Question

我试图制作一个小帐户注册程序。它会提示用户userID和password。我正在userID部分工作，但它看起来并不正确。如果我输入< 8字符userID，它会正常工作，但当我输入userID字符超过16时，会向用户询问不同的userID {1}}打印两次。为什么？

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

int main(void)
{
    // Setting up variables
    char userID[18];
    char passcode[51];

    // Firstly promt the user for user ID
    do // This loop is for asking the user to type another userID if his chosen one isn't approprite
    {
        printf("Please choose your user ID! (it must be between 6 - 16 charaters long)\n");
        for (int i = 0; i < 17; i++) // This loop store the userID into the character array
        {
            userID[i] = getchar(); // Store userID into array character by         character
            if (userID[i] == '\n')
            {
                userID[i+1] = '\0';
                break;
            }

        }
    }
    while (strlen(userID) < 7  ||  (strlen(userID) > 16  &&  userID[16] != '\n'));

    // Printing info on the screen.
    printf("Your acount info are:\n\tuserID  : %s\tpasscode: ", userID);

Answer 1

如果我输入< 5个字符userID，它会正常工作，但当我输入userID个字符超过16时，会询问用户的消息不同的userID打印两次。为什么？

当您输入的16个字符数超过17时，如果您未输入userID，则'\n'个字符会被扫描到字符串userID中，不会出现空字符放在字符串strlen(userID)的末尾。
然后，当您使用\0时，会调用未定义的行为，因为最后没有放置终止index + 1个字符。试着这样做：
当你重新开始迭代时，剩余的额外整数将被接收。

解决方案：

为避免上述问题：

在每次迭代时在flush位置放置一个空字符。
stdin每次迭代结束时的do { printf("Please choose your user ID! (it must be between 6 - 16 charaters long)\n"); for (int i = 0; i < 17; i++) { userID[i] = getchar(); userID[i+1] = '\0'; //placing null character at the position index+1 if(userID[i] == '\n') { userID[i] = '\0'; break; } } fflush(stdin); //flushing the extra input at the end of each iteration } while (strlen(userID) < 7 || (strlen(userID) > 16 && userID[16] != '\n'));。
```
more than 16 123456789
1123
123456789
```

示例输入：

Please choose your user ID! (it must be between 6 - 16 charaters long)
more than 16 123456789
Please choose your user ID! (it must be between 6 - 16 charaters long)
1123
Please choose your user ID! (it must be between 6 - 16 charaters long)
123456789

Your acount info are:
    userID  : 123456789
    passcode:

示例输出：

fflussh(stdin)

注意：

避免使用fflush(stdin)因为@JonathanLeffler中的 comments 所示的可移植性问题。另外，这里有一个解释性的帖子，为什么不使用fflush(stdin)：click

以下是您的代码实现，避免使用do { int i; printf("Please choose your user ID! (it must be between 6 - 16 charaters long)\n"); for (i = 0; (userID[i] = getchar()); i++) { userID[i+1] = '\0'; //placing null character at the position index+1 if(userID[i] == '\n') { userID[i] = '\0'; break; } //if user enters more than 16 characters then, //scan in 17th character for `strlen(userID)` purpose and //consume all other characters using an integer `c` if( i > 17) { int c; while( ((c = getchar())!= '\n') && (c!=EOF) ); break; } } } while ( (strlen(userID) < 7) || (strlen(userID) > 16 ) );

@implementation IntroductViewCell

- (void)awakeFromNib {
    // Initialization code
}

- (void)setSelected:(BOOL)selected animated:(BOOL)animated {
    [super setSelected:selected animated:animated];

    // Configure the view for the selected state
}

- (void)resizeWithWidth:(NSInteger)width {
    self.contentView.bounds = CGRectMake(0, 0, width, width / 3);

    [self setNeedsUpdateConstraints];
    [self updateConstraintsIfNeeded];

}

@end

如果用户输入超过16个字符，为什么我的C程序会打印两次消息？

1 个答案: