C语言中Do while循环结合switch case无法正常运行的问题排查

Let's walk through what's going wrong with your code—there are a few key issues here that are keeping your switch branches from firing, plus some input handling quirks to straighten out.

Your switch statement uses case '3' and case '0', but your variable I is an integer (int), and you're reading it with scanf_s("%d", &I)—which reads numeric integer values, not character ASCII codes.

The character '3' has an ASCII value of 51, and '0' is 48. When you type the number 3 or 0, scanf_s stores the integer 3 or 0 in I—so these will never match the character constants in your case labels. That's why your switch branches never trigger!

Fix: Replace the character constants with integer constants:

case 3:
case 0:

Even after fixing the switch, your input logic has some bugs that will confuse users or cause unexpected behavior:

• Redundant getchar() calls: You call getchar() right after scanf_s, which eats the newline left in the input buffer—but then you add another getchar() inside the case 3 branch. This makes the program wait for an extra keypress that users won't expect. Instead, use a helper function to clear the input buffer consistently.
• Broken default branch logic:
  • The condition if (scanf_s("%d", &I) != 3 || scanf_s("%d", &I) != 0) makes no sense. scanf_s returns the number of successfully read values (here, 1 if it reads an integer, 0/EOF if not)—it will never return 3 or 0. This condition will always be true, and calling scanf_s twice forces users to input twice, which isn't what you want.
  • fflush(stdin) is undefined behavior in C. The standard only allows fflush on output streams. To clear the input buffer, read all remaining characters until you hit a newline or EOF.
• Readability issue: The while (I != 0); line is separated from the do block's closing brace. It works, but grouping them together makes the code easier to follow.

Here's a cleaned-up version of your code that addresses all these issues:

#include <stdio.h>

// Helper function to clear leftover characters in the input buffer
void clear_input_buffer() {
    int c;
    while ((c = getchar()) != '\n' && c != EOF);
}

int main() {
    int I = 1;
    printf("Press enter to start the loop...");
    getchar(); // Wait for user to press enter

    do {
        printf("\nYou are in a LOOP. Would you like to stay in it, or leave it? \nPress 0 to leave the loop or press 3 to stay in it: ");
        
        // Handle invalid input (non-numeric values)
        if (scanf_s("%d", &I) != 1) {
            printf("\nInvalid input! Please enter a number.\n");
            clear_input_buffer();
            continue;
        }
        clear_input_buffer(); // Clear the newline after the integer input

        switch (I) {
            case 3:
                printf("\nYou are STILL inside the LOOP. Next time press 0 to exit.\n");
                break;
            case 0:
                printf("\nExiting the LOOP...");
                break;
            default:
                printf("\nPlease, enter a valid command (0 to exit, 3 to stay): ");
                break;
        }
    } while (I != 0);

    printf("\nCongratulations! You are OUT of the LOOP!");
    return 0;
}

• Always match your case constant types to your switch variable type: integers use integer constants, chars use character constants.
• Be careful with input buffers—avoid redundant reads and never use fflush(stdin).
• Encapsulate repetitive logic (like clearing the input buffer) into helper functions to keep your code clean.

内容的提问来源于stack exchange，提问作者Azu Serra