fstream pointer arithmetic

[Gary Wessle]

Hi

ofter I need to get to a line in a text file just before a line with a
known string. but by the time getline ( ifs, line ) gets to the line
with the knows string, the last line as already passed.
sure I can remember the line and then while( getline .. till it flags
then recall last remembered, but I was thinking, is there a method by
which one can move getline back one line,
filestream_ pointer--;
kind of an increment?

thanks

[Alan Johnson]

Gary Wessle wrote:

Hi

ofter I need to get to a line in a text file just before a line with a
known string. but by the time getline ( ifs, line ) gets to the line
with the knows string, the last line as already passed.
sure I can remember the line and then while( getline .. till it flags
then recall last remembered, but I was thinking, is there a method by
which one can move getline back one line,
filestream_ pointer--;
kind of an increment?

thanks

The only facilities for doing something like that are the tellg and
seekg member functions, which will tell you the position in the stream,
and move you to a position in the stream.

In any case, a solution based on tellg/seekg is going to be way more
complex than just remembering the previous line. And likely to be much
less efficient too since you have to reread the previous line.

Using swap, you can remember the previous line without much overhead.
Maybe something like:

template <typename Predicate>
std::string find_previous_line(std::istream & in, Predicate matches)
{
std::string line, previous_line ;
while (std::getline(in, line))
{
if (matches(line))
return previous_line ;
line.swap(previous_line) ;
}

return "" ;
}

--
Alan Johnson

[jamx]

On 14 feb, 06:05, Alan Johnson <[e-mail]:

Gary Wessle wrote:

Hi

ofter I need to get to a line in a text file just before a line with a
known string. but by the time getline ( ifs, line ) gets to the line
with the knows string, the last line as already passed.
sure I can remember the line and then while( getline .. till it flags
then recall last remembered, but I was thinking, is there a method by
which one can move getline back one line,
filestream_ pointer--;
kind of an increment?

thanks

The only facilities for doing something like that are the tellg and
seekg member functions, which will tell you the position in the stream,
and move you to a position in the stream.

In any case, a solution based on tellg/seekg is going to be way more
complex than just remembering the previous line. And likely to be much
less efficient too since you have to reread the previous line.

Using swap, you can remember the previous line without much overhead.
Maybe something like:

template <typename Predicate>
std::string find_previous_line(std::istream & in, Predicate matches)
{
std::string line, previous_line ;
while (std::getline(in, line))
{
if (matches(line))
return previous_line ;
line.swap(previous_line) ;
}

return "" ;

}

--
Alan Johnson

swap() does look like an overhead to me, since there is no reason to
put the content of previous_line into line.

line.swap(previous_line);

could better be written as

previous_line = line;

[Rolf Magnus]

jamx wrote:

On 14 feb, 06:05, Alan Johnson <[e-mail]:

>Gary Wessle wrote:

Hi

ofter I need to get to a line in a text file just before a line with a
known string. but by the time getline ( ifs, line ) gets to the line
with the knows string, the last line as already passed.
sure I can remember the line and then while( getline .. till it flags
then recall last remembered, but I was thinking, is there a method by
which one can move getline back one line,
filestream_ pointer--;
kind of an increment?

thanks

The only facilities for doing something like that are the tellg and
seekg member functions, which will tell you the position in the stream,
and move you to a position in the stream.

In any case, a solution based on tellg/seekg is going to be way more
complex than just remembering the previous line. And likely to be much
less efficient too since you have to reread the previous line.

Using swap, you can remember the previous line without much overhead.
Maybe something like:

template <typename Predicate>
std::string find_previous_line(std::istream & in, Predicate matches)
{
std::string line, previous_line ;
while (std::getline(in, line))
{
if (matches(line))
return previous_line ;
line.swap(previous_line) ;
}

return "" ;

}

--
Alan Johnson

swap() does look like an overhead to me, since there is no reason to
put the content of previous_line into line.

line.swap(previous_line);

could better be written as

previous_line = line;

It depends. I could easily imagine an implementation where swap() is faster.

[John Harrison]

Rolf Magnus wrote:

jamx wrote:

>>On 14 feb, 06:05, Alan Johnson <[e-mail]:

>>>Gary Wessle wrote:

Hi

ofter I need to get to a line in a text file just before a line with a
known string. but by the time getline ( ifs, line ) gets to the line
with the knows string, the last line as already passed.
sure I can remember the line and then while( getline .. till it flags
then recall last remembered, but I was thinking, is there a method by
which one can move getline back one line,
filestream_ pointer--;
kind of an increment?

thanks

The only facilities for doing something like that are the tellg and
seekg member functions, which will tell you the position in the stream,
and move you to a position in the stream.

In any case, a solution based on tellg/seekg is going to be way more
complex than just remembering the previous line. And likely to be much
less efficient too since you have to reread the previous line.

Using swap, you can remember the previous line without much overhead.
Maybe something like:

template <typename Predicate>
std::string find_previous_line(std::istream & in, Predicate matches)
{
std::string line, previous_line ;
while (std::getline(in, line))
{
if (matches(line))
return previous_line ;
line.swap(previous_line) ;
}

return "" ;

}

--
Alan Johnson

swap() does look like an overhead to me, since there is no reason to
put the content of previous_line into line.

line.swap(previous_line);

could better be written as

previous_line = line;

It depends. I could easily imagine an implementation where swap() is faster.

I would expect swap to be at least as fast, but 'previous_line = line;'
expresses the intent more clearly, so I would prefer it.

john