How do I fix buffer errors in while?

Question

When you run this code, it somehow works as well as the number of characters entered by scanf after the first operation. What's the problem?

I filled in the blank space before %c because I thought it might be a buffer problem, but the error still occurs.

void EX18() {
    float x, y;
    char op;
    int i = 0;
    while (1) {
        /*
        printf("\n******************** \n");
        printf("A ---- Add \n");
        printf("S ---- Subtract \n");
        printf("M ---- Multiply \n");
        printf("D ---- Divide \n");
        printf("Q ---- Quit \n");
        printf("******************** \n");
        */
        i++;
        printf("\n%dth iteration \n",i);
        printf("Select an operation: ");
        scanf(" %c", &op);

        if (op == 'Q') break;
        else {
            switch (op) {
            case 'A':
                printf("Enter two numbers separated by a space: ");
                scanf("%f %f", &x, &y);
                printf("%.2f \n", x + y);
                break;
            case 'S':
                printf("Enter two numbers separated by a space: ");
                scanf("%f %f", &x, &y);

                printf("%.2f \n", x - y);
                break;
            case 'M':
                printf("Enter two numbers separated by a space: ");
                scanf("%f %f", &x, &y);

                printf("%.2f \n", x * y);
                break;
            case 'D':
                printf("Enter two numbers separated by a space: ");
                scanf("%f %f", &x, &y);

                printf("%.2f \n", x / y);
                break;
            default:
                printf("\nUnknown command. Please try again.\n");
            }
        }
    }
}

int main()
{
    EX18();
    //TEST5();
    return 0;
}

Result Screen :

1th iteration Select an operation: 00

Unknown command. Please try again.

2th iteration Select an operation: Unknown command. Please try again.

3th iteration Select an operation: 000

Unknown command. Please try again.

4th iteration Select an operation: Unknown command. Please try again.

5th iteration Select an operation: Unknown command. Please try again.

6th iteration Select an operation:

Answer 1

The problem is that you read the "command" character by character. If you input two invalid characters the first scanf(" %c", &op) call will read the first, but the second invalid input is still in the buffer to be read by the next call.

As I advised in a comment, use fgets to read whole lines, and then e.g. sscanf to parse the input:

char buffer[256];  // No need to skimp on space

if (!fgets(buffer, sizeof buffer, stdin))
{
    // Failed to read input
    // TODO: Handle this case
}

if (sscanf(" %c", &op) != 1)
{
    // Invalid input
    // TODO: Handle this case
}

It's more code, but it will be more robust and safer than your current code.

Do something similar for the numeric input.

I also suggest you put these in separate functions, which makes it easier to read the code, to test it and to debug it when needed.

---

Please note that while the code describe above will make your input handling better and safer, it's not perfect.

An obnoxious user could input a line longer than 255 characters, in which case we're basically back where we started, where you get left over input in the buffers.

To be really safe from it you should check if the last character in the input from fgets is a newline, and if not then tell the user about the invalid input and use fgets (of perhaps even just fgetc) in a loop until you have read the newline.

Answer 2

The key part of your program is the two lines

printf("Select an operation: ");
scanf(" %c", &op);

Suppose you type A. scanf reads the one character, assigns it to op, the later part of your program recognizes A for addition and does its thing.

Suppose you type O. scanf reads the one character, assigns it to op, but the later part of your program doesn't recognize the character and so prints "Unknown command".

Suppose you type XYZ. scanf reads the first character, X, and assigns it to op. But, again, the later part of your program doesn't recognize the character and so prints "Unknown command". But this time, when the program loops back to the beginning, those two unread characters YZ are still sitting on the input stream. So scanf can immediately read the next character, Y, and assign it to op. Which the later part of your program doesn't recognize. And then when the program loops a third time, again, it's already got that Z on the input stream to read.

The key things to understand are:

1. scanf does not read lines of input.
2. Unread or unrecognized input is left on the input stream.

So it's very common for a program that uses scanf to get confused when some unread or unrecognized input, typed on what you thought was the "previous line", is actually still hanging around in the input buffer, where it gets read by a later call to scanf, seemingly immediately, without even waiting for you to type anything.

Although scanf is reasonably simple to use for simple inputs, it's actually a pretty strange function and, as you've seen, can be very confusing. In particular, if you're thinking that the user is typing a line of text and hitting Return, it's often better to have your program work that way too, by reading input a line at a time using something like fgets, as suggested in several of the comments. See this answer for more about reading input using something other than scanf.

See also this previous question for some more discussion of how scanf actually works.