanks.</p>