Re: Overwrite a file

From: Joachim Schimpf <>
Date: Mon 09 Jun 2003 11:41:48 AM GMT
Message-ID: <>
Jordan C N Chong wrote:
> I have a file, temp
> which has the content:
> p(1,2,3).
> I have run the following:
> open(temp,update,File),
> read(File,p(X,Y,Z)),
> Q is X + Y + Z,
> write(File,p(X,Y,Q)),
> close(File).
> My question is, is it possible to overwrite p(1,2,3) to p(1,2,6)?
> I have tried the above, it only appends p(1,2,6) at to the file.

Update-streams have only one "file-pointer" which marks the
"current position" in the file. After reading your p(X,Y,Z) term,
the pointer points behind the term, and this is where the subsequent
write operation writes.

Depending on your exact problem, you could
- open the file twice, once for reading and once for writing (so
  you have two file pointers)
- use update mode as you have done, but use at/2 to remember the
  position before reading, and seek/2 to go back to that position
  before writing.

 Joachim Schimpf              /             phone: +44 20 7594 8187
 IC-Parc                     /
 Imperial College London    /
Received on Mon Jun 09 12:43:30 2003

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