Help creating alphanumeric string

Projects Programming
12 
  char chr;

// ............ snip

// ............ somewhere in void loop ---- How to filter data to handle by type

  if (( chr >= '0' ) && ( chr <= '9' )  { chr is a text digit, value = chr - '0';  do something; }

  else   if (( chr >= 'A' ) && ( chr <= 'Z' )  { chr is text uppercase alpha;  do something; }
  
  else   if (( chr >= 'a' ) && ( chr <= 'z' )  { chr is text lowercase alpha;  do something; }

  else  chr = 0;  // not an alpha-numeric

you could do

  for ( chr = '0'; chr < ( '9' + 1 ); chr++ )  { print( chr ); }
  print( "\n\n" );

  for ( chr = 'A'; chr < ( 'Z' + 1 ); chr++ )  { print( chr ); }
  print( "\n\n" );

  for ( chr = 'a'; chr < ( 'z' + 1 ); chr++ )  { print( chr ); }
  print( "\n\n" );