BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  something I came to think about ... (logical operators in if Goto page 1, 2, 3  Next          BorlandTalk.com Forum Index -> comp.lang.pascal.delphi.misc View previous topic :: View next topic   Author Message Nicolai Hansen Guest Posted: Wed Nov 12, 2003 1:01 pm    Post subject: something I came to think about ... (logical operators in if NB! This is not something I have tried - just a thought that struck me after reading some c++ NG (you sometimes get weird thoughts then) :) Some logic operator law states that (a or b) is the same as (not (a and b)). However, I now have doubt that this holds true for if statements ;) Consider the following two statements: 1) if (p=nil) or (p->d=nil) then 2) if not ((p<>nil) and (p->d<>nil)) then (consider p as a pointer to an object with one element, d, being another pointer. The above is then a general check for NULL data) 1) If either p=nil, or the element d=nil, this returns true. But because of the compiler's interpretation of OR, we know not if (p=nil)? or (p->d=nil)? is executed first. If (p->d=nil) is executed first when (p=nil) is actually true, this will result in an access violation. 2) With AND it is guaranteed that (p<>nil) is executed first. (p->d<>nil) is only ever checked IF p<>nil holds true. This leads to a different behaviour than case 1). And then I got scared :p Cos I seem to recall reading somewhere that the compiler do adjust these IF statements, exchanging "not (a AND b)" with "(a OR b)" automatically. But it is not the same? Is this true? (haha, got that one? "true" :p) -or- (again! haha) am I just starting another useless theoretical thread about nothing? Back to top AlanGLLoyd Guest Posted: Wed Nov 12, 2003 2:17 pm    Post subject: Re: something I came to think about ... (logical operators i In article <d96764ff.0311120501.48c2abfa (AT) posting (DOT) google.com>, [email]nic (AT) aub (DOT) dk[/email] (Nicolai Hansen) writes: Quote: 1) If either p=nil, or the element d=nil, this returns true. But because of the compiler's interpretation of OR, we know not if (p=nil)? or (p->d=nil)? is executed first. If (p->d=nil) is executed first when (p=nil) is actually true, this will result in an access violation. Not so (at least not in D3), D3 Help says ... "Object Pascal supports two different models of code generation for the and and or operators: complete evaluation and short-circuit (partial) evaluation. Complete evaluation means that every operand of a Boolean expression built from the and and or operators is guaranteed to be evaluated, even when the result of the entire expression is already known. <snip> Short-circuit evaluation guarantees strict left-to-right evaluation and that evaluation stops as soon as the result of the entire expression becomes evident. <snip> Short-circuit evaluation also makes possible the evaluation of constructs that would not otherwise be legal." Alan Lloyd [email]alanglloyd (AT) aol (DOT) com[/email] Back to top Dave Langers Guest Posted: Wed Nov 12, 2003 2:18 pm    Post subject: Re: something I came to think about ... (logical operators i Quote: Some logic operator law states that (a or b) is the same as (not (a and b)). Of course you meant (a or b) equals (not ((not a) and (not b))) ! Quote: However, I now have doubt that this holds true for if statements ;) Consider the following two statements: 1) if (p=nil) or (p->d=nil) then 2) if not ((p<>nil) and (p->d<>nil)) then (consider p as a pointer to an object with one element, d, being another pointer. The above is then a general check for NULL data) I don't know the "p->d" notation; is it Delphi, and if so, is it the same as something like "p^.d"? Quote: 1) If either p=nil, or the element d=nil, this returns true. But because of the compiler's interpretation of OR, we know not if (p=nil)? or (p->d=nil)? is executed first. If (p->d=nil) is executed first when (p=nil) is actually true, this will result in an access violation. 