[fpc-pascal] string concatenation speed

[Michael Van Canneyt]

On Tue, 21 Jun 2005, Marc Santhoff wrote:

> Hi,
> 
> in an application of mine occurs a lot of string separation and
> re-concatenation. Since it is using masses of ANSI strings this is some
> performance problem.
> 
> The strings get stored and loaded from  TStringList's and concatenated
> by simply using '+' and separated with the function copy().
> 
> What I'd like to know is: What's are fastest ways of handling ANSI
> strings?

It's hard to say in general; You would have to supply some example code.

if you're doing things like
  For I:=X to Y do
   S:=S+L[i];  // S string, L list.

then
  It might be better to do

  Len:=0;
  For I:=X to Y do
    Inc(Len,Length(L[i]));  // S string, L list.
  SetLength(S,Len);
  P:=1;
  For I:=X to Y do
    begin
    T:=L[i];
    Len:=Length(T);
    Move(T[1],S[P],Len);
    inc(P,Len)
    end;

This will avoid a lot of calls to uniquestring, get/setlength etc.

Also, keep in mind that getting the I-th string from a list is an expensive operation.

So it is better to do
   T:=L[i];
   S:=Copy(T,X,Length(T)-X);
than to do
   S:=Copy(L[i],X,Length(L[i])-X);

The first option will call GetString only once, the second will call it twice.

This is the kind of thing you should pay attention to.

Michael.