Hi,
In C++, how can I count lines using the standard classes fstream, ifstream?
Thank you, Mohammad
+10
A:
You read the file line by line. Count the number of lines you read.
Martin York
2010-06-18 20:21:24
That sounds like a _huge_ overcomplication! ;-)
James McNellis
2010-06-18 20:34:35
+13
A:
How about this :-
std::ifstream inFile("file");
std::count(std::istreambuf_iterator<char>(inFile),
std::istreambuf_iterator<char>(), '\n');
Abhay
2010-06-18 20:26:55
That's what I'd say, if you didn't say it before me. Still, your way is slightly better... +1
fingerprint211b
2010-06-18 20:39:38
Just remembered something, What if the last line doesn't have a \n?
fingerprint211b
2010-06-18 20:42:48
@fingerprint211b: Add one to the result :-) There is always this tradeoff whether u have a newline at the end of file.
Abhay
2010-06-18 20:44:45
@fingerprint211b: Then there is one less line. Just because the last line is blank does not mean it's any different than any other line.
Billy ONeal
2010-06-18 20:44:58
@wheaties: I also think think this is a faster way of counting lines in file if not the fastest. If you are reading lines, the system library (be it fgets or std::readline) will copy the data to *your* buffer; but an optimal solution will access the data in the buffer where it is read from the system, like memory mapping. So the truly optimal solution by the library could be to create a mmap_streambuf to mmap the file. A count_lines method would then just iterate over it there would never be a need to copy the data at all.
Abhay
2010-06-18 20:56:42
I cannot find "std:count" I just included <iostream>, <string> and <fstream>.
MIH1406
2010-06-18 21:08:35
@MIH: A quick Google search would have told you that `std::count()` is in `<algorithm>`.
James McNellis
2010-06-18 21:10:05
+1 for `istream` **buf** `_iterator`, as made famous by Item 29 of Effective STL!
Cubbi
2010-06-18 22:46:54
A:
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.
Craig W. Wright
2010-06-18 20:31:38
`while ( ! in.eof() )` <--- NOOOOO! This will give garbage if there's any type of failure reading the file. Put `std::getline` as the condition of the while.
Billy ONeal
2010-06-18 20:36:55
It's still invalid after your edit because if `std::getline` fails, you increment the line count, and don't check if it succeeded until after. Move `std::getline` into the condition of the while.
Billy ONeal
2010-06-18 20:46:04
A:
This is the correct version of Craig W. Wright's answer:
int numLines = 0;
ifstream in("file.txt");
while ( std::getline(in, std::string()) )
++numLines;
Billy ONeal
2010-06-18 20:48:25
How does one determine the average number of characters per line without reading the file? If your counting characters, you might as well just count the newlines instead; which kind of invalidates your answer.
Thomas Matthews
2010-06-19 01:27:31