ISBN-10

#include<stdio.h>
#include<conio.h>
#include<ctype.h>
#include<stdlib.h>

void menu();
int checkdigit(int *);
void strint(char *, int *);

void main()
{
    menu();
}

void strint(char *input, int *output) //string to int array
{
    for (int index = 0; index<9; index++)
    {
        if (isdigit(input[index]))
        {
            output[index] = input[index] - '0';
        }
        else
        {
            printf("\n\nInvalid input, nondigit character detected");
            break;
        }
    }
}

int checkdigit(int *input)
{
    int sum = 0;
    for (int index = 0; index<9; index++)
    {
        sum += (index + 1)*input[index];
    }
    return sum % 11;
}

void menu()
{
    char cinput[12];
    int output[12];
    int choice = 1;
    do {
        system("cls");
        printf("\n\t\t\tPlease choose an option;\n\n");
        printf("\t\t1.\tGenerate an ISBN-10 check digit\n");
        printf("\t\t2.\tValidate an ISBN-10\n\n");
        printf("\t\t0.\tExit program\n\n\t\t");
        scanf_s("%d", &choice);
        getchar();
        switch (choice)
        {
        case 1:
            system("cls");
            printf("\n\t\tEnter the 9 digit partial ISBN-10:\n\t\t");
            fgets(cinput, 10, stdin);
            strint(cinput, output);

            printf("\n\n\t\tThe checkdigit for %9s is %c", cinput, checkdigit(output)<10 ? checkdigit(output) +'0':'X');
            printf("\n\t\tPress any key to continue");
            _getch();
            break;
        case 2:
            system("cls");
            printf("\n\t\tEnter ISBN-10 for validation:\n\t\t");
            fgets(cinput, 11, stdin);
            if (cinput[9] == 'X' || cinput[9] == 'x')
            {
                cinput[9] = '1';
                cinput[10] = '0';
                cinput[11] = '\0';
            }
            strint(cinput, output);
            if (checkdigit(output) == atoi(&cinput[9]))
                printf("\n\n\t\tThe ISBN-10 is valid. \n\t\tActual Check Digit %2d = %c",checkdigit(output), atoi(&cinput[9])<10? atoi(&cinput[9])+'0':'X');
            else printf("\n\n\t\tThe ISBN-10 is invalid. \n\t\tActual Check Digit %2d =/= %c", checkdigit(output), atoi(&cinput[9])<10 ? atoi(&cinput[9]) + '0' : 'X');
            printf("\n\t\tPress any key to continue");
            _getch();
            break;
        }
    } while (choice);
}