BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Re: 2Q: Danger of using FillMem() on a string? Speed of Abs(          BorlandTalk.com Forum Index -> comp.lang.pascal.delphi.misc View previous topic :: View next topic   Author Message J French Guest Posted: Fri Dec 12, 2003 3:35 pm    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( On Fri, 12 Dec 2003 15:25:54 +0100, Quivis <quivis (AT) ask (DOT) me.first> wrote: <snip> Quote: My second question is best illustrated as follows: //Quivis, 2003-12: Reverse string (S) function ReverseStr(const S: string): string; var I, iLen: integer; sTmp: string; begin iLen := Length(S); SetLength(sTmp, iLen); for I := 1 to iLen do sTmp[I] := S[Abs(I - (iLen + 1))]; Result := sTmp; end; Abs() is not slow - but this code is daft ..... bizarre is more likely Draw a string on a bit of paper, as a load of boxes - draw arrows - try a string of length two - then one of length three Think hard about what 'Div' does Who Knows (Quivis) what you will realize ? Back to top Rob Kennedy Guest Posted: Fri Dec 12, 2003 4:21 pm    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( Quivis wrote: Quote: Anyway, what I'm concerned about is, are there any dangers in using this function? Like: "it's a bad idea because [...]," and similar. The code is fine. However, it will *never* return False. If SetLength or FillMemory fail, you will get an exception and you will have no return value at all. Your error checking seems to assume that FillMemory will start filling at the beginning of the buffer and finish at the end, but it's allowed to fill the buffer backward if it wants to, in which case checking the last character won't be a good indication of success anyway. You should check for the case when iNumChars is zero or negative. A zero value will be fine for SetLength and FillMemory, but it will fail when you check S[iNumChars]. A negative value is invalid input; use Assert to check for it: Assert(iNumChars >= 0); I would declare S as an out parameter instead of var; it's more expressive of how the parameter is used. FillMemory is just a thin wrapper around the built-in FillChar procedure. Save yourself a CALL/RET and use FillChar directly. Quote: I've seen similar approaches used in many code-snippets on how to reverse a string, but so far I have not seen anyone using Abs() to do the "dirty work" of reversing the string. So my question is: Is there a reason for this? Is Abs() very slow, or what? It's not slow, but it's not needed, either. Within your loop, iLen+1 is always greater than I, so the result of subtraction will always be negative. If the result is always negative, then you can save the call to Abs and just reverse the operands: sTmp[I] := S[iLen + 1 - I]; ILen+1 is also an expression with a constant result, so you should perform the operation outside the loop. I'd remove the use of sTmp and work on Result directly. Abs is actually not a function at all. It's compiler magic, so there is no CALL/RET involved; it's about as fast as it could possibly be. -- Rob Back to top AlanGLLoyd Guest Posted: Fri Dec 12, 2003 7:04 pm    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( In article <MPG.1a43ee611d5f529a989845 (AT) 207 (DOT) 14.113.17>, Quivis <quivis (AT) ask (DOT) me.first> writes: Quote: In some of my apps I do this: //Quivis, 2002-03, edited 2003-12: Fill a string (S) with a //character (cCharacter) up to desired length (iNumChars), //returns boolean function FillString(var S: string; iNumChars: integer; cCharacter: Char): boolean; begin Result := False; //assume failure SetLength(S, iNumChars); FillMemory(pointer(S), iNumChars, Ord(cCharacter)); if S[iNumChars] = cCharacter then Result := True; end; ...and it seems to be working. To those who may wonder: I don't have a clue as to how fast this is, I've never tested it since it's mainly used on fairly short strings. I just thought it was a waste not to use already built-in stuff, and, to tell you the truth, I didn't really expect it to run... :-) But if anyone has a clue as to how fast this function might be, I'd appreciate to hear about it, although this is not why I'm posting. Anyway, what I'm concerned about is, are there any dangers in using this function? Like: "it's a bad idea because [...]," and