Count lines in ASCII file using C

问题

I would like to count the number of lines in an ASCII text file. I thought the best way to do this would be by counting the newlines in the file:

for (int c = fgetc(fp); c != EOF; c = fgetc(fp)) {  /* Count word line endings. */
    if (c == '\n') ++lines;
}

However, I'm not sure if this would account for the last line on all both MS Windows and Linux. That is if my text file finishes as below, without an explicit newline, is there one encoded there anyway or should I add an extra ++lines; after the for loop?

cat
dog

Then what about if there is an explicit newline at the end of the file? Or do I just need to test for this case by keeping track of the previously read value?

回答1:

If there is no newline, one won't be generated. C tells you exactly what's there.

回答2:

Text files are always expected to end with a line feed. There's no canonical way of handling files that don't.

Here's how some tools choose to deal with characters after the last line feed:

wc doesn't count it as a line (so you have good precedence for that)
Vim marks the file as [noeol], and saves the file without a trailing line feed
GNU sed treats the file as if it had a last line feed
sh's read exits with error, but still returns the data

Since behaviour is pretty much undefined, you can just do whatever's convenient or useful to you.

回答3:

First, there will not be any implicitly encoded newline at the end of the last line. The only way there will be a newline is if the software or person that produced the file put it there. Putting it there is generally considered good practice, however.

The ultimate answer for what you should report as the line count depends on the convention that you need to follow for the software or people that will be using this line count, and probably what you can assume about the behavior of the input source as well.

Most command-line tools will terminate their output with a newline character. In this case, the sensible answer may be to report the number of newline characters as the number of actual lines.

On the other hand, when a text editor is displaying a file, you will see that the line numbering in the margin (if supported) contains a number for the last line whether it is empty or not. This is in part to tell the user that there is a blank line there, but if you want to count the number of lines displayed in the margin, it is one plus the number of newline characters in the file. It is typical for some coders to not terminate their last lines with a newline character (sometimes due to sloppiness), so in this case this convention would actually be the right answer.

I'm not sure any other conventions make much sense. For example, if you choose not to count the last line unless it is non-empty, then what counts as non-empty? The file ending after newline? What if there is whitespace on that line? What if there are several empty lines at the end of the file?

回答4:

If you're going to use this method, you could always keep a separate counter for how many letters on the line you are at. If the count at the end is greater than 1, then you know there is stuff on the last line that wasn't counted.

int letters = 0

for (int c = fgetc(fp); c != EOF; c = fgetc(fp)) {  /* Count word line endings. */
    letters++; // Increase count on character

    if (c == '\n')
    {
        ++words;
        letters = 0; // Set back to 0 after new line
    }
}

if (letters > 0)
{
    ++words;
}

回答5:

Your concern is real, the last line in the file may be missing the final end of line marker. The end of line marker is a single '\n' in Linux, a CR LF pair in Windows that the C runtime converts automatically into a '\n'.

You can simplify your code and handle the special case of the last line missing a linefeed this way:

int c, last = '\n', lines = 0;

while ((c = getc(fp)) != EOF) {  /* Count word line endings. */
    if (c == '\n')
        lines += 1;
    last = c;
}
if (last != '\n')
    lines += 1;

Since you are concerned with speed, using getc instead of fgetc will help on platforms where it is defined as a macro that handles the stream structures directly and calls a function only to refill the buffer, every BUFSIZ characters or so, unless the stream is unbuffered.

回答6:

How about this:

Create a flag for yourself to keep track of any non \n characters following a \n that is reset when c=='\n'. After the EOF, check to see if the flag is true and increment if yes.

bool more_chars = false;
for (int c = fgetc(fp); c != EOF; c = fgetc(fp)) {  /* Count word line endings. */
            if (c == '\n') {
              more_chars = false;
              ++words;
            } else more_chars = true;
 }
 if(more_chars) words++;

回答7:

Windows and UNIX/Linux style line breaks make no difference here. On either system a text file may or may not have a newline at the end of the last line.

If you always add 1 to the line count, this effectively counts the empty line at the end of the file when there is a newline at the end (i.e., file "foo\n" will count as having two lines: "foo" and ""). This may be an entirely reasonable solution, depending on how you want to define a line.

Another definition of a "line" is that it always ends in a newline, i.e., the file "foo\nbar" would only have one line ("foo") by this definition. This definition is used by wc.

Of course you could keep track of whether the newline was the last character in file and only add 1 to the count in case it wasn't. Then a "line" would be defined as either ending in a newline or being non-empty at the end of the file, which sounds quite complex to me.

来源：https://stackoverflow.com/questions/30278113/count-lines-in-ascii-file-using-c

标签

ascii

text-files