2) With AND it is guaranteed that (p<>nil) is executed first. (p->d<>nil) is only ever checked IF p<>nil holds true. This leads to a different behaviour than case 1). I think I recognize this from a C-type language where there is a difference between the operators & and | versus && and ||, in Delphi it is controlled by the {$B} compiler directive. However, if a compiler is intelligent enough to know that he doesn't have to evaluate (p^.d <> nil) after (p <> nil) has already evaluated to FALSE in an and-statement, why wouldn't he be intelligent enough to know that he doesn't have to evaluate (p^.d = nil) after (p = nil) has already evaluated to TRUE in an or-statement? (I haven't tried this in a piece of code, and the help section on complete versus short-circuit Boolean evaluation only shows a few 'and'-examples, but it would be kind of silly if 'or' would behave differently, wouldn't it?) Quote: And then I got scared :p Cos I seem to recall reading somewhere that the compiler do adjust these IF statements, exchanging "not (a AND b)" with "(a OR b)" automatically. But it is not the same? Is this true? (haha, got that one? "true" :p) -or- (again! haha) am I just starting another useless theoretical thread about nothing? Yeah, haha ;-S -- M.vr.gr. Dave ("d-dot-langers-at-wxs-dot-nl") Back to top Nicolai Hansen Guest Posted: Wed Nov 12, 2003 3:07 pm    Post subject: Re: something I came to think about ... (logical operators i "Dave Langers" <spam.messages (AT) annoy (DOT) us> skrev i en meddelelse news:botffj$1hpsg4$1 (AT) ID-86745 (DOT) news.uni-berlin.de... Quote: Some logic operator law states that (a or b) is the same as (not (a and b)). Of course you meant (a or b) equals (not ((not a) and (not b))) ! I just might :) Quote: I don't know the "p->d" notation; is it Delphi, and if so, is it the same as something like "p^.d"? Oops - as I started, I got to think about this while reading a post in a c++ NG replace "p->d" with "p.d" :) Quote: However, if a compiler is intelligent enough to know that he doesn't have to evaluate (p^.d <> nil) after (p <> nil) has already evaluated to FALSE in an and-statement, why wouldn't he be intelligent enough to know that he doesn't have to evaluate (p^.d = nil) after (p = nil) has already evaluated to TRUE in an or-statement? I was frightened it might evaluate it from right to left if it felt like it But from reading Alan's post, I think I worry without a cause :D Anyway, it was just one of these silly small things I was thinking about while I was supposed to work ;) Back to top Nicolai Hansen Guest Posted: Wed Nov 12, 2003 3:07 pm    Post subject: Re: something I came to think about ... (logical operators i Quote: Short-circuit evaluation guarantees strict left-to-right evaluation and that evaluation stops as soon as the result of the entire expression becomes evident. All I can say is 'phew', my world lasts another day :p Back to top René Allan Larsen Guest Posted: Wed Nov 12, 2003 6:41 pm    Post subject: Re: something I came to think about ... (logical operators i In article <d96764ff.0311120501.48c2abfa (AT) posting (DOT) google.com>, Nicolai Hansen wrote: Quote: NB! This is not something I have tried - just a thought that struck me after reading some c++ NG (you sometimes get weird thoughts then) :) Some logic operator law states that (a or b) is the same as (not (a and b)). However, I now have doubt that this holds true for if statements ;) Consider the following two statements: 1) if (p=nil) or (p->d=nil) then 2) if not ((p<>nil) and (p->d<>nil)) then (consider p as a pointer to an object with one element, d, being another pointer. The above is then a general check for NULL data) 1) If either p=nil, or the element d=nil, this returns true. But because of the compiler's interpretation of OR, we know not if (p=nil)? or (p->d=nil)? is executed first. If (p->d=nil) is executed first when (p=nil) is actually true, this will result in an access violation. That is why you should always use "Boolean short-circuit evaluation" (which Delphi defaults to anyway): The Delphi compiler supports two modes of evaluation for the and and or operators: complete evaluation and short-circuit (partial) evaluation. Complete evaluation means that each conjunct or disjunct is evaluated, even when the result of the entire expression is already determined. Short-circuit evaluation means strict left-to-right evaluation that stops as soon as the result of the entire expression is determined. For example, if the expression A and B is evaluated under short-circuit mode when A is False, the compiler won’t evaluate B; it knows that the entire expression is False as soon as it evaluates A. Short-circuit evaluation is usually preferable because it guarantees minimum execution time and, in most cases, minimum code size. Complete evaluation is sometimes convenient when one operand is a function with side effects that alter the execution of the program. Short-circuit evaluation also allows the use of constructions that might otherwise result in illegal runtime operations. Quote: 2) With AND it is guaranteed that (p<>nil) is executed first. (p->d<>nil) is only ever checked IF p<>nil holds true. This leads to a different behaviour than case 1). No. If you switch to "complete evaluation", (p->d<>nil) is checked EVEN if p<>nil holds true. Regards, René Back to top AlanGLLoyd Guest Posted: Wed Nov 12, 2003 7:09 pm    Post subject: Re: something I came to think about ... (logical operators i In article <3fb24cfe$0$27402$edfadb0f (AT) dread16 (DOT) news.tele.dk>, "Nicolai Hansen" <nic (AT) aub (DOT) dk> writes: Quote: But from reading Alan's post, I think I worry without a cause :D It's always useful to read and understand as much as one cane before asking questions, then the questions become really helpful and can confirm (or deny) the mental concept one has built from the printed words. Alan Lloyd [email]alanglloyd (AT) aol (DOT) com[/email] Back to top VBDis Guest Posted: Sat Nov 15, 2003 6:57 pm    Post subject: Re: something I came to think about ... (logical operators i Im Artikel <d96764ff.0311120501.48c2abfa (AT) posting (DOT) google.com>, [email]nic (AT) aub (DOT) dk[/email] (Nicolai Hansen) schreibt: Quote: 1) if (p=nil) or (p->d=nil) then Apart from the "complete boolean eval" topic there's another trick in this condition. Exchange both comparisons, and you'll get an AV if p=nil. Conditions of the above form often are used to check an pointer for non-nil, before accessing members of the data structure that is referenced by the pointer. Of course such cascaded tests only work properly with short-circuit evaluation. If you want to make such code bullet proof, you must split the condition into nested IF statements, which are executed in a predictable manner. Perhaps you now see the difference between boolean logic in theory, and the way how conditions are used and evaluated in programming practice. Pure and simple math often is not sufficient to write working code, as soon as numerical constraints, dependencies or side effects come into play. DoDi Back to top Martin Harvey (Demon Acco Guest Posted: Sat Nov 15, 2003 7:42 pm    Post subject: Re: something I came to think about ... (logical operators i On 12 Nov 2003 05:01:18 -0800, [email]nic (AT) aub (DOT) dk[/email] (Nicolai Hansen) wrote: Quote: Some logic operator law states that (a or b) is the same as (not (a and b)). De Morgan's law. Quote: However, I now have doubt that this holds true for if statements Short circuit evaluation may change the legality of some constructs. MH. Back to top David Reeve Guest Posted: Sun Nov 16, 2003 12:52 pm    Post subject: Re: something I came to think about ... (logical operators i "VBDis" <vbdis (AT) aol (DOT) com> wrote Quote: Im Artikel <d96764ff.0311120501.48c2abfa (AT) posting (DOT) google.com>, [email]nic (AT) aub (DOT) dk[/email] [snip] Quote: Perhaps you now see the difference between boolean logic in theory, and the way how conditions are used and evaluated in programming practice. Pure and simple math often is not sufficient to write working code, as soon as numerical constraints, dependencies or side effects come into play. Too true..... Have you ever used a hardware descriptor language like AHDL or VHDL? Looks a bit like Pascal, but compiles the code to yield logic gates in silicon. Thus IF A == cat THEN OP = Vcc; ELSE OP = Gnd; END IF; Though written sequentially, the behaviour it describes is simultaneously 'in place'. The problem with implementation of logic via a sequential machine, ie a processor, is just that, sequential, and you need to understand how the compiler will construct that sequence. Dave Back to top Nicolai Hansen Guest Posted: Sun Nov 16, 2003 5:15 pm    Post subject: Re: something I came to think about ... (logical operators i Quote: Have you ever used a hardware descriptor language like AHDL or VHDL? Looks a bit like Pascal, but compiles the code to yield logic gates in silicon. Thus Fun that you mention it .. One of our hardware engineers (or electronic engineer, or something just like that) is trying to learn me VHDL at the moment. Fear, cause soon even weirder ideas will pop in my twisted mind :D Back to top Martin Harvey (Demon Acco Guest Posted: Mon Nov 17, 2003 2:30 am    Post subject: Re: something I came to think about ... (logical operators i On Sun, 16 Nov 2003 12:52:10 GMT, "David Reeve" <dree4456 (AT) big-pond (DOT) net.au> wrote: Quote: Have you ever used a hardware descriptor language like AHDL or VHDL? Looks a bit like Pascal, but compiles the code to yield logic gates in silicon. Thus IF A == cat THEN OP = Vcc; ELSE OP = Gnd; END IF; Though