How to count lines of a file in C++?

Question

How can I count lines using the standard classes, fstream and ifstream?

Answer 1

Divide the file size by the average number of characters per line!

Answer 2

kernel methods following @Abhay

A complete code I've done :

size_t count_line(istream &is)
{
    // skip when bad
    if( is.bad() ) return 0;  
    // save state
    std::istream::iostate state_backup = is.rdstate();
    // clear state
    is.clear();
    std::istream::streampos pos_backup = is.tellg();

    is.seekg(0);
    size_t line_cnt;
    size_t lf_cnt = std::count(std::istreambuf_iterator<char>(is), std::istreambuf_iterator<char>(), '\n');
    line_cnt = lf_cnt;
    // if the file is not end with '\n' , then line_cnt should plus 1  
    is.unget();
    if( is.get() != '\n' ) { ++line_cnt ; } 

    // recover state
    is.clear() ; // previous reading may set eofbit
    is.seekg(pos_backup);
    is.setstate(state_backup);

    return line_cnt;
}

it will not change the origin file stream state and including '\n'-miss situation processing for the last line.

Answer 3

This is the correct version of Craig W. Wright's answer:

int numLines = 0;
ifstream in("file.txt");
std::string unused;
while ( std::getline(in, unused) )
   ++numLines;

Answer 4

How about this :-

  std::ifstream inFile("file"); 
  std::count(std::istreambuf_iterator<char>(inFile), 
             std::istreambuf_iterator<char>(), '\n');

Answer 5

int numLines = 0;
ifstream in("file.txt");
//while ( ! in.eof() )
while ( in.good() )
{
   std::string line;
   std::getline(in, line);
   ++numLines;
}

There is a question of how you treat the very last line of the file if it does not end with a newline. Depending upon what you're doing you might want to count it and you might not. This code counts it.

See: http://www.cplusplus.com/reference/string/getline/

Answer 6

int aNumOfLines = 0;
ifstream aInputFile(iFileName); 

string aLineStr;
while (getline(aInputFile, aLineStr))
{
    if (!aLineStr.empty())
        aNumOfLines++;
}

return aNumOfLines;