问如何在代码中包含负值？数制转换

EN

int main() {

    int num, base, i, r, digit, p, count = 0;

    char a[100];

    printf("Enter a decimal number: \n");
    scanf("%d", &num);

    printf("Enter the base to convert to: \n");
    scanf("%d", &base);

    p=num;
    do {
        r = p % base;
        digit = '0' + r;

        if (digit > '9')
            digit = digit+7;
        a[count]=digit;
        count++;
        p=p/base;
    } while (p!=0);

    printf("\n%d in base %d is: \n",num, base);

    for (i=count-1;i>=0;--i)
        printf("%c",a[i]);

    printf(".\n");

    return 0;
}