C - scanf 不会在每次循环迭代中执行

Question

我正在写一个有趣的程序(不是为了上学)，我很难弄清楚为什么scanf函数不是在我的循环的每次迭代中执行-我已经玩弄了'for'循环和'while'循环。

我知道这取决于我如何写scanf函数(即scanf("%s", &variablename); VS scanf("%99[^n]s", &variablename);)会有所不同，但我已经尝试了一切，我很绝望!

当我对scanf的输入进行printf检查时，每次迭代它只输入一个字符串，所以如果我在第一次输入中输入两个单词，那么它需要两次迭代来处理-每次一个单词。下面是我要描述的代码段:

int main(void){
    int tries = 0;
    int score = 0;
    char question[100];
    char useranswer[100];
    const char *phrase = {"our favorite saying"};
    printf("nQuestion #3 (10 points): What is our secret saying?n");
        sleep(1);
        tries = 1;
    while (tries<=3){
        printf("YOUR ANSWER:");
        scanf("%s[^n]", useranswer); 
        if(strncmp(useranswer, phrase, 15) != 0){
            printf ("Nope, try again!n");
            printf("You have used %d out of 3 tries!n", tries);
            if (tries == 2){
                printf("Here's your final hint:xxx...n");
            }
            if (tries == 3){
            printf("You didn't get it. The answer is: our favorite saying!n");
            }
            tries++;
        }   
        if (strncmp(useranswer, phrase, 15) == 0){
            printf("Damn, you're good.  Well done.n");
            score += 10;
            break;
        }
    }   

该代码的输出是:

Question #3 (10 points): What is our secret saying?
YOUR ANSWER:our favorite saying
Nope, try again!
You have used 1 out of 3 tries!
YOUR ANSWER:Nope, try again!
You have used 2 out of 3 tries!
Here's your final hint:xxx...
YOUR ANSWER:Nope, try again!
You have used 3 out of 3 tries!
You didn't get it. The answer is: our favorite saying!

(它只允许我输入一次，我输入了"我们最喜欢的说法")

Answer 1

在注释中你可以找到为什么你的scanf格式说明符不起作用

另一种选择是使用fgets，可能在辅助函数中处理读取用户输入时可能出现的一些极端情况:

#include <ctype.h>
char *read_line( char *buf, size_t n, FILE *pfin )
{
    ssize_t length = 0;
    int ch;
    if ( !buf || n == 0 )
        return NULL;
    /*  Consume trailing control characters, like '','n', 'r', 'f'...
        but also 't'. Note that ' ' is not skipped. */
    while ( (ch = fgetc(pfin)) != EOF  &&  iscntrl(ch) ) { }
    if ( ch == EOF )
        return NULL;
    /*  At least one char is printable  */
    *buf = ch;
    ++length;
    /*  Read from file till a newline or up to n-2 chars. The remaining chars
        are left in the stream buffer. Return NULL if no char is read.      */
    if ( fgets(buf + 1, n - 1, pfin) )
    {
        /*  Trim the string to the first control character                  */
        while ( !iscntrl(buf[length]) ) 
        {
            ++length;
        }
        buf[length] = '';       
    }
    return buf;
}

我也会改变下面的逻辑。OP多次使用strncmp(useranswer, phrase, 15)，但是这个神奇的数字15比phrase的大小小，所以它最终只比较一个子字符串。

while ( tries <= 3 ) {
    printf("YOUR ANSWER:");
    if ( !read_line(useranswer, sizeof useranswer, stdin) ) {
        printf("Error: Unexpected end of input.n");
        exit(EXIT_FAILURE);
    }
    if( strcmp(useranswer, phrase) == 0 ) {
        printf("Damn, you're good.  Well done.n");
        score += 10;
        break;
    } else {
        printf ("Nope, try again!n");
        printf("You have used %d out of 3 tries!n", tries);
        if (tries == 2) {
            printf("Here's your final hint:xxx...n");
        }
        if (tries == 3) {
            printf("You didn't get it. The answer is: our favorite saying!n");
        }
        tries++;
    }
}

---

最后要说明的是，我发现phrase的OP声明有点奇怪(可能是拼写错误):

const char *phrase = {"our favorite saying"};
//            string literals are already ^^ null terminated...

可以使用简单的数组声明，如:

const char phrase[] = "our favorite saying";

同时考虑sizeof phrase在这两种不同情况下返回的值。

---

感谢@chux提供的所有有价值的提示和有趣的链接:
https://stackoverflow.com/a/27729970/4944425
https://stackoverflow.com/a/28462221/4944425
感谢@Dmitri在他的评论中指出，一旦我们确定两个字符串都是空终止的，我们可以使用strcmp而不是strncmp。