Entering a character except for numbers will make my program to output infinitely. How do I fix this?

Question

When I enter any characters except numbers in the "Enter your choice" it will infinitely loop. For example: Typing in a character. Result As, you can see it will just infinitely loop, unless I input a number between 1 to 10 as represented for each product choice. Or when typing in any number not between numbers 1 to 10, it will be recognized as an Invalid choice. P.S. Newbie Coder.

This is the code of the program.

#include <stdio.h>

int main(void)
{
    int choice, quantity, total = 0, price = 0;
    char end;

    do
    {
        printf("\nWelcome to our store!\n\n");
        printf("Welcome to our store!\n");
        printf("Please select a product from the following list:\n");
        printf("1. Oishi Prawn Crackers - 7 PHP\n");
        printf("2. Piattos - 16 PHP\n");
        printf("3. Coca-Cola - 40 PHP\n");
        printf("4. Sting Energy Drink - 25 PHP\n");
        printf("5. Gatorade - 43 PHP\n");
        printf("6. Nature Spring 500mL - 10 PHP\n");
        printf("7. KitKat - 30 PHP\n");
        printf("8. Snickers - 44 PHP\n");
        printf("9. Oishi Prawn Crackers - 7 PHP\n");
        printf("10. M&M's - 80 PHP\n");
        printf("Enter 0 to finish.\n");
        printf("\nProduct                 Quantity   Price\n");
        printf("----------------------------------------\n");

        do
        {
            printf("Enter your choice: ");
            scanf(" %d", &choice);

            if (choice == 0)
            {
                break;
            }

            printf("Enter the quantity: ");
            scanf(" %d", &quantity);

            switch (choice)
            {
            case 1:
                printf("Oishi Prawn Crackers        %d        %d\n", quantity, price = 7 * quantity);
                total += 7 * quantity;
                break;
            case 2:
                printf("Piattos                     %d        %d\n", quantity, price = 16 * quantity);
                total += 15 * quantity;
                break;
            case 3:
                printf("Coca-Cola                   %d        %d\n", quantity, price = 40 * quantity);
                total += 40 * quantity;
                break;
            case 4:
                printf("Sting Energy Drink          %d        %d\n", quantity, price = 25 * quantity);
                total += 25 * quantity;
                break;
            case 5:
                printf("Gatorade 500mL              %d        %d\n", quantity, price = 43 * quantity);
                total += 43 * quantity;
                break;
            case 6:
                printf("Nature Spring 500mL         %d        %d\n", quantity, price = 10 * quantity);
                total += 10 * quantity;
                break;
            case 7:
                printf("KitKat                      %d        %d\n", quantity, price = 30 * quantity);
                total += 30 * quantity;
                break;
            case 8:
                printf("Snickers                    %d        %d\n", quantity, price = 44 * quantity);
                total += 44 * quantity;
                break;
            case 9:
                printf("M&M's                       %d        %d\n", quantity, price = 40 * quantity);
                total += 40 * quantity;
                break;
            case 10:
                printf("Pringles                    %d        %d\n", quantity, price = 80 * quantity);
                total += 80 * quantity;
                break;
            default:
                printf("Invalid choice.\n");
                break;
            }
        } while (choice != 0);

        printf("----------------------------------------\n");
        printf("Total cost: %d PHP\n", total);
        printf("Thank you for shopping with us!\n");
        printf("\nWant to Buy Again?\n");
        printf("Y if Yes\n");
        printf("Type any key if No\n");
        scanf(" %c", &end);

        switch (end) {
        case 'Y':
            printf("\nOK!\n");
            break;
        default:
            printf("\nBYE!\n");
            break;
        }
    } while (end == 'Y');
    return 0;
}

So, I typed in numbers from 1 to 10 and it seems to recognize every product and it will ask for the quantity. And typing in any numbers it will do what it should and will output Invalid Choice. I tried changing the variables expecting it to be fixed but it won't work at all. It seems that I overlooked something but I don't know where.

Answer 1

Consider using a structure to store the name and cost of each product. That will avoid errors in repetitions.
scanint and scanchar will scan and discard pending characters in the input stream. Extra non-whitespace characters will cause a re-prompt.

#include <stdio.h>

typedef struct item item_t;
struct item {
    char const *name;
    int cost;
};

int scanint ( char const *prompt);
int scanchar ( char const *prompt);
void menu ( size_t items, item_t product[]);

int main(void)
{
    item_t products[] = {
          { "Oishi Prawn Crackers",  7}
        , { "Piattos",              16}
        , { "Coca-Cola",            40}
        , { "Sting Energy Drink",   25}
        , { "Gatorade",             43}
        , { "Nature Spring 500mL",  10}
        , { "KitKat",               30}
        , { "Snickers",             44}
        , { "Pringles",              7}
        , { "M&M's",                80}
    };

    int choice = 0;
    int quantity = 0;
    int total = 0;
    char end = 0;
    size_t items = sizeof products / sizeof *products;

    do {
        printf("\nWelcome to our store!\n\n");
        menu ( items, products);

        while ( 1) {
            choice = scanint ( "Enter your choice: ");

            if (choice == 0) {
                break;
            }
            --choice;

            if ( choice < items) {
                quantity = scanint ( "\tEnter the quantity: ");

                printf ( "%-22s%3d %7d\n"
                , products[choice].name
                , quantity
                , products[choice].cost * quantity);
                total += products[choice].cost * quantity;
            }
            else {
                printf("Invalid choice.\n");
            }
        }

