Thread: counting number of lines in my output file

Open file for sequential reading (see fopen for more information) and read line-by-line and count until you reach the end.

Originally Posted by zesty

opening the file and reading the number of lines myself would not be of any use,as i need to know it beforehand to allocate the required memory. I read somewhere something like

No, you don't. Just use a sizeable array of say 10k or so and read line-by-line to find out how many lines there are.
If you just need to find the size, you can just open it, seek to the end (fseek) and retrieve the file pointer position (ftell).
There's nothing preventing you from reading the file and deciding how much memory to allocate before reading it into memory at a later time.

Alternatively, you can use a dynamic array - extend your array size as it needs to grow (realloc).

Code:

int c, lines = 0;
FILE *f;

...

/* Assume f points to a valid FILE opened for reading... */

while((c = fgetc(f)) != EOF)
{
    if(c == '\n')
    {
        lines++;
    }
}

Easier than you may think.

But also more inefficient than reading line-by-line with fgets.

Code:

int lines = 0;
char buffer[10000];
FILE *f;

...

/* Assume f points to a valid FILE opened for reading... */

while( fgets(buffer, sizeof(Buffer), f) )
{
	lines++;
}

I'd say that's coming close to abusing the size of variables on the stack -- also I'm pretty sure fgets() would do something similar to MacGyver's code

It also means if a line is larger than 10,000 bytes you'll get incorrect results, perhaps the fastest way is (by far not the most efficient):

Code:

* seek the end of the file
* find out how many bytes you are from the start (ie how big the file is)
* allocate a buffer big enough to hold the file
* read in the file into the buffer
* loop through the buffer char-by-char looking for new lines (while keeping a running total)
* free the buffer

Well, if you're concerned about stack space, you can always allocate on the heap.
fgets is also supposed to return a char*, but msdn docs never say what it returns if there's not an error. I'm thinking you can find out whether fgets was able to read an entire line or not.

You said so yourself that you don't like supporting IE because it's not standards compliant. Hence don't use the MSDN docs for standard C -- they seem to 'miss-label' things (ie POSIX compliant functions), or totally forget to mention things.

Originally Posted by man fgets (3)

Upon successful completion, fgets() and gets() return a pointer to the
string.

In other words, NULL on failure else the string.

Originally Posted by man fgets (3)

The fgets() function reads at most one less than the number of characters
specified by n from the given stream and stores them in the string s.
Reading stops when a newline character is found, at end-of-file or error.
The newline, if any, is retained. If any characters are read and there
is no error, a `\0' character is appended to end the string.

They suffice well enough. Despite severals flaws, they do describe the functions.
I wouldn't recommend it 100% to newbies, but it will suffice since it's the only doc I have offline.
But in case it returns a pointer to the string, then all you have to do is:

Code:

int lines = 0;
char buffer[10000];
FILE *f;

/* Assume f points to a valid FILE opened for reading... */

while( fgets(buffer, sizeof(Buffer), f) )
{
	/* Only increment lines if fgets read an entire line, otherwise continue reading the line */
	if ( Buffer[ strlen(Buffer) ] == '\n' )
		lines++;
}

Or...

Code:

if(strchr(ret, '\n') != NULL)
    lines++;

Don't forget, fgets() stops at a newline, so there's never going to be a newline in the string other than at the end

Indeed, so if there isn't a newline, the entire line was not read.