c - fscanf 读取最后一个整数两次

Question

我有以下简单的程序可以从文本文件 (num.txt) 中读取数据。文本文件的每一行都有数字 1 2 3 4 5。当我运行该程序时，它会打印 5 两次。谁能告诉我为什么会这样，以及如何解决？提前致谢

int main(void)
{
   int number;
   FILE *file;

   int i = 0;;

   file = fopen("num.txt", "r");

   while (!feof(file)){

      fscanf(file, "%d", &number);
      printf("%d\n", number);
      }

   return 0;
}

这是我的文本文件 num.xtx

1
2
3
4
5

这是程序输出

1
2
3
4
5
5

还有5个

Answer 1

来自 scanf 函数族的手册页，

The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs, in which case the error indicator for the stream is set, and errno is set to indicate the error.

这意味着最后一次成功的 fscanf 调用从流 file 中读取最后一行，之后是 while 循环条件 ! feof(file) 为真，因为尚未满足文件结束条件。这意味着循环会额外执行一次，并且会再次打印变量 number 的先前值。

请阅读此 - while(!feof(file)) is always wrong

您应该检查 scanf 的返回值，而不是检查文件流上的文件结束指示器。

#include <stdio.h>   

int main(void) {
   int number;
   FILE *file = fopen("num.txt", "r");

   // check file for NULL in case there
   // is error in opening the file
   if(file == NULL) {
      printf("error in opening file\n");
      return 1;
   }      

   // check if fscanf call is successful 
   // by checking its return value for 1.
   // fscanf returns the number of input
   // items successfully matched and assigned
   while(fscanf(file, "%d", &number) == 1)
      printf("%d\n", number);

   return 0;
}