How to eliminate this extra element?

Question

I am writing a c++ function for reading the nth column of a tab delimited text file, here is what I have done:

typedef unsigned int  uint;


inline void fileExists (const std::string& name) {
    if ( access( name.c_str(), F_OK ) == -1 ) {
        throw std::string("File does not exist!");
    }
}

size_t bimNCols(std::string fn) {
    try {
        fileExists(fn);
        std::ifstream in_file(fn);
        std::string tmpline;
        std::getline(in_file, tmpline);
        std::vector<std::string> strs;
        strs = boost::split(strs, tmpline, boost::is_any_of("\t"), boost::token_compress_on);
        return strs.size();
    } catch (const std::string& e) {
        std::cerr << "\n" << e << "\n";
        exit(EXIT_FAILURE);
    }
}

typedef std::vector<std::string> vecStr;

vecStr bimReadCol(std::string fn, uint ncol_select) {
    try {
        size_t ncols = bimNCols(fn);
        if(ncol_select < 1 or ncol_select > ncols) {
            throw std::string("Your column selection is out of range!");
        }

        std::ifstream in_file(fn);
        std::string tmpword;
        vecStr colsel; // holds the column of strings
        while (in_file) {
            for(int i=1; i<ncol_select; i++) {
                in_file >> tmpword;
            }
            in_file >> tmpword;
            colsel.push_back(tmpword);
            in_file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        }
        return colsel;

    } catch (const std::string& e) {
        std::cerr << "\n" << e << "\n";
        exit(EXIT_FAILURE);
    }
}

The problem is, in the bimReadCol function, at the last line, after

in_file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

in_file.good() still evaluates to true . So, suppose I have a text file test.txt like this:

a 1 b 2
a 1 b 2
a 1 b 2

bimReadCol("test.txt", 3) would return a vector (b, b, b, b) , with an extra element. Any idea how to fix this?

Answer 1

The usual solution for line oriented input is to read line by line, then parse each line:

std::string line;
while ( std::getline( in_file, line ) ) {
    std::istringstream parser( line );
    for ( int i = 1; parser >> tmpword && i <= ncol_select; ++ i ) {
    }
    if ( parser ) {
        colsel.push_back( tmpword );
    }
    //  No need for any ignore.
}

The important thing is that you must absolutely test after the input (be it from in_file or parser ) before you use the value. A test before the value was read doesn't mean anything (as you've seen).

Answer 2

Ok, I got it. The last line of the text file does not contain a newline, that's why in_file evaluates to true at the last line.

I think I should calculate the number of lines of the file, then replace while(in_file) with a for loop.

If someone has a better idea, please post it and I will accept.

Update

The fix turns out to be rather simple, just check if tmpword is empty:

vecStr bimReadCol(std::string fn, uint ncol_select) {
    try {
        size_t ncols = bimNCols(fn);
        if(ncol_select < 1 or ncol_select > ncols) {
            throw std::string("Your column selection is out of range!");
        }

        std::ifstream in_file(fn);
        vecStr colsel; // holds the column of strings
        std::string tmpword;
        while (in_file) {
            tmpword = "";
            for(int i=1; i<=ncol_select; i++) {
                in_file >> tmpword;
            }
            if(tmpword != "") {
                colsel.push_back(tmpword);
            }
            in_file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        }
        return colsel;

    } catch (const std::string& e) {
        std::cerr << "\n" << e << "\n";
        exit(EXIT_FAILURE);
    }
}

As @James Kanze has pointed out, even if the last line contains a newline, in_file would still evaluate to true , but since we are at the end of file, the next reading into tmpword will be empty, so we will be fine as long as we check that.