无限循环 Do While

Question

I am trying to create a program, in which at the beginning it shows a menu to the user, which consists in a do{... }while; which reads an int., with a switch inside.

It works perfectly to read and check the integer, the problem is when writing a character or string, which gets stuck in an infinite loop showing the default message of the switch loop. The code is as follows:

int op;
printf("Choose an option:\n 1. option 1\n 2. option 2\n 3. option 3\n");
do{
    scanf("%d", &op);
    switch(op){
        case 1: (instruction); break;
        case 2: (instruction); break;
        case 3: (instruction); break;
        default: printf("\nPlease enter a valid option\n");
    }
}while(op<1 || op>3);

Answer 1

It works perfectly to read and check the integer, the problem is when writing a character or string, which gets stuck in an infinite loop showing the default message of the switch loop.

scanf("%d", &op); does nothing when the input is not a valid int, you need to check the return value is 1 and if not to decide what to do like for instance read a string or flush up to the end of line, also managing the EOF case

Note in case scanf does nothing op is not set

So can be:

int op;
printf("Choose an option:\n 1. option 1\n 2. option 2\n 3. option 3\n");
do{
    if (scanf("%d", &op) != 1) {
       // flush all the line
       while ((op = getchar()) != '\n') {
         if (c == EOF) {
           puts("EOF, abort");
           exit(0); /* what you want */
         }
       }
       op = -1; /* invalid value */
     }
     switch(op){
        case 1: (instruction); break;
        case 2: (instruction); break;
        case 3: (instruction); break;
        default: puts("\nPlease enter a valid option");
       }
    }           
}while(op<1 || op>3);

I encourage you to never trust on an input and always check return value of scanf etc

Answer 2

The %d conversion specifier is seeking for decimal input only. It does not work to consume characters or strings. If you input a character instead of an decimal value, the directive will fail, and because op isn´t initialized you have undefined behavior .

To catch a string with scanf() use the %s conversion specifier or use fgets() . For only catching one character use the %c conversion specifier with scanf() .