Loops in ada and the implementation

Question

Below is a piece of code shown and doubts are regarding the implementation of loops

               C := character'last; --'// SO code colorizer hack
               I := 1;
               K : loop
                  Done := C = character'first; --'
                  Count2 := I;
                  Exit K when Done;
                  C := character'pred(c);  --'
                  I := I + 1;
               end loop K;

Can anyone please tell me what does 'K' stands for.I guess its not a variable.How does 'K' control the execution of the loop?

Answer 1

K is the name of the loop. The end loop and Exit statements refer to that name, to make it clear what loop is being exited.

The Ada Reference Manual calls it a "loop_statement_identifier".

Answer 2

K is a label that names the loop. Wow, it has been a long time since I've seen any Ada...

Answer 3

As noted, K is the loop's label. It allows you to identify a particular loop to aid readability, and also to selectively exit a specific loop from a set of nested enclosing ones (i.e. being a "goto"...shhh! :-)

Here's a contrived example (not compiled checked):

   S : Unbounded_String;
   F : File_Type;
   Done_With_Line : Boolean := False;
   All_Done       : Boolean := False;
begin
    Open(F, In_File, "data_file.dat");
  File_Processor:
    while not End_Of_File(F) loop
        S := Get_Line(F);
       Data_Processor:
        for I in 1 .. Length(S) loop
           Process_A_Character
                (Data_Char => Element(S, I),   -- Mode in
                 Line_Done => Done_With_Line,  -- Mode out
                 Finished  => All_Done);       -- Mode out

           -- If completely done, leave the outermost (file processing) loop
           exit File_Processor when All_Done;

           -- If just done with this line of data, go on to the next one.
           exit Data_Processor when Done_With_Line;
        end loop;
    end loop File_Processor;
    Close(F);
 end;

Answer 4

K is essentially the name of the loop. The exit k tells the code to stop looping and go to the next statement after loop k ends.

You usually don't need to name loops, as you can just say exit and it will exit the enclosing loop. However, if you have a loop nested inside another loop, and you'd like to exit not the one immediately around the exit statement, but the outermost one, then doing something like this may be required.