similar. What's the problem with using Delphi's StringOfChar() ? Alan Lloyd [email]alanglloyd (AT) aol (DOT) com[/email] Back to top J French Guest Posted: Sat Dec 13, 2003 11:58 am    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( On Sat, 13 Dec 2003 11:28:19 +0100, Quivis <quivis (AT) ask (DOT) me.first> wrote: Quote: In article <3fd9dd86.10662948 (AT) news (DOT) btclick.com>, [email]erewhon (AT) nowhere (DOT) com[/email] says... 8>< /snip Abs() is not slow - but this code is daft .... bizarre is more likely See my response to Rob where it came from. 8>< /snip Think hard about what 'Div' does I came up with this: iLen := Length(S) + 1; Result := S; for I := 1 to iLen div 2 do begin //is there a faster way to swap two characters? C := Result[I]; Result[I] := Result[iLen - I]; Result[iLen - I] := C; end; Yes, that is more like it, but the code is slightly obfuscated Just because something looks terser, does not mean that it is faster - I also missed a trick - I thought you were reversing a string L := Length( S ); For L9 := 1 To L Div 2 Do Begin C := S[L9]; S[ L9 ] := Result[ L ]; S[ L ] := C; L := L - 1; End; However you are actually making a COPY of the String L := Length( S ); SetLength( Result, L ); For L9 := 1 To L Do Begin Result[ L9 ] := S[ L ]; L := L - 1 ; End; Notice that I am using a simple counter rather than doing the maths time and time again - this makes the code far easier to understand - and probably faster if the compiler can 'remember' the results of simple math - then it can certainly 'remember' to substitute a Register for a variable BTW, the reason why I would recomend using SetLength() if you are returning a Result, is because Delphi definitely creates a new unique Result, whereas assigning a copy of the string in S to Result _might_ result in reference counting - probably not, as the compiler is pretty smart - but in principle it is better to tell the compiler to do exactly what you want, rather than 'misleading' it - it also makes the code clearer to read As far as swapping characters goes, it is probably not worth worrying about - there is the ASM XCHG - but it is unlikely to be one of the 'optimized' Op codes that have had a RISC element since the .486 Quote: Who Knows (Quivis) what you will realize ? Well, it seems you have realized what my "name" means... I wasn't sure for some time ... BTW apologies for being rather sharp yesterday - it is just that I've spent much of my coding life trying to unravel 'clever' code by other people - and avoid writing it myself Back to top Rob Kennedy Guest Posted: Sat Dec 13, 2003 5:50 pm    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( Quivis wrote: Quote: In article <vtjqohcq2p3a2 (AT) corp (DOT) supernews.com>, Rob Kennedy <.> says... FillMemory is just a thin wrapper around the built-in FillChar procedure. Save yourself a CALL/RET and use FillChar directly. Are you sure? FillMemory() is, if I'm not mistaken, a system API. I am certain. Go check it yourself in the source code. Another indication is in the documentation. Unlike for most other API functions, MSDN doesn't tell you which C library to link to your code since the function doesn't appear in any library. Its implementation is left to the compiler vendor. The same goes for CopyMemory, MoveMemory, and ZeroMemory. They're implemented using Delphi's Move and FillChar. Quote: From Win32 help: [...] VOID FillMemory ( PVOID Destination, // pointer to block to fill DWORD Length, // size, in bytes, of block to fill BYTE Fill // the byte value with which to fill ); [...] The I in API stands for interface, not implementation. -- Rob Back to top Dr John Stockton Guest Posted: Sat Dec 13, 2003 9:26 pm    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( JRS: In article <MPG.1a45082b98f29eb9989849 (AT) 207 (DOT) 14.113.17>, seen in news:comp.lang.pascal.delphi.misc, Quivis <quivis (AT) ask (DOT) me.first> posted at Sat, 13 Dec 2003 11:28:19 :- Quote: iLen := Length(S) + 1; Result := S; for I := 1 to iLen div 2 do begin //is there a faster way to swap two characters? C := Result[I]; Result[I] := Result[iLen - I]; Result[iLen - I] := C; end; The entire string is copied and then reversed; it bight be better to copy it backwards. Consider something like K := 1 ; L := Length(S) ; SetLength(Result, L) ; for J := L downto 1 do begin Result[K] := S[J] ; Inc(K) end ; or K := 1 ; L := Length(S) ; SetLength(Result, L) ; repeat Result[K] := S[L] ; Inc(K) ; Dec(L) until L=0 ; or L := Length(S) ; SetLength(Result, L) ; PS = @S[L] ; PR = @R[1] ; repeat PR^ := PS^ ; Inc(PR) ; Dec(PS) until ??? ; -- © John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Delphi 3 Turnpike 4 © <URL:http://www.merlyn.demon.co.uk/> TP/BP/Delphi/&c., FAQqy topics & links; <URL:http://www.bancoems.com/CompLangPascalDelphiMisc-MiniFAQ.htm> clpdmFAQ; <URL:http://www.borland.com/newsgroups/guide.html> news:borland.