Print last n number of lines of a file using c

Question

Below is my function to display with fp the file pointer and num be the number of lines to print. Right now, it again displays the complete file, not required lines, which I do not want.

void dispfile(FILE *fp, int num)
{

    long int pos;char s[100];
    int count;
    fseek(fp, 0, SEEK_END);
    pos=ftell(fp);

    while(pos)
    {
        fseek(fp, --pos, SEEK_SET);
        if(fgetc(fp)=='\n')
        {   
            if(count++ == num) 
            break;
        }
    }

    while(fgets(s, sizeof(s), fp))
    {
        printf("%s",s);
        //fputs(s, stdout);
    }

}

how did it work.... to me it is showing the complt file again except the frst character. — Barara, Oct 20 '16 at 18:21
Possible duplicate of [reading last n lines from file in c/c++](http://stackoverflow.com/questions/17877025/reading-last-n-lines-from-file-in-c-c) — mr.tarsa, Oct 20 '16 at 18:23
Don't know why it's not working for you, unless the stream isn't seekable. You should check the return value of `fseek` to see if it's working. — Barmar, Oct 20 '16 at 18:23
There is a reason for compiler warnings: they point you at problems in your code. If your compiler did not warn already, enable all recommended warnings and pay heed to them! — too honest for this site, Oct 20 '16 at 18:48

score 2 · Accepted Answer · answered Oct 20 '16 at 18:28

2

The count is not initialized. It contains rather unpredictable garbage, and the condition count++ == num is satisfied in an unpredictable moment (strictly speaking, you have the UB).

answered Oct 20 '16 at 18:28

user58697

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Weather Vane · Answer 2 · 2016-10-20T18:06:27.677

1

You can do it by reading the file twice, the first time to count its lines, the second to skip some lines and then print up to num lines. If the file does not have num lines, all are printed. Assuming s[] will hold the longest line in the file.

void dispfile(FILE *fp, int num)
{
    int lines = 0;
    char s[100];

    rewind(fp);
    while(fgets(s, sizeof s, fp) != NULL) {
        lines++;                                // count the lines
    }

    rewind(fp);
    lines -= num;                               // lines to skip
    while(lines-- > 0) {
        if(fgets(s, sizeof s, fp) == NULL) {
            return;                             // unexpected EOF
        }
    }

    while(fgets(s, sizeof s, fp) != NULL) {     // print the rest
        printf("%s", s);                        // newline is already included
    }
}

edited Oct 20 '16 at 18:06

answered Oct 20 '16 at 18:03

Weather Vane

33,872
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1

There is no need to read the file twice actually. – too honest for this site Oct 20 '16 at 18:05
@Olaf by caching in an array? – Weather Vane Oct 20 '16 at 18:07
1

Ringbuffer of `char *` actually. But I'd not give code to OP, I don't see much effort in the code. Nor a specific problem. There's currently too many homework stuff being asked. – too honest for this site Oct 20 '16 at 18:08
@Olaf you can submit that as an answer. – Weather Vane Oct 20 '16 at 18:11
@Olaf he has at least tried, tried to count the `\n` characters. – Weather Vane Oct 20 '16 at 18:12
@Olaf teacher won't believe that is the work of OP. What did you say about homework? It would be educational but not something that can be handed in as own work ;) – Weather Vane Oct 20 '16 at 18:13
Also SEEK_END could be used as it was shown [here](http://stackoverflow.com/questions/17877025/reading-last-n-lines-from-file-in-c-c) – mr.tarsa Oct 20 '16 at 18:22

Print last n number of lines of a file using c

2 Answers2