如何使用 C 代码查找文件的尾部

Question

我编写了代码来查找尾巴，但是，它有一个例外，当我尝试将整行打印为尾部时，它不会打印起始字符。例如，如果我的文件有 14 行，并且我输入n的值也是 14，那么它不会打印起始字符。

请帮助修改我的代码：

#include <stdio.h>
#include <stdlib.h>
int main(int argc,char *argv[])
{
    FILE *in, *out;
    int count = 0,lines;
    long int pos;
    char s[100];
    char ch,c;
    if(argc<2)
    {
        printf("Arguments which is to be passed should be 2");
    }
    else if(argc>2)
    {
        printf("too many argumnets are passed");
    }
    in = fopen("anj.txt", "r");
    out = fopen("output.txt", "w");
    if(in==NULL || out==NULL)
    {
        printf("unable to open");
        exit(0);
    }
    else
    {
        int n=atoi(argv[1]);
        if(n>=1)
        {
            fseek(in, 0, SEEK_END);
            pos = ftell(in);
            while (pos)
            {
                fseek(in, --pos, SEEK_SET);
                if (fgetc(in) == 'n')
                {
                    //count=count+1;
                    if (count++==n)
                    break;
                }
            }
            if(count<n)
            {
                printf("no. of lines in file %d is less than enterd",count);
            }
            c = fgetc(in);
            while (c != EOF)
            {
                fputc(c, out);
                c = fgetc(in);
            }
        }
        else
        printf("renter the value of n");
    }
    fclose(in);
    fclose(out);
    return 0;
}

Answer 1

如果行数与输入的值相同，您的代码将读取文件中的第一个字符，由 fgetc 使用，并因pos == 0而结束循环！

在此极端情况的 while 循环之后添加此检查：

if (pos == 0)
{
    fseek(in, 0, SEEK_SET);
}

Answer 2

如果您所说的尾部是指

文件的最后一行，那么有一种更简单的方法可以获取文件的最后一行，并使用 fgets（）' 将其写入输出文件：

int CountLines(FILE *fp);
int main(void)
{
    FILE *in, *out;
    char string[509];
    char input[80];
    char *dummy;
    char *fmt = "%[^n]%*c";
    int count=0;
    int index = 0;
    int tailCnt=0;
    in = fopen("anj.txt", "r");
    out = fopen("output.txt", "w");
    if(in==NULL || out==NULL)
    {
        printf("unable to open");
        exit(0);
    }
    else
    {
        //get count of lines
        tailCnt = -1;
        count = CountLines(in);
        rewind (in);
        while((tailCnt<=0) || (tailCnt > count))
        {
            printf("File has %d lines.nEnter lines from bottom of file to output (1 to %d).n", count, count);
            scanf(fmt, input);
            tailCnt = strtol(input, &dummy, 10);
        }
        while(fgets(string, 100, in)) 
        {
            if(index >= (count-tailCnt))
            {
                fputs(string, out);
            }
            index++;
        }
        //check for fgets() error
        if(errno != 0)
        {
             printf("Error returned from fgets().  Exitingn");
             return 0;
        }
    }
    fclose(in);
    fclose(out);
    return 0;
}
int CountLines(FILE *fp)//simple example line count, 
{                       //no error checking...
    int count=0;
    char line[509] = {0};//ANSI compatibility requires a compiler to accept minimum of 509 characters
    while(fgets(line, 509, fp)) count++;
    return count;
}

针对以下 anj.txt 的极端情况 0、1 和 17 进行了测试：

This is line 1
This is line 2
This is line 3
This is line 4
This is line 5
This is line 6
This is line 7
This is line 8
This is line 9
This is line 10
This is line 11
This is line 12
This is line 13
This is line 14
This is line 15
This is line 16
This is line 17