* Guidelines Back to top Maarten Wiltink Guest Posted: Sat Dec 13, 2003 11:09 pm    Post subject: Re: Some tests on variations of the theme (a bit long) [ Re: "Quivis" <quivis (AT) ask (DOT) me.first> wrote Quote: In article <3fdaf94d.83320989 (AT) news (DOT) btclick.com>, [email]erewhon (AT) nowhere (DOT) com[/email] says... However you are actually making a COPY of the String Yes, but it seems Delphi refuses to work in this way directly on TMemo.Text, ... That is correct. You are not allowed to pass a property as a var parameter, because it _may_ be backed by accessor methods. Even if it's backed by a field in the current class, and constructing a reference would be possible, a descendant class may change that. So the only safe thing to do is to forbid it altogether. Groetjes, Maarten Wiltink Back to top J French Guest Posted: Sun Dec 14, 2003 1:33 pm    Post subject: Re: Some tests on variations of the theme (a bit long) [ Re: On Sat, 13 Dec 2003 18:10:51 +0100, Quivis <quivis (AT) ask (DOT) me.first> wrote: Quote: In article <3fdaf94d.83320989 (AT) news (DOT) btclick.com>, [email]erewhon (AT) nowhere (DOT) com[/email] says... 8>< /snip However you are actually making a COPY of the String Yes, but it seems Delphi refuses to work in this way directly on TMemo.Text, so I had to. Either inside the function or before I called it. I opted for doing it inside the function. They way I use it, it will only be called once for each string. TMemo.Text is a Property - in general you are probably right in making StrReverse a function - my version works on the string itself (a procedure) - this is historical, since the original was in ASM from about 1987 Quote: 8>< /snip BTW, the reason why I would recomend using SetLength() if you are returning a Result, is because Delphi definitely creates a new unique Result, whereas assigning a copy of the string in S to Result _might_ result in reference counting That's something to keep in mind. IMO/IME it is better to be totally 'honest' when writing routines - it makes them cleaner and clearer Quote: Some tests I've done, using the following setup: snip Quote: Conclusion: There are more ways than one to skin a cat, especially when the cats are so similar in size as these. True, but let's face it, the second version of the routine that I posted was 'blindingly obvious' code Quote: Well, it seems you have realized what my "name" means... ;-) I wasn't sure for some time ... Well, it actually means whoever, at least in my Latin dictionary, e.g. as in "whoever did that shall be keelhauled" Did the Romans keelhaul ? - I doubt that there boats were sufficiently barnacled Quote: BTW apologies for being rather sharp yesterday No problems. Back to top Dr John Stockton Guest Posted: Sun Dec 14, 2003 8:12 pm    Post subject: Re: 2Q: Danger of using FillMem() on a string? Speed of Abs( JRS: In article <MPG.1a465d13cde147b3989850 (AT) 207 (DOT) 14.113.17>, seen in news:comp.lang.pascal.delphi.misc, Quivis <quivis (AT) ask (DOT) me.first> posted at Sun, 14 Dec 2003 11:42:51 :- Quote: In article <uBvns5AMQ42$Ewwb (AT) merlyn (DOT) demon.co.uk>, [email]spam (AT) merlyn (DOT) demon.co.uk[/email] says... The entire string is copied and then reversed; it bight be better to ^^^ Have you caught a cold? The cause is that I am not a good typist; however, your supposition is unfortunately correct. The latter situation, unlike the former, will probably improve. -- © John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Delphi 3 Turnpike 4 © <URL:http://www.merlyn.demon.co.uk/> TP/BP/Delphi/&c., FAQqy topics & links; <URL:http://www.bancoems.com/CompLangPascalDelphiMisc-MiniFAQ.htm> clpdmFAQ; <URL:http://www.borland.com/newsgroups/guide.html> news:borland.