title: How to convert a string array to string type in C++, as in concatenate every element together into one string, and use substring on the string?
body: <p>for example:</p>
  
  <p>Reading file input and stored into </p>
  
  <pre><code>char fileInput[200];
  </code></pre>
  
  <p>then I converted it to some sort of string array using</p>
  
  <pre><code>string fileStrArr(fileInput);
  </code></pre>
  
  <hr>
  
  <p>the testing output from the file looks like this : 50014002600325041805
  how can I use substring with a loop to get every 4-digit char and convert it to a number such as "5001" "4002" "6003"...? so I think I need to make string array into a string first?</p>