#004
07 января 1987 |
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Shoutburst - program structure
PROGRAM STRUCTURE By Brian Cavers _________________ I wish somebody would explain what the difference is between a procedure and a subroutine. No, I don't mean the superficial difference I mean the structural difference. Let's look at a definition:-. A BASIC PROGRAM IS A LIST OF INSTRUCTIONS WHICH IS FOLLOWED IN SEQUENCE BY THE COMPUTER. THERE IS A SPECIAL INSTRUCTION, FOLLOWED BY A LABEL,WHICH CAUSES THE COMPUTER TO JUMP OUT OF THE SEQUENCE AND TO CONTINUE FROM A PLACE IN THE PROGRAM POINTED TO BY THE LABEL. THE COMPUTER CONTINUES TO CARRY OUT THE INSTRUCTIONS FOLLOWING THIS POINT, IN SEQUENCE, UNTIL IT MEETS ANOTHER SPECIAL COMMAND WHICH CAUSES IT TO JUMP BACK AND CONTINUE WITH THE ORIGINAL SEQUENCE.. It seems to me that this could equally be the definition of a procedure or of a subroutine. The "special instruction": PROC with a label is no different to GOSUB with a label and ENDPROC is the exact equivalent of RETURN. I realize that the Spectrum doesn't support true labels but they can be simulated with variables and other machines do provide labelled GOSUBS. I own an Atari ST with FAST BASIC which provides both procedures and subroutines. The start of the subroutine is marked with a label and the finish with a RETURN. It is called with GOSUB LABEL in the . usual way. The start of a procedure is marked with DEF PROC LABEL and the end with ENDPROC in the usual way. It is called from the main program with PROC LABEL. The manual actively discourages GOSUBS as if they contribute to bad programming and it is stated that they were only included in the BASIC to be compatible with other BASICS! However, it does not give any rational argument as to why it is thought that procedures are structurally superior. After a week or two programming using procedures I discovered that FAST BASIC executes subroutines FASTER than procedures! I know what I shall use in future!. Actually this is what infuriates me about what I call "structure" fanatics. There is this smug, superior claim, as if it is self-evident, that subroutines are obsolete and that one "should" use procedures but I HAVE NEVER YET SEEN ANY EXPLANATION! THEY REALLY DO LOOK LIKE IDENTICAL COMMANDS TO ME! I CHALLENGE ANYONE TO PROVIDE A RATIONAL EXPLANATION AS TO WHY PROCEDURES ARE STRUCTURALLY SUPERIOR TO SUBROUTINES. The poor old GO TO takes even more of a hammering from the structural purists. I realize that numerous jumps back and forth can turn a program into a debugger's nightmare but I. cannot see what is wrong in having several main routines making up a program, each one being jumped to as needed. For example, in a database: INPUT ROUTINE, SORT ROUTINE, PRINT ROUTINE, DISPLAY ROUTINE etc. Additionally, why is: 10 LET A = 1 20 LET A = A + 1 30 PRINT A 40 IF A <25 THEN GO TO 20 any less structurally sound than: 10 FOR A = 1 TO 25 20 PRINT A 30 NEXT A. or: 10 LET A = 1 20 WHILE A <=25 30 PRINT A 40 LET A = A + 1 50 WEND. In every case what the program is actually doing is jumping back in a loop. The loop is the structure - the labels: GO TO, FOR NEXT, WHILE WEND, to me, are cosmetic - irrelevant to the structure of the program. (This is not to say, of course, that these commands don't all have their uses in the right place - they do but it's not STRUCTURALLY RELEVANT in my view!).. Another thing! (I'm really getting annoyed now - but then, this is SHOUTBURST isn't it!). In many BASICS, the computer keeps track of the program's position in FOR NEXT loops by use of a stack, I believe. This means it could cause problems if you jump out of the middle of a FOR NEXT loop without finishing it properly. This has given rise to the nonsense that it is structurally inefficient to escape FOR NEXT loops in that way. On the Spectrum, the FOR NEXT variable is just another variable and can be abandoned at any time and another loop started using the same variable, for example:-. 10 FOR N = 1 TO 25 20 GO TO 50 30 NEXT N 40 STOP 50 FOR N = 1 TO 4 60 NEXT N NO PROBLEM TO THE SPECTRUM AND NO STRUCTURAL PROBLEM EXCEPT IN SOMEONE'S IMAGINATION! To sum up, I think what make's my blood boil most is not the claim that certain features are structurally superior but that such claims never seem to be accompanied by rational, objective supporting evidence or arguments. After all, it is not up to me to support my position - I'm not the one who's. SUPPORTING anything (in this particular topic). It's the people who claim that certain labels somehow CONSTITUTE correct structure who need to prove what they say is true! To finish, I'd just like to emphasise how important I believe structure is in programming - I take a great deal of care when writing the structure of my own programs. However, I believe that structure lies in the WAY THE PROGRAMMER ARRANGES HIS INSTRUCTIONS AND NOT IN SUPERFICIAL LABELS! Grrrrrrrrrrrrrrr!.
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