* Guidelines Back to top Michael Brown Guest Posted: Tue Dec 16, 2003 2:31 am    Post subject: Re: Some tests on variations of the theme (a bit long) [ Re: Quivis wrote: Quote: In article <3fdaf94d.83320989 (AT) news (DOT) btclick.com>, [email]erewhon (AT) nowhere (DOT) com[/email] says... [...] Some tests I've done, using the following setup: XP-Pro, 1.3 GHz CPU (Celeron Mobile), 256 MB RAM Iterations: 100,000 for each code snippet on separate runs. String to reverse: Many websites assume that every visitor will have a browser capable of Javascript. [...] //Tick count: 761, stabilized around this value, first run: 781 [...] What? There's no heavily-unrolled, hand-optimised MMX version in there? Aww (estimated tick count: 20-50) [...] -- Michael Brown www.emboss.co.nz : OOS/RSI software and more Add michael@ to emboss.co.nz - My inbox is always open Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> comp.lang.pascal.delphi.misc All times are GMT Page 1 of 1   Jump to: Select a forum C++ Builder Newsgroups----------------C++ BuilderC++ Builder (ActiveX)C++ Builder (Announcements)C++ Builder (Command Line Tools)C++ Builder DatabasesC++ Builder Databases (ADO)C++ Builder Databases (Desktop)C++ Builder Databases (InterBase Express)C++ Builder Databases (Multi-Tier)C++ Builder Databases (SQL Servers)C++ Builder (Graphics)C++ Builder (IDE)C++ Builder (Internet)C++ Builder (Internet Socket)C++ Builder (Internet Web)C++ Builder (Jobs)C++ Builder (Language)C++ Builder (Language C++)C++ Builder (Compatibility with MFC)C++ Builder (Native API)C++ Builder (Non-Technical)C++ Builder (Students)C++ Builder (Third Party Tools)C++ Builder (Upgrade)C++ Builder (VCL)C++ Builder (VCL Components Usage)C++ Builder (VCL Components Development)C++ Builder (VCL Usage)C++ Builder (WinAPI)C++ Builder Installation Delphi Newsgroups----------------Delphi (General)Delphi ActiveX Controls UsageDelphi ActiveX Controls DevelopmentDelphi As400Delphi DatabasesDelphi Databases (ADO)Delphi Databases (dbExpress)Delphi Databases (Desktop)Delphi Databases (InterBase Express)Delphi Databases (Multi-Tier)Delphi Databases (SQL Servers)Delphi DeploymentDelphi GraphicsDelphi IDEDelphi InternationalizationDelphi InternetDelphi Internet Isapi-WebBrokerDelphi Internet WebSnapDelphi Internet WinsockDelphi JobsDelphi Language BASMDelphi Language ObjectPascalDelphi Model Driven Architecture (General)Delphi Model Driven Architecture (Third Party)Delphi MultimediaDelphi Native APIDelphi Non-TechnicalDelphi ObjectPascalDelphi OLE AutomationDelphi OO designDelphi OpenToolsAPIDelphi Reporting-ChartingDelphi RTLDelphi StudentsDelphi TeamSourceDelphi Thirdparty ToolsDelphi Thirdparty Tools (General)Delphi Thirdparty Tools (IntraWeb)Delphi Thirdparty Tools (RaveReports)Delphi UpgradeDelphi VCLDelphi VCL ComponentsDelphi VCL Components UsingDelphi VCL Components WritingDelphi WebServices SOAPDelphi WinAPIDelphi XMLDelphi Installationcomp.lang.pascal.delphi.components.misccomp.lang.pascal.delphi.components.usagecomp.lang.pascal.delphi.components.writingcomp.lang.pascal.delphi.databasescomp.lang.pascal.delphi.misccomp.sources.delphi JBuilder Newsgroups----------------JBuilderJBuilder AnnouncementsJBuilder Applet IssuesJBuilder As400JBuilder CompilerJBuilder CORBA/RMIJBuilder DatabasesJBuilder DB SwingJBuilder DebuggerJBuilder DeploymentJBuilder DocumentationJBuilder EnterpriseJBuilder HandheldJBuilder IDEJBuilder JavaJBuilder JavaAPIJBuilder Java LanguageJBuilder Java SwingJBuilder Java BeansJBuilder Java Beans UsageJBuilder Java Beans DevelopmentJBuilder JBCLJBuilder JDataStoreJBuilder JobsJBuilder LinuxJBuilder MacintoshJBuilder & MobileSetJBuilder MultiLingual AppsJBuilder Non-TechnicalJBuilder OpenToolsJBuilder Servlets (JSP)JBuilder StudentsJBuilder Team DevelopmentJBuilder Thirdparty ToolsJBuilder UpgradeJBuilder Installation InterBase Newsgroups----------------InterBaseInterBase Bug DiscussionInterBase GeneralInterBase IBConsoleInterBase OpenSourceInterBase ReplicationInterBase SQLInterBase ThirdpartyInterBase Installation Miscellaneous Borland Newsgroups----------------AppCenterAppServerEnteraVisiBroker  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.