Solved

Split a string: needed efficient algoritme

Posted on 1998-06-07
5
145 Views
Last Modified: 2012-05-07
Having no time to do this myself right now,
I would like to have the following slow code modified so that it becomes
much, much quicker.
Function Reserve can go away, but the procedure SplitFromEnd must give the exact same result as it does know.
The typical length of the string to process in SplitFromEnd is about 20 Kb.
So Reserve is very very inefficient.
Please post your answer ASAP, and in the form of a procedure I can cut and paste
in my program right away and do not give mere hints.

With Regards,
 Donker


Function Reverse(Inputstring: String): String;
var x : integer;
    ToProcess : String;
begin
    ToProcess:='';
    for x:= 1 to length(Inputstring)
    do ToProcess:= copy(Inputstring,x,1) + ToProcess;
    Reverse:=ToProcess;

end;

procedure SplitFromEnd(SplitAt: String; var InputString, PostString: String) ;
var TmpPos: integer;
    Before, AfterIncl, ToProcess: String;
    tmpSplitAt: string;
begin
   ToProcess:=Reverse(Inputstring);
   tmpSplitAt:=Reverse(SplitAt);

   TmpPos := pos(tmpSplitAt, ToProcess);

   if tmppos = 0
   then begin
        MessageDlg('SplitFromEnd: NOT FOUND', mtInformation,[mbOk], 0);
        end;

   AfterIncl:= copy(ToProcess, 1, tmppos + length(SplitAt));
   AfterIncl:=reverse(AfterIncl);
   Before :=  copy(ToProcess, tmppos +length(SplitAt), length(ToProcess));
   Before := reverse(Before);

   InputString:=Before;
   Poststring:= AfterIncl;
end;
0
Comment
Question by:padonker
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
5 Comments
 
LVL 5

Accepted Solution

by:
inter earned 350 total points
ID: 1334779
Hi friend,
This routine does no reversion and it only do one copy operation. May be it full fill you needs.

procedure SplitFromEnd(SplitAt: string; var InStr, PostStr: string);
var
  P : PChar;
  C : PChar;
  CLen : integer;
  Found : boolean;
begin
  // Append #0 for null terminated processing
  C := @SplitAt[1];
  CLen := Length(SplitAt);
  P := @InStr[Length(InStr) - Length(SplitAt)];
  SplitAt := SplitAt + #0;
  // try to find the SplitAt from right
  Found := false;
  while (P <> @InStr[1]) and not Found do  //while not hit to the begining
  begin
    Dec(P);
    Found := StrLComp(P, C, CLen) = 0;
  end;
  if Found then
  begin
    //copy post
    PostStr := StrPas(P);
    //Truncate the InStr
    SetLength(InStr, Longint(P) - Longint(@InStr[1]));
  end else
    MessageDlg('SplitFromEnd: NOT FOUND', mtInformation, [mbOk], 0);
end;

Awaiting comments
Regards,
Igor
0
 
LVL 5

Expert Comment

by:ronit051397
ID: 1334780
Maybe this Reverse function will work faster:

Function Reverse(Inputstring: String): String;
var P,S: pChar;
begin
  S:=pChar(Inputstring);
  P:=StrEnd(S)-1;
  Dec(S);
  while P<>S do
  begin
    Result:=Result+P[0];
    Dec(P);
  end;
end;
0
 

Author Comment

by:padonker
ID: 1334781
I say: Yes, this is a excellent piece of code.
Thank you Inter.



With regards,
   Donker
0
 
LVL 6

Expert Comment

by:Holger101497
ID: 1334782
I guess inter's function will be the fastest... although it does 20000 string compares if the string is 20000 long... I don't know the internal implementation of POS, so I don't know how fast it is...

some hints about your code:
copy(Inputstring,x,1) is InputString[x] ! I don't know how "smart" the compiler is in optimization, but you definitely don't need a function call here! InputString[x] is (almost) directly translated into an address


ronit: Your code will only be marginally faster - if at all.
The problem is this: for x:= 1 to length(Inputstring) do ToProcess:= copy(Inputstring,x,1) + ToProcess; OR Result:=Result+P[0];

They both append the characters one by one and re-allocate the string 20000 times! The "correct" way of reversing it would be something like this:

L:=Length(InputString); //store it to save 20000 function calls in the loop
SetLength(result,L); //allocate the string;
FOR P:=1 TO L DO result[L]:=InputString[L+1-P];

you can use PChars, but I'm not sure if that's faster...

P.S.: Ooops. While I was writing, the answer was accepted. I wonder if my comment still makes it :-))
0
 
LVL 5

Expert Comment

by:inter
ID: 1334783
Thanks,
If you need a faster routine I can program a FindLast routine which immediatelly finds the right most matching string and we can get rid of the StrLComp, but this may take a while. Regards, Igor
0

Featured Post

Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
Intraweb download file link ? 1 184
Delphi: Connect to running MS Outlook 4 211
Delphi Yen format 3 76
RESTRequest Parameter 4 86
Have you ever had your Delphi form/application just hanging while waiting for data to load? This is the article to read if you want to learn some things about adding threads for data loading in the background. First, I'll setup a general applica…
Hello everybody This Article will show you how to validate number with TEdit control, What's the TEdit control? TEdit is a standard Windows edit control on a form, it allows to user to write, read and copy/paste single line of text. Usua…
With Secure Portal Encryption, the recipient is sent a link to their email address directing them to the email laundry delivery page. From there, the recipient will be required to enter a user name and password to enter the page. Once the recipient …

710 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question