如何使用scanf检查输入？

Question

我有以下代码：

#include <stdio.h>

void printMenu() {
    printf("********************************\n");
    printf("*Enter a number (-1 to exit):  *");
    printf("\n* 1 - Add new student          *");
    printf("\n* 2 - Delete a student         *");
    printf("\n* 3 - Update student           *");
    printf("\n* 4 - Print a student          *");
    printf("\n* 5 - Print all students       *");
    printf("\n********************************\n");
    printf("\nEnter: ");
}

int main (int argc, char **argv) {
    int a;

    printMenu();
    scanf("%d", &a);

    while(a != -1) {
        switch(a) {
            case 1:     
                printf("1\n");
                break;
            case 2:     
                printf("2\n");
                break;
            case 3:         
                printf("3\n");
                break;
            case 4:         
                printf("4\n");
                break;
            case 5:         
                printf("5\n");
                break;
            default:       
                printf("\nInvalid answer!\n");
                break;
        }
       
        puts("");
        printMenu();
        scanf("%d", &a);  
    }
    return 0;
} 

我想检查scanf 的输入。例如，如果用户输入一个数字，它会成功运行，但是如果用户输入一个字符串或字符之类的东西，它会一次又一次地不停地打印菜单。

我想检查用户是否输入了数字。如果没有，我想再次询问输入。 我试过这样的事情：

if(scanf("%d", &a)!=1) {
     printf("Enter again: ");
     scanf("%d", &a);
}

或者：

assert(scanf("%d", &a);

assert 函数程序停止执行，但我希望它在一段时间后继续执行。 有什么想法吗？

Answer 1

在处理交互式输入时，我觉得总是很方便 消耗输入行，然后分析它。
这样，无论在输入上给出什么（符合预期 与否），它不会留在输入流中阻止进一步分析。

一般是这样的

/**
  gcc -std=c99 -o prog_c prog_c.c \
      -pedantic -Wall -Wextra -Wconversion \
      -Wc++-compat -Wwrite-strings -Wold-style-definition -Wvla \
      -g -O0 -UNDEBUG -fsanitize=address,undefined
**/

#include <stdio.h>

int
main(void)
{
  for(;;)
  {
    printf("Please, provide something interesting: ");
    char input_line[0x100];
    if(!fgets(input_line, sizeof(input_line), stdin))
    {
      printf("no more input\n");
      break; // leave the loop
    }
    int answer;
    if(sscanf(input_line, "%d", &answer)!=1)
    {
      printf("an integer was expected.\n");
      continue; // go back to the prompt
    }
    printf("%d + %d = %d\n", // make use of the provided information
           answer, answer, answer+answer);
  }
  return 0;
}

Answer 2

但如果用户输入类似字符串或字符的内容（例如 a 或 grebv），它会一次又一次地不间断地打印菜单

这是因为当 scanf 失败时没有读取任何内容，因此您必须绕过非数字，例如使用 getchar/fgets 读取到行尾。警告您还必须管理 EOF 的情况，例如，因为 stdin 在文件中被重定向。

待办事项：

if(scanf("%d", &a)!=1) {
     printf("Enter again: ");
     scanf("%d", &a);
}

即使在第二个 scanf 之前添加无效输入的刷新也是不够的，因为您可能有两个或多个连续错误（第二个 scanf 也失败）。

一种方式可以是例如：

#include <stdio.h>

void printMenu() {
  fputs("********************************\n"
        "*Enter a number (-1 to exit):  *\n"
        "* 1 - Add new student          *\n"
        "* 2 - Delete a student         *\n"
        "* 3 - Update student           *\n"
        "* 4 - Print a student          *\n"
        "* 5 - Print all students       *\n"
        "********************************\n"
        "\nEnter: ", stdout);
}

int main()
{
  int a;

  do {
    printMenu();

    if (scanf("%d", &a) != 1) {
      /* flush invalid input up to the end of line */
      while ((a = getchar()) != '\n') {
        if (a == EOF) {
          puts("...EOF...");
          return -1;
        }
      }
      a = 0; /* any 'invalid' value managed in the 'default' case of the 'switch'*/
    }

    switch (a) {
    case -1:
      break; /* or 'return 0;' and the 'do while' is a 'for(;;)' */
    case 1:     
      printf("1\n");
      break;
    case 2:     
      printf("2\n");
      break;
    case 3:         
      printf("3\n");
      break;
    case 4:         
      printf("4\n");
      break;
    case 5:         
      printf("5\n");
      break;
    default:
      puts("\nInvalid answer!");
      break;
    }
  } while (a != -1);

  return 0;
}

编译和执行：

/tmp % gcc -Wall a.c
/tmp % ./a.out
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: 1
1
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: aze

Invalid answer!
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: 2
2
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: -1
/tmp % 
/tmp % ( echo 1 ; echo aze ; echo 2 ) | ./a.out
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: 1
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: 
Invalid answer!
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: 2
********************************
*Enter a number (-1 to exit):  *
* 1 - Add new student          *
* 2 - Delete a student         *
* 3 - Update student           *
* 4 - Print a student          *
* 5 - Print all students       *
********************************

Enter: ...EOF...
/tmp %