c - C编程中如何用异常过滤掉字符

Question

我正在尝试编写一个 C 程序，要求用户输入，如果输入介于给定的 float 之间，则会出现输出。在这种情况下，如果输入速度为 20.5，则输出将是您处于二档。我想过滤掉所有无效输入。

我已经有了某种过滤器，但效果不佳。如果输入是“afgegq”，它将被过滤并再次询问我输入。但是如果是156FFAGFAE。它不会被过滤。

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

int main()
{
    for (;;)
    {
        int versnelling[5] = {1, 2, 3, 4, 5};

        float speed = 0;
        printf("Please enter a speed: ");
        if (scanf("%f", &speed) != 1)
        {
            fputs("error: invalid input. Please enter a number\n",stderr);
            return 0;
        }

        if (0.0 < speed && speed < 10.0)
        {
            printf("The gear you are in is: %i\n", versnelling[0]);
        }
        else if (speed >= 10.0 && speed < 30.0)
        {
            printf("The gear you are in is: %i\n", versnelling[1]);
        }
        else if (speed >= 30.0 && speed < 60.0)
        {
            printf("The gear you are in is: %i\n", versnelling[2]);
        }
        else if (speed >= 60.0 && speed < 80.0)
        {
            printf("The gear you are in is: %i\n", versnelling[3]);
        }
        else if (speed >= 80.0 && speed <= 100)
        {
            printf("The gear you are in is: %i\n", versnelling[4]);
        }
        else if (speed > 100)
        {
            printf("I can not go faster then 100km/h \n");
        }
        else if (speed == 0)
        {
            printf("The gear you are in is: Neutral\n");
        }
        else if (speed < 0 && speed > -15)
        {
            printf("The gear you are in is: R\n");
        }
        else if (speed < -15)
        {
            printf("I can not go that fast in reverse\n");
        }
    }
}

我希望它过滤掉除数字和字母 Q 之外的所有内容。我需要字母 Q，因为我想创建输入的平均值(稍后我将尝试找出方法)。但要知道我需要这个。提前谢谢你。

Answer 1

处理错误用户输入的最佳方法是首先读取行，然后使用strtof() 将其解析为float。

    printf("Please enter a speed: ");
    fflush(stdout); // Insure prompt is printed first

    char buf[100];
    if (fgets(buf, sizeof buf, stdin) == NULL) {
      printf("End of file\n");
      return 0;
    }

    if (toupper((unsigned char) *buf) == 'Q') == NULL) {
      printf("Q detected\n");
      break;
    }

    char *endptr; // location to store end of parsing
    errno = 0;
    *f = strtof(buf, &endptr); 
    bool no_conversion = buf == endptr;

    // Allow trailing white-space
    while (isspace((unsigned char) *endptr)) {
      endptr++;
    }

    if (no_conversion || *endptr) {
      fputs("error: invalid input. Please enter a number\n", stderr);
      continue; // No conversion or junk at the end
    }

    // Add additional tests if one wants to detect out-of-range (not shown)

    ....