C结构不扫描所有输入

Question

我有这个 C 代码：

#include "stdio.h"

main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1,book2;

    printf("Enter details of first book\n");
    gets(book1.name);
    gets(book1.author);
    scanf("%d%d",&book1.year,&book1.copies);

    printf("Enter details for second book\n");
    gets(book2.name);
    gets(book2.author);
    scanf("%d%d",&book2.year,&book2.copies);

    printf("%s\n%s\n%d\n%d\n",book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",book2.name,book2.author,book2.year,book2.copies);  
}  

这里发生的是它只扫描到第二本书的作者姓名。之后，它直接打印输出。

这是我的输入：（前两行是最初的 printf 语句）

Enter details of first book
warning: this program uses gets(), which is unsafe.
the c programmign laguagne
dfadsda
3432
23
Enter details for second book
ruby on rails
mark hammers  

之后它直接打印输出：

the c programmign laguagne
dfadsda
3432
23

ruby on rails
0
0

这里有什么问题？我们还可以看到，第二本书的名称被分配给了作者。

我在 Mac OS X ML 上使用 gcc 作为编译器。

Answer 1

请注意，您可能应该使用 fgets() 而不是 gets()，因为由于缓冲区安全问题，它现在已被弃用。

这是因为scanf() 在读取下一个条目的数据之前会吃掉最后一个\n。

Answer 2

在每个输入语句之前使用fflush(stdin)。此方法将清除输入缓冲区。 修改后您的代码将是-

#include "stdio.h"

int main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1,book2;

    printf("Enter details of first book\n");
    gets(book1.name);
    fflush(stdin);

    gets(book1.author);
    fflush(stdin);

    scanf("%d%d",&book1.year,&book1.copies);
    fflush(stdin);

    printf("Enter details for second book\n");
    gets(book2.name);
    fflush(stdin);

    gets(book2.author);
    fflush(stdin);
    scanf("%d%d",&book2.year,&book2.copies);

    printf("%s\n%s\n%d\n%d\n",book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",book2.name,book2.author,book2.year,book2.copies);  
    return 0;
} 

您可以查看fflush()here的详细信息。

更新： 在 scanf() 语句之后，您需要刷新输入缓冲区。 fflush() 方法在这里没有用，因为它只为输出流定义。您可以在每个 scanf() 行之后使用单行代码来使用部分读取的行的其余部分，例如 -

while((c = getchar()) != '\n' && c != EOF);

你的代码将是：

#include "stdio.h"

int main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1,book2;
    char c;
    printf("Enter details of first book\n");
    gets(book1.name);
    gets(book1.author);

    scanf("%d%d",&book1.year,&book1.copies);
    while((c = getchar()) != '\n' && c != EOF);

    printf("Enter details for second book\n");
    gets(book2.name);
    gets(book2.author);
    scanf("%d%d",&book2.year,&book2.copies);
    while((c = getchar()) != '\n' && c != EOF);

    printf("%s\n%s\n%d\n%d\n",book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",book2.name,book2.author,book2.year,book2.copies);  
    return 0;
} 

输出：

Enter details of first book
warning: this program uses gets(), which is unsafe.
sadsadas
asa
12
34
Enter details for second book
zxczxc
sds
23
22
sadsadas
asa
12
34
zxczxc
sds
23
22

Answer 3

在您的源代码中，

scanf("%d%d",&book1.year,&book1.copies);

不读取“23”之后的“\n”，因为这只是读取两个整数。

这个问题的一个解决方案是在阅读第二本书之前执行gets()，例如：

#include "stdio.h"

main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1,book2;

    printf("Enter details of first book\n");
    gets(book1.name);
    gets(book1.author);
    scanf(" %d %d",&book1.year,&book1.copies);
    char a[100];
    gets(a);

    printf("Enter details for second book\n");
    gets(book2.name);
    gets(book2.author);
    scanf("  %d  %d",&book2.year,&book2.copies);

    printf("%s\n%s\n%d\n%d\n",book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",book2.name,book2.author,book2.year,book2.copies);  
}  

因此，使用gets读取整数，然后使用atoi是更简单的方法。

#include "stdio.h"

main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1,book2;

    printf("Enter details of first book\n");
    gets(book1.name);
    gets(book1.author);
    char buff[100];
    gets(buff);
    book1.year = atoi(buff);
    gets(buff);
    book1.copies = atoi(buff);

    printf("Enter details for second book\n");
    gets(book2.name);
    gets(book2.author);
    gets(buff);
    book2.year = atoi(buff);
    gets(buff);
    book2.copies = atoi(buff);

    printf("%s\n%s\n%d\n%d\n",book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",book2.name,book2.author,book2.year,book2.copies);  
}  

Answer 4

试试这个

#include <stdio.h>
#include <string.h>

int main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1 = { 0 },book2 = { 0 }; // initialize to 0

    printf("Enter details of first book\n");
    printf( "name>" ); 
    fgets(book1.name, sizeof(book1.name), stdin);
    // remove \n
    book1.name[strlen(book1.name)-1] = '\0';

    printf( "author>"); 
    fgets(book1.author, sizeof(book1.author), stdin);
    book1.author[strlen(book1.author)-1] = '\0'; // remove \n
    printf( "year copies>");
    scanf("%d %d",&book1.year,&book1.copies);
    fflush(stdin); // remove any garbage remaining like \n

    printf("Enter details for second book\n");
    printf( "name>" );
    fgets(book2.name, sizeof(book2.name), stdin);
    book2.name[strlen(book2.name)-1] = '\0';

    printf( "author>"); 
    fgets(book2.author, sizeof(book2.author), stdin);
    book2.author[strlen(book2.author)-1] = '\0';
    printf( "year copies>");
    scanf("%d %d",&book2.year,&book2.copies);
    printf("%s\n%s\n%d\n%d\n",
      book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",
      book2.name,book2.author,book2.year,book2.copies);  
    return 0;
}  

Answer 5

解决方案：

#include <stdio.h>  /* Using fgets(), scanf(), printf() in this program */
#include <string.h> /* Using strlen() in this program */

int main()
{
    struct books
    {
        char name[100],author[100];
        int year,copies;
    }book1,book2;

    char c;
    char read_new_line;

    printf("Enter details of first book\n");
    if (fgets(book1.name, sizeof(book1.name), stdin) == NULL)
    {
        fprintf(stderr, "error reading name of book 1\n");
        return -1;
    }
    /* Strip out \n character added by fgets */
    book1.name[strlen(book1.name) - 1] ='\0';

    if (fgets(book1.author, sizeof(book1.author), stdin) == NULL)
    {
        fprintf(stderr, "error reading author of book 1\n");
        return -1;
    }
    /* Strip out \n character added by fgets */
    book1.author[strlen(book1.author) - 1] ='\0';

    scanf("%d %d",&book1.year,&book1.copies);
    /* Strip out \n character left out in input stream */
    while ((read_new_line = getchar()) != EOF && read_new_line != '\n')                                             
        ;

    printf("Enter details for second book\n");
    if (fgets(book2.name, sizeof(book2.name), stdin) == NULL)
    {
        fprintf(stderr, "error reading name of book 2\n");
        return -1;
    }
    /* Strip out \n character added by fgets */
    book2.name[strlen(book2.name) -1 ] = '\0';

    if (fgets(book2.author, sizeof(book2.author), stdin) == NULL)
    {
        fprintf(stderr, "error reading author of book 2\n");
        return -1;
    }
    /* Strip out \n character added by fgets */
    book2.author[strlen(book2.author) - 1] ='\0';

    scanf("%d %d",&book2.year,&book2.copies);
    /* Strip out \n character left out in input stream */
    while((c = getchar()) != '\n' && c != EOF)
        ;

    printf("%s\n%s\n%d\n%d\n",book1.name,book1.author,book1.year,book1.copies);
    printf("%s\n%s\n%d\n%d\n",book2.name,book2.author,book2.year,book2.copies);  
    return 0;
}

---

观察问题中发布的代码：

让我们试着理解为什么你的代码不工作：

从下面的语句调用 scanf 之后

scanf("%d%d",&book1.year,&book1.copies);

你的输入是

3432\n
23\n

scanf 读入3432 并存储在&book1.year 中，随后\n 在输入中被遗漏 溪流。然后，第二个%d 丢弃前导空格（此上下文中的空格包括空格、制表符、换行符等）并读入23 并将其存储在&book1.copies 中，随后\n 被排除在外输入流。

当gets(book2.name) 在输入流中被称为\n 时，输入流匹配gets() 标准，因此“空字符串”被分配给book2.name，并且为book2.name 提供的任何含义和用户输入都存储在@ 987654339@。

后跟用于book2.author 的任何字符串，并在用户输入时键入分配给book2.year%d 转换已完成，但它失败了，因为没有输入正确的整数并且scanf() 返回失败。

注意：

• 使用gets() 本身就很糟糕。当您使用%s 时，您需要确保防止缓冲区溢出，而gets() 则无法做到这一点。阅读：Why does everyone say not to use gets()?

• 一些已发布的答案似乎建议fflush() 用于清除
  stdin 流。阅读，Why Shouldn't I use fflush(stdin)?