c - 在 while 循环中使用 "continue"，跳过 scanf ("%d"，var)

Question

所以我正在为我的 C 程序使用 visual studio。

while 循环以提示和 scanf 开始，如果用户输入非数字响应，循环中的 switch/case 将变为默认值。这将打印错误并“继续”循环。

问题是，当循环继续到下一次迭代时，它会完全跳过“scanf”，然后无限循环遍历默认情况。我已经搜索了几个小时，但似乎找不到解决方案。

我的目标是跳过 switch/case 之后的代码，然后回到开头。任何帮助将不胜感激。

while (userInput != 'N' && userInput != 'n') {

    printf("Enter input coefficients a, b and c: "); // prompt user input
    scanf_s("%d %d %d", &a, &b, &c); // look for and store user input

    /* ----- Break up the quadratic formula into parts -----*/

    inSqRoot = (pow(b, 2) - (4.0 * a * c)); // b^2 - 4ac
    absInSqRoot = abs((pow(b, 2) - (4.0 * a * c))); // absolute value of b^2 - 4ac
    denom = 2.0 * a; // get denomiator 2.0 * a
    negB = -1.0 * b; // take negative of b

    /*------ Determine number of roots -------*/

    if (!isdigit(a) || !isdigit(b) || !isdigit(c)) {

        rootNum = 4;

    }  // end if

    else if (a == 0 && b == 0 && c == 0) {

        rootNum = 0;
    } // end if

    else if (inSqRoot == 0) {

        rootNum = 1;
    } // end if

    else if (inSqRoot > 0) {

        rootNum = 2;
    } // end if

    else if (inSqRoot < 0) {

        rootNum = 3;

    } // end if


    /*------ Begin switch case for rootNum ------*/

    switch (rootNum) {

    case 0: // no roots

        printf("The equation has no roots.\n");
        break;

    case 1: // one root

        root1 = (-b + sqrt(inSqRoot)) / denom;

        printf("The equation has one real root.\n");
        printf("The root is: %.4g\n", root1);
        break;

    case 2: // two roots

        root1 = (-b + sqrt(inSqRoot)) / denom;
        root2 = (-b - sqrt(inSqRoot)) / denom;

        printf("The equation has two real roots.\n");
        printf("The roots are: %.4g and %.4g\n", root1, root2);
        break;

    case 3: // imaginary roots

        printf("The equation has imaginary roots.\n");
        printf("The roots are: %.1g + %.4gi and %.1g - %.4gi \n", negB / denom, sqrt(absInSqRoot) / denom, negB / denom, sqrt(absInSqRoot) / denom);
        break;

    default:
        printf("ERROR: The given values are not valid for a quadratic equation.\n");
        continue;
    }

    printf("Enter Y if you want to continue or N to stop the program: ");
    scanf_s("%*c%c", &userInput);
    printf("\n");
}

Answer 1

scanf 实际上并没有被跳过，它只是一遍又一遍地读取相同的错误输入。您可以使用 scanf 的返回值来查看读取了多少参数。在错误输入的情况下，它将小于 3。要清除错误输入，您可以调用这样的函数

void flushInput() {
    int c;
    while ((c = getchar()) != '\n' && c != EOF);
}

并在默认情况下添加对 flushInput 的调用

default:
    printf("ERROR: The given values are not valid for a quadratic equation.\n");
    flushInput();
    continue;