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fstream question

Started by scarface-cD Apr 21, 2002 at 2:13 PM 13 replies 850+ views

Original Post

Apr 21, 2002 at 2:13 PM

I am having trouble getting the following sample code to output in the desired format. I want it to copy from one file to another without using the member function 'get()' SAMPLE INPUT Bob Dillon 73 43 43 64 85 CODE char next; while ( fin >> next ) { fout << next << " "; } OUTPUT will be B o b D i l l o n 7 3 4 3 4 3 6 4 8 5 I was under the understanding that when using 'fin >> next' or anything related to this style; all spaces and line breaks are ignored and that the only way to copy spaces and linebreaks is to use the member function 'get()' But it appears I am wrong. So how do I achieve what I want? Which is to take input from one file using a different method other than 'get()' and outputting it to an output file formatted exactly as it is seen in the input file. nick [edited by - scarface-cd on April 21, 2002 3:25:36 PM]

SabreMan

127

SabreMan

Apr 21, 2002 at 2:30 PM

I'm not 100% clear on what you are asking. Do you want the output to include or exclude the whitespace from the input file? The following code will make an exact copy of the file contents in input.txt and write it to output.txt:

    
#include <iostream>

#include <fstream>

#include <iterator>

#include <algorithm>

using namespace std;

int main()
{
	ifstream is("input.txt");
	ofstream os("output.txt");

	copy(istreambuf_iterator<char>(is), 
		istreambuf_iterator<char>(), 
		ostream_iterator<char>(os));
}

If you want to ignore whitespace, then use an istream_iterator rather than an istreambuf_iterator.

Edit: fscking source boxes.

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[edited by - SabreMan on April 21, 2002 3:30:59 PM]

scarface-cD

Author

scarface-cD

Apr 21, 2002 at 2:45 PM

I would like to copy the input file verbatim.

thank you for your response sabreman.
Your code will work, however I would like to see it done using the library

SabreMan

127

SabreMan

Apr 21, 2002 at 2:49 PM

quote:
Original post by scarface-cD
Your code will work, however I would like to see it done using the library

Using which library?

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scarface-cD

Author

scarface-cD

Apr 21, 2002 at 3:36 PM

sorry

fstream

alexk7

Apr 21, 2002 at 3:52 PM

Why do you want to avoid get() ?

scarface-cD

Author

scarface-cD

Apr 21, 2002 at 4:11 PM

I want to avoid get() because when I process the input file, I am discarding the first two elements. (i.e. the first and last name) the remaining elements (int) I then have to average (double).

Oluseyi

526

Oluseyi

Apr 21, 2002 at 4:21 PM

quote:
Original post by scarface-cD
Your code will work, however I would like to see it done using the library <fstream>

His code uses the library. istream_iterator and ostream_iterator are generic iterator types for stream I/O (in this case instatiated directly from the file streams is and os), and copy is a generic algorithm that takes the supplied iterators and replicates sequence data in the output iterator.

You may not consider it "using the library", but it''s actually the most efficient form of using the library in terms of efficiency per lines of code.

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Thanks to Kylotan for the idea!

Fruny

203

Fruny

Apr 21, 2002 at 5:05 PM

How about the following snippet ?

  
ifstream ifs( "whatever" );
stringstream ss;
string data;
ss << ifs.rdbuf();
// read the whole file into ss

data = ss.str();  
// strings are reference counted, this is fast

[source]

DrPizza

Apr 21, 2002 at 5:28 PM

quote:
strings are reference counted, this is fast

Mine aren''t (you''ll find that most, if not all, multithreaded string libraries are not reference-counted). The standard doesn''t require any such thing.

  
#include <fstream>
int main(void)
{
    using namespace std;
    ifstream fin("input.txt");
    ofstream fout("output.txt");
    // the default buffer size for many implementations is small

    // this will tend to speed up such implementations significantly

    // testing will reveal the best value to use

    const streamsize bufferSize = 4096;
    char inBuffer[bufferSize];
    char outBuffer[bufferSize];
    fin.rdbuf()->pubsetbuf(inbuffer, bufferSize);
    fout.rdbuf()->pubsetbuf(outbuffer, bufferSize);
    fout << fin.rdbuf();
}

Kylotan

Mod

3,487

Kylotan

Apr 21, 2002 at 5:31 PM

Fewest lines of code?


ifstream is("input.txt");
ofstream os("output.txt");
os << is.rdbuf();

DrPizza

Apr 21, 2002 at 5:47 PM

Works fine in VS.NET, but, like I said, is fairly slow. Enlarging the buffers helps.

scarface-cD

Author

scarface-cD

Apr 21, 2002 at 6:16 PM

thank you everyone I appreciate all of your help, i will be applying a few of the suggestions.

Kylotan

Mod

3,487

Kylotan

Apr 22, 2002 at 5:00 PM

Is there any way of knowing the optimal buffer size? Short of experimentation, of course. Technically the disk access should be buffered at both ends anyway, so I assume the stream buffer size only really helps with regards to the library''s implementational methods...

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SabreMan

127

SabreMan

Apr 22, 2002 at 5:49 PM

quote:
Original post by Kylotan
Is there any way of knowing the optimal buffer size? Short of experimentation, of course.

And this experimentation should be carried out once there is a performance problem identified, not before. There''s no point obfuscating intent without a good reason.

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