        printf ( "----------------------------------------\n");
        printf ( "Total cost: %d PHP\n", total);
        printf ( "Thank you for shopping with us!\n");

        end = scanchar ( "\nWant to Buy Again?\nY if Yes\nType any key if No\n");

        switch (end) {
        case 'Y':
            printf("\nOK!\n");
            break;
        default:
            printf("\nBYE!\n");
            break;
        }
    } while (end == 'Y');
    return 0;
}

void menu ( size_t items, item_t product[]) {
    printf ( "Please select a product from the following list:\n");
    for ( int each = 0; each < items; ++each) {
        printf ( "%2d. %-22s%3d PHP\n"
        , each + 1, product[each].name, product[each].cost);
    }
}

int scanint ( char const *prompt) {
    char newline[2] = "";
    int extra = 0;
    int scanned = 0;
    int value = 0;

    do {
        printf ( "%s", prompt);
        fflush ( stdout);
        scanned = scanf ( "%d", &value);
        if ( scanned == EOF) {
            return 0;
        }
        extra = 0;
        scanf ( "%*[ \f\t\v]"); // scan and discard whitespace except newline
        scanf ( "%*[^\n]%n", &extra); // scan discard and count not a newline
        scanned += scanf ( "%1[\n]", newline);
    } while ( value < 0 || scanned != 2 || extra != 0);

    return value;
}

int scanchar ( char const *prompt) {
    char newline[2] = "";
    int extra = 0;
    int scanned = 0;
    char value = 0;

    do {
        printf ( "%s", prompt);
        fflush ( stdout);
        scanned = scanf ( " %c", &value);
        if ( scanned == EOF) {
            return 0;
        }
        extra = 0;
        scanf ( "%*[ \f\t\v]"); // scan and discard trailing whitespace except newline
        scanf ( "%*[^\n]%n", &extra); // scan discard and count not a newline
        scanned += scanf ( "%1[\n]", newline);
    } while ( scanned != 2 || extra != 0);

    return value;
}

Answer 2

Here you go:

#include <stdio.h>

int main(void)
{
    int choice = 0, quantity, total = 0, price = 0;
    char end;

    do
    {
        printf("\nWelcome to our store!\n\n");
        printf("Welcome to our store!\n");
        printf("Please select a product from the following list:\n");
        printf("1. Oishi Prawn Crackers - 7 PHP\n");
        printf("2. Piattos - 16 PHP\n");
        printf("3. Coca-Cola - 40 PHP\n");
        printf("4. Sting Energy Drink - 25 PHP\n");
        printf("5. Gatorade - 43 PHP\n");
        printf("6. Nature Spring 500mL - 10 PHP\n");
        printf("7. KitKat - 30 PHP\n");
        printf("8. Snickers - 44 PHP\n");
        printf("9. Oishi Prawn Crackers - 7 PHP\n");
        printf("10. M&M's - 80 PHP\n");
        printf("Enter 0 to finish.\n");
        printf("\nProduct                 Quantity   Price\n");
        printf("----------------------------------------\n");

        do
        {
            int choice = 0;
            
            printf("Enter your choice: ");
            scanf(" %d", &choice);
            while ( getchar() != '\n' );
            if (choice == 0 || choice > 10)
            {
                break;
            }

            printf("Enter the quantity: ");
            scanf(" %d", &quantity);

            switch (choice)
            {
            case 1:
                printf("Oishi Prawn Crackers        %d        %d\n", quantity, price = 7 * quantity);
                total += 7 * quantity;
                break;
            case 2:
                printf("Piattos                     %d        %d\n", quantity, price = 16 * quantity);
                total += 15 * quantity;
                break;
            case 3:
                printf("Coca-Cola                   %d        %d\n", quantity, price = 40 * quantity);
                total += 40 * quantity;
                break;
            case 4:
                printf("Sting Energy Drink          %d        %d\n", quantity, price = 25 * quantity);
                total += 25 * quantity;
                break;
            case 5:
                printf("Gatorade 500mL              %d        %d\n", quantity, price = 43 * quantity);
                total += 43 * quantity;
                break;
            case 6:
                printf("Nature Spring 500mL         %d        %d\n", quantity, price = 10 * quantity);
                total += 10 * quantity;
                break;
            case 7:
                printf("KitKat                      %d        %d\n", quantity, price = 30 * quantity);
                total += 30 * quantity;
                break;
            case 8:
                printf("Snickers                    %d        %d\n", quantity, price = 44 * quantity);
                total += 44 * quantity;
                break;
            case 9:
                printf("M&M's                       %d        %d\n", quantity, price = 40 * quantity);
                total += 40 * quantity;
                break;
            case 10:
                printf("Pringles                    %d        %d\n", quantity, price = 80 * quantity);
                total += 80 * quantity;
                break;
            default:
                printf("Invalid choice.\n");
                break;
            }
        } while (choice != 0);

        printf("----------------------------------------\n");
        printf("Total cost: %d PHP\n", total);
        printf("Thank you for shopping with us!\n");
        printf("\nWant to Buy Again?\n");
        printf("Y if Yes\n");
        printf("Type any key if No\n");
        scanf(" %c", &end);

        switch (end) {
        case 'Y':
            printf("\nOK!\n");
            break;
        default:
            printf("\nBYE!\n");
            break;
        }
    } while (end == 'Y');
    return 0;
}