C 编程。尝试循环诱使用户输入正数

Question

这里是初学者。我试图诱使用户输入正数。然而，当用户输入错误的数字时，while 循环似乎不起作用。

输出:

Please enter a positive integer: -3
I'm sorry, you must enter a positive integer greater than zero: why?
I'm sorry, you must enter a positive integer greater than zero: -42
I'm sorry, you must enter a positive integer greater than zero: 42
The positive integer was : 42
Press any key to continue....

代码:

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
void clear_keyboard_buffer(void);
int main()
{
    int k;
    printf("Please enter a positive integer: ");
    scanf("%d", &k);
    if (k > 0)
    {
        clear_keyboard_buffer();
        printf("The positive integer was: %d ", k);

    }
    while (k<=0)
    {
        printf("I'm sorry, you must enter a positive integer greater than zero: ");
        scanf("%d", &k);
        return 0;
    }
}

void clear_keyboard_buffer(void)
{
    char ch;
    scanf("%c", &ch);
    while (ch != '\n')
    {
        scanf("%c", &ch);
    }
}

Answer 1

好吧，我认为最简单的方法是向您展示一些可以实现您想要的功能的代码，以及一些关于为什么应该这样做的注释:

#include <stdio.h>

int main(void) /* note the void here, it says "no parameters"! */
{
    int k;

    /* here we don't use printf() because there is no formatting to do */
    fputs("Please enter a positive integer: ", stdout);

    scanf(" %d", &k); /* note the space, it consumes any whitespace */

    while (k < 1)
    {
        fputs("I'm sorry, you must enter a positive integer greater "
                "than zero: ", stdout);
        scanf(" %d", &k);
    }

    printf("You entered `%d'\n", k);

    return 0;
}

您仍然应该检查scanf()的返回值以获取生产质量代码，因为可能出现错误(例如，用户输入了某些内容)这不是一个数字...)

话虽如此，对于真正可靠的用户输入，我建议完全放弃 scanf() 并仅使用例如fgets() 读取一行输入(无论什么)，然后解析自己。 strtol() 可以派上用场......

只是为了让您了解我在说什么，这里有一个非常简单但可靠的解决方案:

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

static int readInt(void)
{
    char buf[1024];
    int input;
    char *endptr;

    fgets(buf, 1024, stdin);

    /* strip off "end of line" characters */
    buf[strcspn(buf, "\r\n")] = 0;

    input = strtol(buf, &endptr, 10);

    /* sanity check, only accept inputs that can be wholly parsed as
     * an integer
     */
    if (buf[0] == 0 || *endptr != 0) input = -1;

    return input;
}

int main(void)
{
    int k;

    fputs("Please enter a positive integer: ", stdout);

    k = readInt();

    while (k < 1)
    {
        fputs("I'm sorry, you must enter a positive integer greater "
                "than zero: ", stdout);
        k = readInt();
    }

    printf("You entered `%d'\n", k);

    return 0;
}