title: concatenate string and running index into string within a loop
body: <p>To use a given graphic package I need to define, book and fill histogram.
  How can I get the name of the histogram which is a string to concatenate 
  with 2 integer as a string ( hts_i_j ) in 3 for loop instead.
  That has to be done in c++ 
  See  the exemple below</p>
  
  <p>to define </p>
  
  <pre><code>TH1F* hts_5_53;
  TH1F* hts_5_54;
  ……
  TH1F* hts_5_69;
  </code></pre>
  
  <p>to book</p>
  
  <pre><code>hts_5_53= HDir.make&lt;TH1F&gt;("hts_5_53")," Title", 100,0.,100.);
  hts_5_54-&gt;HDir.make&lt;TH1F&gt;("hts_5_54")," Title", 100,0.,100.);
  ……
  hts_16_69-&gt;HDir.make&lt;TH1F&gt;("hts_16_69")," Title", 100,0.,100.);
  </code></pre>
  
  <p>to fill</p>
  
  <pre><code>hts_5_53-&gt;Fill(f)
  hts_5_54-&gt;Fill(f)
  ……
  hts_16_69-&gt;Fill(f)
  </code></pre>
  
  <p>Instead I would like to define, book and fill in 3 for loops.
  e.g
  .</p>
  
  <pre><code>for(int i=5, i&lt;17, ++i){
    for(int j=53, j&lt;70, ++j){
  
     hts_i_j 
  
   } 
  }
  </code></pre>
  
  <p>how can I get the string hts to concatenate with the 
  indices ( i,j) in a simple short way while defining, 
  booking and filling in 3 for loop instead</p>