SECTION D - PRIMER                                   FILE PRIMER.D
                                          LOOPS
                                          *****

              A LOOP is when an instruction or a set of instructions keeps 
          repeating itself. It has a variety of uses and is a very important 
          part of the programmers armoury. Unfortunately, however, a loop can 
          be formed accidentally and can be alarming to the beginner. At one 
          extreme, the screen may fill rapidly with repeated characters, at 
          the other the screen may go static or blank. 
              Dealing with ACCIDENTAL LOOPS ===  Most times all that is 
          required when working with pure BASIC  is to press the STOP key, 
          then investigate the cause. A prime cause is careless use of the 
          GOTO command.For example the single line program  { 10 PRINT 
          "Hello";: GOTO 10 } will cause the word Hello to be printed until it 
          fills the screen. The only way to stop it is to press the STOP key( 
          or switch off) This is bad enough but if the line was { 10 GOTO 10} 
          the screen would become blank. Try it if you like! In practice ,the 
          cause may not be so obvious, and may need a little detective work.
              When working closer to the fundamental controls of the computer 
          such as when using machine code with BASIC, accidental loops are in 
          general more pernicious and may not respond to the "STOP" treatment. 
          In this case it may be necessary to reboot CP/M using 
          SHIFT/EXTRA/EXIT or even switch off and then on again before 
          restarting CP/M. In both cases all work that has not been saved to 
          disc will be lost. The golden rule is therefore always to SAVE a 
          program to disc before attempting to run it. This message is 
          included here for information in case you copy a BASIC program 
          containing machine code from a magazine. It should never happen with 
          this course,except perhaps when dealing with ESCAPE CODES
              Getting back to intentional loops; they can be conditional or 
          unconditional. A conditional loop is one in which the looping will 
          end when a predetermined condition is met. For example we could 
          arrange for the loop to repeat itself a set number of times or to 
          stop when a codeword is typed into the keyboard. An unconditional 
          loop is one which continues to repeat itself until stopped by the 
          operator. One example of this is an advertising program which runs 
          through then has a GOTO to the start again. You have probably seen 
          these in shop windows.
              In PRIMER we shall only deal with the most common loop, the 
          FOR....NEXT loop. This takes takes the form { FOR a=1 to 500: NEXT } 
          Nothing will appear on the screen, but this loop is used as a time 
          delay , as the machine will count from 1 to 500 before carrying on 
          with the program. Two more examples of this can be seen in the 
          listing in Part 1- Introductory text..
              A useful variation on the above is {FOR a=1 TO 500 STEP 2:NEXT } 
          when the count will go 1,3,5 etc.,in steps of 2. Counts can also be 
          decreased as { FOR a =500 TO 1: NEXT } . Practical uses for the 
          FOR...NEXT loop are given in the next listing coming up. Please take 
          the opportunity to experiment with the examples given and also to 
          find and recognise FOR...NEXT loops in other BASIC programs.      

          END of file PRIMER.D
 and also to 
          find and recognise FOR...NEXT loops in other