How to calculate number of lines in file?

Question

I work in C-language at first time and have some question.

How can I get the number of lines in file?

FILE *in;
char c;
int lines = 1;
...
while (fscanf(in,"%c",&c)  == 1) {
  if (c == '\n') {
    lines++;
  }
}

Am I right? I actually don't know how to get the moment , when string cross to the new line.

Answer 1

OP's code functions well aside from maybe an off-by-one issue and a last line issue.

Standard C library definition

A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. C11dr §7.21.2 2

A line ends with a '\n' and the last line may or may not end with a '\n'.

If using the idea that the last line of a file does not require a final '\n, then the goal is to count the number of occurrences that a character is read after a '\n'.

// Let us use a wide type
// Start at 0 as the file may be empty
unsigned long long line_count = 0;

int previous = '\n';
int ch;
while ((ch = fgetc(in)) != EOF) {
  if (previous == '\n') line_count++;
  previous = ch;
}

printf("Line count:%llu\n", line_count);

---

Reading a file one character at a time may be less efficient than other means, but functionally meets OP goal.

This answer uses (ch = fgetc(in)) != EOF instead of fscanf(in,"%c",&c) == 1 which is typically ""faster", but with an optimizing compiler, either may emit similar performance code. Such details of speed can be supported with analysis or profiling. When in doubt, code for clarity.

Answer 2

Can use this utility function

/*
* count the number of lines in the file called filename
*
*/
int countLines(char *filename)
{

  FILE *in = fopen(filename,"r");
  int ch=0;
  int lines=0;

  if(in == NULL){
     return 0; // return lines;
  }
  while((ch = fgetc(in)) != EOF){

     if(ch == '\n'){
        lines++;
     }
  }
    fclose(in);
  return lines;
}

Answer 3

When counting the 'number of \n characters', you have to remember that you are counting the separators, and not the items. See 'Fencepost Error'

Your example should work, but:

• if the file does not end with a \n, then you might be off-by-one (depending on your definition of 'a line').
• depending on your definition of 'a line' you may be missing \r characters in the file (typically used by Macs)
• it will not be very efficient or quick (calling scanf() is expensive)

The example below will ingest a buffer each time, looking for \r and \n characters. There is some logic to latch these characters, so that the following line endings should be handled correctly:

• \n
• \r
• \r\n

#include <stdio.h>
#include <errno.h>

int main(void) {
    FILE *in;
    char buf[4096];
    int buf_len, buf_pos;
    int line_count, line_pos;
    int ignore_cr, ignore_lf;

    in = fopen("my_file.txt", "rb");
    if (in == NULL) {
        perror("fopen()");
        return 1;
    }

    line_count = 0;
    line_pos = 0;
    ignore_cr = 0;
    ignore_lf = 0;

    /* ingest a buffer at a time */
    while ((buf_len = fread(&buf, 1, sizeof(buf), in)) != 0) {

        /* walk through the buffer, looking for newlines */
        for (buf_pos = 0; buf_pos < buf_len; buf_pos++) {

            /* look for '\n' ... */
            if (buf[buf_pos] == '\n') {
                /* ... unless we've already seen '\r' */
                if (!ignore_lf) {
                    line_count += 1;
                    line_pos = 0;
                    ignore_cr = 1;
                }

            /* look for '\r' ... */
            } else if (buf[buf_pos] == '\r') {
                /* ... unless we've already seen '\n' */
                if (!ignore_cr) {
                    line_count += 1;
                    line_pos = 0;
                    ignore_lf = 1;
                }

            /* on any other character, count the characters per line */
            } else {
                line_pos += 1;
                ignore_lf = 0;
                ignore_cr = 0;
            }
        }
    }

    if (line_pos > 0) {
        line_count += 1;
    }

    fclose(in);

    printf("lines: %d\n", line_count);

    return 0;
}