如何在C中添加无限循环？

Question

我必须使其成为无限循环，并在输入错误字符时使其返回到开头。该怎么办？

到目前为止，这是我的代码：

#include <stdio.h>
#include <cs50.h>

int main ()
{
// Prompt user for input last choice contender
   char c;
   printf("Last choice: ");
   c = getchar();

      // If last choice is R, loser should play S
    if (c == 'R'|| c == 'r')
    {
        printf("Your next move: S");
    }
// Else if last choice is P, loser should play R
    else if (c == 'P' || c == 'p')
    {
        printf("Your next move: R");
    }
// Else if last choice is S, loser should play P
    else if (c == 'S' || c == 's')
    {
        printf("Your next move: P");
    }
// TO DO: If another character start over
    else if (c != 'S'|| c != 's' || c != 'P' || c != 'p' || c != 'R'|| c != 'r')
}
// TODO: make an infinite loop / start over

Answer 1

如果我确实正确理解了您的需求，您可以这样做：

#include <stdio.h>
#include <cs50.h>

int main ()
{
    while(true) // or while(1) if you don't want to include stddef or any other lib containing true and false declaration
    // Prompt user for input last choice contender
    {
        char c;
        printf("Last choice: ");
        c = getchar();

        // If last choice is R, loser should play S
        if (c == 'R'|| c == 'r')
        {
            printf("Your next move: S");
        }
        // Else if last choice is P, loser should play R
        else if (c == 'P' || c == 'p')
        {
            printf("Your next move: R");
        }
        // Else if last choice is S, loser should play P
        else if (c == 'S' || c == 's')
        {
            printf("Your next move: P");
        }
        // TO DO: If another character start over
        else if (c != 'S'|| c != 's' || c != 'P' || c != 'p' || c != 'R'|| c != 'r')
    }
}

另外，如果没有满足授权字符，您不需要做任何其他事情就可以在循环开始时重新启动代码。您甚至不需要用最新的 else if 子句捕获它们，因为无论如何到达循环中代码的末尾，一旦到达那里就不会再做任何事情。
（否则 if (c != 'S'|| c != 's' || c != 'P' || c != 'p' || c != 'R'|| c != 'r' ））
但是，如果您希望保留空（无子 {}）else if 子句，则应该添加一个 ;在子句的末尾，否则我不知道你当前的代码将如何编译。

我欢迎您使用 stack Overflow，但我建议您遵循一些关于 c 语言的基本指南，看起来您缺少一些严重的基础，这将继续阻碍您当前的学习道路。总的来说，我鼓励您继续学习:)祝您好运！

Answer 2

我有多种方法可以做到这一点，我推荐使用 while 循环。

添加一段时间(){

}
围绕您想要重申的所有代码，在 while() 参数内您可以有一个像这样的布尔值
布尔值赢=假，
而（！赢了）
当玩家获胜或您不希望玩家继续游戏时添加 won = true ！
但我也建议阅读有关 while 循环的内容，谷歌 while 循环 in google

Answer 3

您可以使用无限循环（例如 for(;;) 或 while(1)），它将永远运行，直到出现显式 break 语句遇到。您可以在确定输入有效后使用 break 语句。在那之前，循环将永远运行。

在这种情况下，您可以简单地使用 switch 语句，而不是使用长链 if...else if使用函数 toupper。

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

int main ()
{
    for (;;) //infinite loop
    {
        //NOTE: The following variable must be an "int" in order to
        //be able to distinguish EOF from an actual character.
        int c;

        printf("Last choice: ");
        c = getchar();

        switch ( toupper( c ) )
        {
            case 'R':
                printf( "Your next move: S" );
                break;
            case 'P':
                printf( "Your next move: R" );
                break;
            case 'S':
                printf( "Your next move: P" );
                break;
            case EOF:
                fprintf( stderr, "unexpected error!\n" );
                exit( EXIT_FAILURE );
            default:
                printf( "Input was not ok, please try again.\n" );
                continue;
        }

        //input was ok, so we can break out of the infinite loop
        break;
    }
}

然而，这个程序还不能正常工作：

Last choice: q
Input was not ok, please try again.
Last choice: Input was not ok, please try again.
Last choice: 

Last choice: This is a test.
Input was not ok, please try again.
Last choice: Input was not ok, please try again.
Last choice: Input was not ok, please try again.
Last choice: Your next move: P

正如你所看到的，程序对于同一行输入多次打印错误消息。这是因为 getchar 将返回该行中的每个字符，包括行末尾的换行符，并且每个字符都将被单独处理，直到找到所需的字符之一。这不是你想要的。由于用户输入是基于行的，因此一次读取一整行输入并拒绝整行输入（如果它包含多个字符（除了换行符之外））会更有意义。为了读取整行输入，我建议使用函数 fgets< /code>：

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

int main ()
{
    for (;;) //infinite loop
    {
        //need space for 1 character, the newline character and
        //the terminating null character
        char line[3];

        printf("Last choice: ");

        //attempt to read one line of input
        if ( fgets( line, sizeof line, stdin ) == NULL )
        {
            fprintf( stderr, "Input error!\n" );
            exit( EXIT_FAILURE );
        }

        //make sure that line was not too long
        if ( strchr( line, '\n' ) == NULL )
        {
            int c;

            printf( "Please only enter a single character!\n" );

            //discard remainder of line
            do
            {
                c = getchar();

                if ( c == EOF )
                {
                    fprintf( stderr, "Input error!\n" );
                    exit( EXIT_FAILURE );
                }

            } while ( c != '\n' );

            continue;
        }

        switch ( toupper( (unsigned char)line[0] ) )
        {
            case 'R':
                printf( "Your next move: S" );
                break;
            case 'P':
                printf( "Your next move: R" );
                break;
            case 'S':
                printf( "Your next move: P" );
                break;
            default:
                printf( "Input was not ok, please try again.\n" );
                continue;
        }

        //input was ok, so we can break out of the infinite loop
        break;
    }
}

现在程序可以正常运行了：

Last choice: This is a test.
Please only enter a single character!
Last choice: q
Input was not ok, please try again.
Last choice: s
Your next move: P