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Read from a specific place in a C ++ file

I have a C ++ program that should return a string in which a specific word appears. For example, if my file looks like this:

the cow jumped over
the moon with the
green cheese in his mouth

and I need to print the line with "c". The entire program receives an offset from the beginning of the file (in this case 24, since "c" is 24 characters from the beginning of the file).

How to print the entire line "Moon using", only the offset?

Thank you so much!

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3 answers

( @Chet Simpson). (, , - , ), . , ( unix- , ios_base::binary Windows).

:

  • ,

( Windows):

std::string GetSurroundingLine(std::istream& f, std::istream::pos_type start_pos)
{
    std::istream::pos_type prev_pos = start_pos;
    std::istream::pos_type pos;
    char buffer[40]; // typical line length, so typical iteration count is 1
    std::istream::pos_type size = sizeof(buffer);

    // Look for the beginning of the line that includes the given position
    while (true)
    {
        // Move back 40 bytes from prev_pos
        if (prev_pos < size)
            pos = 0;
        else
            pos = prev_pos - size;
        f.seekg(pos);

        // Read 40 bytes
        f.read(buffer, prev_pos - pos);
        if (!f)
            throw;

        // Look for a newline byte, which terminates previous line
        int eol_pos;
        for (eol_pos = sizeof(buffer) - 1; eol_pos >= 0; --eol_pos)
            if (buffer[eol_pos] == '\n')
                break;

        // If found newline or got to beginning of file - done looking
        if (eol_pos >= 0 || pos == (std::istream::pos_type)0)
        {
            pos += eol_pos + 1;
            break;
        }
    }

    // Position the read pointer
    f.seekg(pos);

    // Read the line
    std::string s;
    std::getline(f, s, '\n');

    return s;
}

: Windows, \r\n, , \r ( , ), .

+2

, . , , .

#include <iostream>
#include <fstream>
#include <string>

std::string LineFromOffset(
    const std::string &filename,
    std::istream::pos_type targetIndex)
{
    std::ifstream input(filename);

    //  Save the start position of the first line. Should be zero of course.
    std::istream::pos_type  lineStartIndex = input.tellg();

    while(false == input.eof())
    {
        std::string   line;

        std::getline(input, line);

        //  Get the end position of the line
        std::istream::pos_type  lineEndIndex = input.tellg();

        //  If the index of the word we're looking for in the bounds of the
        //  line, return it
        if(targetIndex >= lineStartIndex && targetIndex < lineEndIndex)
        {
            return line;
        }

        // The end of this line is the start of the next one. Set it
        lineStartIndex = lineEndIndex;
    }

    //  Need a better way to indicate failure
    return "";
}

void PrintLineTest()
{
    std::string str = LineFromOffset("test.txt", 24);

    std::cout << str;
}
+3

There are functions of each fopen operation - open a file

fseek - search for a file at the desired offset

fread - read the number of bytes you want

fclose - close file

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