Search for a string backwards in a file.

[SK]

Hey folks,

I am searching for a string (say "ABC") backwards in a file.
First I seek to the end.
Then I try to make a check like -

do {
file.clear ();
file.get(c);
file.seekg(-2, std::ios::cur);
} while (c != 'A' && c.readback() != 'B' && c.readback != 'C')
// readback, hypotheticl func, problem here)
Is there some function/trick like peek that can instead read backwards
one character at a time?
Or is there a better way to solve this problem.

Thank you.

[Old Wolf]

sk******@rediff mail.com (SK) wrote:

I am searching for a string (say "ABC") backwards in a file.
First I seek to the end.
Then I try to make a check like -
Is there some function/trick like peek that can instead read backwards
one character at a time?
Or is there a better way to solve this problem.

it goes in several mbs.

Wish I could redesign the file format, but can't actually :-(
But what I am parsing are timestamps in a file. I need to know the
first and the last timestamp in a given file.

If the file is in ascii format (ie. it is made up of lines), then
you can read forwards a line at a time (it won't take long even to
parse several megs that way).
If you really must start at the end, then start, say, 50k from the
end and find the first newline, and then read forwards from there
(assuming your file does not contain lines longer than 50k).

[Julie]

Karl Heinz Buchegger wrote:
<snip>

The best thing the OP could do is: Avoid that topic at all by redesigning
the file format.

This comment really has no value to the discussion because it assumes that: the
OP has the liberty to arbitrarily redesign the file format, that the
requirements for the existing file format are secondary to the (code)
implementation details, and that reading from the end of the file isn't a
suitable solution to the problem as expressed by the OP.

[Karl Heinz Buchegger]

Julie wrote:

Karl Heinz Buchegger wrote:
<snip>

The best thing the OP could do is: Avoid that topic at all by redesigning
the file format.

This comment really has no value to the discussion because it assumes that: the
OP has the liberty to arbitrarily redesign the file format, that the
requirements for the existing file format are secondary to the (code)
implementation details, and that reading from the end of the file isn't a
suitable solution to the problem as expressed by the OP.

On the other hand it happens quite frequently, that posters post
questions about how to do something and it turns out later that
a simple change in the assignment (if possible) is the far better
solution.

--
Karl Heinz Buchegger
kb******@gascad .at

[Julie]

Karl Heinz Buchegger wrote:

Julie wrote:

Karl Heinz Buchegger wrote:
<snip>

The best thing the OP could do is: Avoid that topic at all by redesigning
the file format.

This comment really has no value to the discussion because it assumes that: the
OP has the liberty to arbitrarily redesign the file format, that the
requirements for the existing file format are secondary to the (code)
implementation details, and that reading from the end of the file isn't a
suitable solution to the problem as expressed by the OP.

On the other hand it happens quite frequently, that posters post
questions about how to do something and it turns out later that
a simple change in the assignment (if possible) is the far better
solution.

Perhaps you should phrase your original comment as a question, rather than an
assertion.

[Hardy]

why just reverse the string and search it in the normal way?:)

"John Harrison" <jo************ *@hotmail.com> дÈëÓʼþ
news:2j******** ****@uni-berlin.de...

"SK" <sk******@redif fmail.com> wrote in message
news:83******** *************** ***@posting.goo gle.com...

Hey folks,

I am searching for a string (say "ABC") backwards in a file.
First I seek to the end.
Then I try to make a check like -

do {
file.clear ();
file.get(c);
file.seekg(-2, std::ios::cur);
} while (c != 'A' && c.readback() != 'B' && c.readback != 'C')
// readback, hypotheticl func, problem here)
Is there some function/trick like peek that can instead read backwards
one character at a time?
Or is there a better way to solve this problem.

Thank you.
Is the file small enough to read the whole file into memory? If so then

read the whole file into a string and search in the string not in the file.
Anything else rapidly gets very complicated and also not terribly efficient. Files aren't designed to be read backwards, I would prefer to redesign your file format so that you don't need to read backwards than to actually
attempt this.

And no there is no quick trick to do this.

john

[Karl Heinz Buchegger]

Hardy wrote:

why just reverse the string and search it in the normal way?:)

Because it isn't very efficient to start searching a 10 MB file
at the beginning, when all you are interested in, is the *last*
occourence of the search string and you know that this
search string is located somewhere in the last 100 Bytes or so.
--
Karl Heinz Buchegger
kb******@gascad .at