[fpc-pascal]ExtractFileDir ?

[Anton Tichawa]

Hi!

Let's have a look for filename "C:A":

>  function ExtractFileDir(const FileName: string): string;
>  var i,j: longint;
>  begin
>    if FileName[2]=':' then j:=3
>                       else j:=1;

Now j = 3, pointing to the letter "A". OK.

>    I := Length(FileName);
>    while (I > 0) and not (FileName[I] in ['/', '\', ':']) do Dec(I);

Error: I is supposed to go from the string end back to the string start, but 
never below j. But exactly this happens here. I is decremented below j, from 
3 to 2, so shit might happen ..

>    if (I > j) and (FileName[I] in ['\', '/']) and
>       not (FileName[I - 1] in ['/', '\', ':']) then Dec(I);

skipped

>    Result := Copy(FileName, j, I-j+1);

Result = Copy(Filename, 3, 0).

Is this an error or wrong documentation? The line result should be '', but it 
actually is 'a:c'.

So there are two errors:

1. I decremented below j, but the programmer had in mind that i was above j. 
the while-statement should read 'while (I > J) ..'

2. The Copy returns the full string for length 0 - 

Note: I don't like the while structure for combined conditions. I prefer a 
repeat-until with several breaks. That usually makes things more clear. I 
know that this is not considered 'well-structured' programming, but the world 
we live in is not 'well-structured'. Should I post a new, tested "repeat - 
break - break - until" version of this function?

Anton.

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-- 

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"Adas Methode war, wie sich zeigen wird, Tagträume in offenbar korrekte 
Berechnungen einzuweben."

Doris Langley Moore: Ada, Countess of Lovelace (London 1977).

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Anton Tichawa
Volkertstrasse 19 / 20
A-1020 Wien
mobil: +43 664 52 07 907
email: anton.tichawa at chello.at

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