3

尝试用 5 个选项编译一个简单的 switch 语句。1-4 产生计算和输出，而#5 退出程序。我做了一个 do/while 循环，所以如果输入选项 5，程序将结束。我收到一个错误：

4_19.c: In function ‘main’:
4_19.c:95: error: ‘choice’ undeclared (first use in this function)
4_19.c:95: error: (Each undeclared identifier is reported only once
4_19.c:95: error: for each function it appears in.)

我不知道为什么它说它未声明，因为我在一开始就声明了它。我做错了什么？谢谢。这是我的代码：

    /* C++ book. 4_19 The speed of sound in gases. 
Create a menu to choose between 4 gases. User then enters number of seconds
it took to travel to destination. The program will calculate how far the source was (from speed that is unique to gas density). Validate input of seconds from 0 to 30 seconds only. 
*/

#include <stdio.h>
int main(void)
{
    do
    {
        // Declare variables
        int choice;
        float speed, seconds = 0, distance;

        //Display program details and menu choice
        printf("\n");
        printf("Choose a gas that you would like to analyze.\n");
        printf("Medium                              Speed(m/s)\n");
        printf("1.Carbon Dioxide                    258.0\n");
        printf("2.Air                               331.5\n");
        printf("3.Helium                            972.0\n");
        printf("4.Hydrogen                          1270.0\n");
        printf("5.Quit Program");
        printf("Enter a choice 1-5: ");
        scanf("%i",&choice);
        while (choice < 1 || choice > 5)        // Validate choice input. 
        {   
            printf("You entered an invalid number. Choose 1,2,3, or 4 only.\n");
            printf("Enter a choice 1-5: ");
            scanf("%i",&choice);
        }

        // Switch statements to execute different choices
        switch(choice)
        {
            case 1:     // Carbon Dioxide
                printf("Enter number of seconds, from 0 to 30, that the sound traveled in carbon dioxide: ");
                scanf("%f", &seconds);
                while (seconds < 0 || seconds > 30)      // Validate time entered
                {
                    printf("The range of input for seconds is only from 0 to 30 seconds.\n");
                    printf("Please enter a valid number for number of seconds: ");
                    scanf("%f", &seconds);
                }
                speed = 258.0;
                distance = speed * seconds;
                printf("The distance from the source of the sound is %.2f meters in carbon dioxide.\n", distance);
                break;

            case 2:     // Air
                printf("Enter number of seconds, from 0 to 30, that the sound traveled in air: ");
                scanf("%f", &seconds);
                while (seconds < 0 || seconds > 30)      // Validate time entered
                {
                    printf("The range of input for seconds is only from 0 to 30 seconds.\n");
                    printf("Please enter a valid number for number of seconds: ");
                    scanf("%f", &seconds);
                }
                speed = 331.5;
                distance = speed * seconds;
                printf("The distance from the source of the sound is %.2f meters in air.\n", distance);
                break;

            case 3:     // Helium
                printf("Enter number of seconds, from 0 to 30, that the sound traveled in helium: ");
                scanf("%f", &seconds);
                while (seconds < 0 || seconds > 30)      // Validate time entered
                {
                    printf("The range of input for seconds is only from 0 to 30 seconds.\n");
                    printf("Please enter a valid number for number of seconds: ");
                    scanf("%f", &seconds);
                }
                speed = 972.0;
                distance = speed * seconds;
                printf("The distance from the source of the sound is %.2f meters in helium.\n", distance);
                break;

            case 4:     // Hydrogen
                printf("Enter number of seconds, from 0 to 30, that the sound traveled in hydrogen: ");
                scanf("%f", &seconds);
                while (seconds < 0 || seconds > 30)      // Validate time entered
                {
                    printf("The range of input for seconds is only from 0 to 30 seconds.\n");
                    printf("Please enter a valid number for number of seconds: ");
                    scanf("%f", &seconds);
                }
                speed = 1270.0;
                distance = speed * seconds;
                printf("The distance from the source of the sound is %.2f meters in hydrogen.\n", distance);
                break;

            case 5:
                printf("End of Program\n");
                break;
        }
    } while (choice != 5);

    return 0;
}
4

4 回答 4

8

You have declared choice inside your do { } while loop; this means that it can only be accessed within those two braces. However, in your while(choice != 5) condition, you reference it again, outside the braces; this is an error. The solution is to move choice up one level, and declare it within the scope for main.

于 2011-07-05T03:39:37.980 回答
3

Move the declaration of choice outside of the loop. Right before the do.

choice is only scoped inside the loop. The while() clause is outside of the loop, so it can't access things that were declared inside the loop's curly braces itself.

于 2011-07-05T03:41:40.127 回答
2

Just declare choice before the do statement. This is because when you declare it within the do loop, its not visible in the while condition as the scope is local and within the loop only

于 2011-07-05T03:40:52.317 回答
1

我知道现在评论这个已经太晚了。但是，如果将来有人遇到这个问题并找到这个问题。您需要在括号外声明变量。这是一个例子。

int main()
{
    char choice;  // Variables defined outside loop. With in the scope of main.
    double a, x, res;
    do
    {


        cout << "Enter a: ";
        cin >> a;
        cout << "Enter x: ";
        cin >> x;

        res = 5.00 * tanh(a, x) + 4.00 * cos(a, x);

        cout << "F = " << res << endl;
        cout << "Would you like to continue? ";
        cin >> choice;

    } while (choice == 'Y' || choice == 'y');

    return 0;
}
于 2020-04-30T18:37:39.290 回答