Question

当逐行浏览文本文件时，我希望能够向前看下一行并在处理当前行时检查它。我在使用C语言。我相信fseek()或其他类似的功能可以帮助我，但我不确定，也不知道如何使用它们。我想要达到以下效果：

    fp = fopen("test-seeking.txt", "r");

    while((fgets(line, BUFMAX, fp))) {
        // Peek over to next line
        nextline = ...;
        printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n",
               line, nextline);
    }

我感谢任何帮助。

Answer 1

以下代码的灵感来自@ Jean-François Fabre comment。它将使用用于保持线条的2D字符数组lineBuffer。第一个读取行写入索引0 lineBuffer[0]，第二行写入lineBuffer[1]。之后，在toggle variable lineSel的帮助下，着作在索引0和1之间交替。最后一步，curLine指针将设置为nextLine。

结果，您可以在循环中使用curLine和nextLine。如果您的文件包含：

line 1
line 2
line 3
...

您将使用：

curLine  = "line 1\n"
nextLine = "line 2\n"

curLine  = "line 2\n"
nextLine = "line 3\n"

...

请参阅live example with stdin instead of a file on ideone。

<强>代码：

#include <stdio.h>

#define BUFMAX       256
#define CURLINE      0
#define NEXTLINE     1
#define TOGGLELINE   (CURLINE ^ NEXTLINE)

int main ()
{
   FILE* fp = fopen("test-seeking.txt", "r");

   char lineBuffer[2][BUFMAX];
   char* curLine;
   char* nextLine;
   int lineSel;

   if (fp != NULL)
   {
      if ((curLine = fgets(lineBuffer[CURLINE], BUFMAX, fp)))
      {
         for (lineSel = NEXTLINE;
              (nextLine = fgets(lineBuffer[lineSel], BUFMAX, fp));
              lineSel ^= TOGGLELINE)
         {
            printf("Current line: \"%s\" / Next line \"%s\"\n",
                   curLine, nextLine);

            curLine = nextLine;
         }
      }

      fclose(fp);
   }

   return 0;
}

Answer 2

确实你可以使用fseek并尝试这样的事情：

fp = fopen("test-seeking.txt", "r");

while ((fgets(line, BUFMAX, fp))) {
    // Get the next line
    fgets(nextline, BUFMAX, fp);

    // Get the length of nextline
    int nextline_len = strlen(nextline);

    // Move the file index back to the previous line
    fseek(fp, -nextline_len, SEEK_CUR); // Notice the - before nextline_len!

    printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n", line, nextline);
}

另一种方法是使用fgetpos和fsetpos，如下所示：

fp = fopen("test-seeking.txt", "r");

while ((fgets(line, BUFMAX, fp))) {
    // pos contains the information needed from
    //   the stream's position indicator to restore
    //   the stream to its current position. 
    fpos_t pos;

    // Get the current position
    fgetpos(fp, &pos);

    // Get the next line
    fgets(nextline, BUFMAX, fp);

    // Restore the position
    fsetpos(fp, &pos);

    printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n", line, nextline);
}

处理输入文件时如何查看（处理2行）

2 个答案: