C代码陷入无限的do-while循环

Question

I'm trying to simulate a vending machine. My program takes an inputted amount from the command line as the price and then continues to prompt until the program is aborted. I'm trying to achieve this through a while-do loop which I now know must be wrong. I need the program to continue to prompt to enter a coin, even after they have inserted the write amount of coins.

An example of how the code should look like complied is this: $ - represents command line

$ pop 225
Price must be from 10 to 100 cents
$ pop 86
Price must be a multiple of 5.
$ pop 50
Welcome to my Vending Machine!
Pop is 50 cents. Please insert nickels, dimes, or quarters.

Enter coin [NDQ]:d
You have inserted a dime
Please insert 40 more cents.
Enter coin [NDQ]:d
You have inserted a dime
Please insert 30 more cents.
Enter coin [NDQ]:d
You have inserted a dime
Please insert 20 more cents.
Enter coin [NDQ]:d
You have inserted a dime
Please insert 10 more cents.
Enter coin [NDQ]:d
You have inserted a dime
Pop is dispensed. Thank you for you business! Please come again.
Enter coin [NDQ]

See how at the end it goes back to being able to enter a coin, until E is selected to exit. I'm trying to do this however, my code is currently stuck in an infinite loop upon initial input of N,D, or Q. Would appreciate any help or guidance. Thank you!

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#define NI 5
#define DI 10
#define QU 25



bool isValid(int num) {
    return (num >= 10 && num <= 100 );
}

bool isMultiple(int num) {
        return (num % 5 == 0);
}

int 
main (int argc, char *argv[])
{  for (int i = 1; i != argc; ++i) {
         int price = atoi(argv[i]);
        if (!isValid(price)) {
            printf("Price muse be from 10 cents to 100 cents.\n");
            break;
        } else if (!isMultiple(price)) {
                printf("Price must be a multiple of 5.\n");
                break;
        } else {
                printf(" Welcome to my Vending Machine!\n");
                printf("Pop is %d cents. Please enter nickels, dimes, or quarters\n", price);


                char coin;

                do
                {
                    printf(" PLease enter a coin [NDQR]\n");
                     scanf (" %c", &coin);


                        int cents = 0;

                        while (cents <= price) {


                        if (coin == 'N'|| coin == 'n') {
                                cents = cents + NI;
                                printf(" You have inserted 5 cents\n");
                        }
                        else if (coin == 'd' || coin == 'D') {
                                cents = cents + DI;
                                printf("You have inserted 10 cents\n");
                        }
                        else if (coin == 'Q' || coin == 'q') {
                                cents = cents + QU;
                                printf("You have entered 25 cents\n");


                        } else {
                                printf("Unknown coin. Rejected.\n");
                        }

                        int balance = price - cents;
                        printf("You have entered a total of %d cents\n", cents);

                        if (balance > 0) {
                        printf("You must enter %d more cents\n", balance);
                    } else {


                         int change = cents - price;
                         int dimes = change/10;
                         int remainder = change % 10;
                         int nickles = remainder/5;
                         int remainder2= nickles % 5;

                        printf("Change returned. %d nickles and %d dimes",nickles, dimes);
                    }

                    }
                } while (coin != 'E' && coin != 'e');


               printf("DONE!\n");
                return 0;
}
}
}

Answer 1

In your while condition: coin != 'E' || coin != 'e' coin != 'E' || coin != 'e' cannot be false , since it's at least different from one of E or e .

You mean coin != 'E' && coin != 'e' , or in that particular case tolower(e)!='e'

Answer 2

Your printf and scanf where you prompt the user for a coin and read the value is outside of your while (cents <= price) loop which processes each coin. That's why you get stuck in an infinite loop.

You need to move these two lines inside of this loop at the start. Also, you probably don't even need the outer do...while loop, since there's no point in the user entering more coins after they've paid the full amount.

           printf(" Welcome to my Vending Machine!\n");
           printf("Pop is %d cents. Please enter nickels, dimes, or quarters\n", price);

           char coin;
           int cents = 0;

           while (cents <= price) {
                printf(" PLease enter a coin [NDQR]\n");
                scanf (" %c", &coin);

                ...
           }
           printf("DONE!\n");

While the condition (coin != 'E' || coin != 'e') is invalid as it will always evaluate to true, that's not the source of the infinite loop. As previously mentioned, you don't need that outer loop at all.