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Get a specific line from ifstream

This is a discussion on Get a specific line from ifstream within the C++ Programming forums, part of the General Programming Boards category; Hiya I'm coding a parser for text files, and need some help figuring out why the stl is playing tricks ...

  1. #1
    glo
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    Get a specific line from ifstream

    Hiya

    I'm coding a parser for text files, and need some help figuring out why the stl is playing tricks on me.

    What I want:
    The parser will go through the text, line by line, and do some stuff.
    When a certain line is reached, I want to store its position so I can return to it later, and continue to parse from that position onwards.

    It's easy to do it by storing all the lines in a string vector, but I don't want to waste memory to hold the entire text file.

    So I read about tellg and seekg, and thought I will be good to go with the following code:

    the function:
    Code:
    void Parse(std::string Filename) {
    	std::ifstream f;
    
    	// Open the file for parsing.
    	f.open(Filename.c_str(), std::ifstream::in);
    	if (f.is_open()) {
    
    		std::string line;
    		std::streampos line_three = std::ios::beg;
    
    		// Read some lines from the file.
    		while (f.good()) {
    			std::streampos pos = f.tellg();
    			std::getline(f, line);
    			// Store the position of the line "hello world".
    			if (line == "hello world") {
    				line_three = pos;
    			}
    		}
    		f.clear();
    		f.seekg(line_three, std::ios::beg);
    		//f.seekg(-2, std::ios::cur);
    		std::getline(f, line);
    		std::cout << "Line#3: " << line << std::endl;
    
    		f.close();
    	}
    	return;
    }
    text.txt:
    Code:
    some text
    some more text...
    hello world
    finish!
    But instead of printing the third line, my output is:
    Code:
    llo world
    I must be doing something stupid, but I couldn't find out what exactly.

    Thanks

  2. #2
    and the hat of wrongness Salem's Avatar
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    I've no idea - the code looks OK and it works here.
    Code:
    $ g++ foo.cpp
    $ ./a.out 
    Line#3: hello world
    $ gcc --version
    gcc (Ubuntu 4.4.3-4ubuntu5.1) 4.4.3
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  3. #3
    glo
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    Thanks for taking a look at this, Salem

    I used mingw to compile the code under Windows 7. The fact that it worked for you under ubuntu had me try something...
    I converted the text file format to unix instead of dos, it works OK.

    But I don't get it. Isn't stl supposed to be portable? How would I make my parser parse both unix and dos text files?

  4. #4
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    you could generate an index of the file using a std::vector<int> to store the beginning position of each line. you would still need to read through the whole file at least once, but then you'd have the ability to jump directly to whichever line you want.

  5. #5
    Registered User hk_mp5kpdw's Avatar
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    Try opening the file in binary mode. I'd bet the newlines are messing you up. There are 2 lines above where your "hello world" is and the output you show is 2 characters off.

    Your loop is also off due to your use of testing good (which is essentially a similar test to eof). You want to test the read op instead of using good. If your last line of the file was the one you are trying to find, it would report an incorrect position. Your loop should probably be something like this:
    Code:
    // Read some lines from the file.
    std::streampos pos = f.tellg();
    while (std::getline(f, line)) {
        // Store the position of the line "hello world".
        if (line == "hello world") {
            line_three = pos;
        }
        pos = f.tellg();
    }
    Last edited by hk_mp5kpdw; 06-05-2012 at 10:32 AM.
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  6. #6
    Registered User manasij7479's Avatar
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    Quote Originally Posted by hk_mp5kpdw View Post
    Try opening the file in binary mode. I'd bet the newlines are messing you up.
    Wouldn't the newline related trouble increase in binary mode ?
    I mean, in stream/text mode, the differences between newline and newline+carriage-return across platforms are easily and automagically handled .
    (Or, it is supposed to be automatic, afaik....I do not have the experience of testing on multiple platforms and compilers.)
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