written sequentially, the behaviour it describes is simultaneously 'in place'. The problem with implementation of logic via a sequential machine, ie a processor, is just that, sequential, and you need to understand how the compiler will construct that sequence. Yep, I used Verilog for a bit - programmed some Xilinx FPGA's with it. Very cool - we had some nice software which would take the verilog, compile it, do all the placing / routing / whatever, and the translate it into counters, gates etc that then got programmed into the chip. Very neat, but you have to be careful if you're a software programmer - the apparent "sequentiality" is of course false - although you can tell it which bits you want clocked, and which bits you don't. Kinda cool: All the statements in your "program" execute simultaneously ;-) <Insert manic discussion about clocked versus asynchronous logic here> MH. Back to top David Reeve Guest Posted: Mon Nov 17, 2003 6:24 am    Post subject: Re: something I came to think about ... (logical operators i "Nicolai Hansen" <nic (AT) aub (DOT) dk> wrote Quote: Have you ever used a hardware descriptor language like AHDL or VHDL? Looks a bit like Pascal, but compiles the code to yield logic gates in silicon. Thus Fun that you mention it .. One of our hardware engineers (or electronic engineer, or something just like that) is trying to learn me VHDL at the moment. Fear, cause soon even weirder ideas will pop in my twisted mind :D Not only is this stuff really good fun, the results are commercially potent. Something that doesn't happen every day You'll enjoy yourself provided you don't lose sight of what it means in hardware terms. Thus A = B, takes a wire from the output of object B and connects it to the input of object A. Its that simple, and that abstract! Dave Back to top David Reeve Guest Posted: Mon Nov 17, 2003 7:36 am    Post subject: Re: something I came to think about ... (logical operators i "Martin Harvey (Demon Account)" <martin (AT) pergolesi (DOT) demon.co.uk> wrote in message news:5gcgrvsuab3d5srbtifqfq15oh351aks97 (AT) 4ax (DOT) com... Quote: [snip] Insert manic discussion about clocked versus asynchronous logic here We're in danger of running way OT here, but might be excused because it does bear upon the original topic of how semantics ultimately must map onto the real world, warts and all. Anyway, my excuse is that I find programming languages so unreasonably exciting The hardware descriptor languages are extreme in this regard, and amaze and delight in the way hardware can be instantiated by means of such a remote and abstract mapping...... not a soldering iron, or bit of Vero board in sight. A little sample.... a preloadable up/down counter....note the distinction between simultaneous and sequential is not made using any keyword but is derived purely from the semantic context. And this is a very simple example. Things get much more complex, but still work very well. Like with Delphi, the trick it seems is to develop a feeling for the underlying ethos and work with it. It's not good to try to outsmart the compiler. Dave ( clk : INPUT; -- declares ins and outs d[11..0] : INPUT; countup : INPUT; clear : INPUT; preset : INPUT; load : INPUT; enctr : INPUT = Vcc; q[11..0] : OUTPUT; ) VARIABLE -- just a var block reg[11..0] : DFF; BEGIN - DEFAULTS reg[].clrn = VCC; reg[].prn = VCC; END DEFAULTS; reg[].clk = clk; -- wires the dflops q[] = reg[]; !reg[].prn = preset OR (load & d[]); -- wires in the load functionality !reg[].clrn = clear OR (load & !d[]); -- this behaviour is asynchronous ie always in place IF countup & enctr THEN -- describes up/down count behaviour reg[] = reg[] + 1; -- this behaviour occurs only on the rising edge of the clock ELSIF !countup & enctr THEN -- ie synchronous logic reg[] = reg[] - 1; ELSE reg[] = reg[]; END IF; END. Back to top Maarten Wiltink Guest Posted: Mon Nov 17, 2003 1:05 pm    Post subject: Re: something I came to think about ... (logical operators i "Martin Harvey (Demon Account)" <martin (AT) pergolesi (DOT) demon.co.uk> wrote in message news:5gcgrvsuab3d5srbtifqfq15oh351aks97 (AT) 4ax (DOT) com... [...] Quote: [Verilog, a hardware description languages is] Very neat, but you have to be careful if you're a software programmer - the apparent "sequentiality" is of course false - although you can tell it which bits you want clocked, and which bits you don't. SEQ x=y y=x PAR x=y y=x _Almost_ a hardware description language (and being abused here, yes). 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