Another idea is to unroll the first iteration of the loop. This way you don't have to test for a flag on subsequent iterations. I believe this is the correct approach optimisation-wise - if one condition is true only once and then guaranteed false for the next 1.000.000 times, checking it programatically (and doing other stuff like modifying the flag variable) 1.000.001 times is a waste. Pseudocode:

if ( CONDITION ) {
  do_iteration();
  while ( CONDITION ) {
    print ",\n";
    do_iteration();
  }
}
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