implementing a bigint calculator c++

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So far I've got an add, subtract and print function as well as a constructor that initializes to zero the array. For some reason the operations(+ and -) made alot of sense to me(i think) so I kind of got ahead of my self and am not too sure how to initialize a big integer, could I get some help with a function such as void assign(const bigint & A) or something like that? Also if there is something already wrong with my code please tell me. Thanks

C#

const int size=30;   //just to make things easier, will change to something big later
class bigint
{
    int digits[size];


public:
    // initializes to zero
    bigint()
    {
        for (int i = 0; i < size; i++)
            digits[i] = 0;

    }

    // prints a big int
    void print()
    {
        bigint B;
        for (int i = size - 1; i >= 0; i--)
        {
            int dig = B.digits[i];
            if (dig != 0)
                std::cout << dig;
        }
    }

    // subtracts a bigint(B) from another(A)
    void subtract(bigint & A, bigint & B)
    {
        for (int i = 0, borrow = 0; i < size; i++)
        {
            if (borrow = ((A.digits[i] -= B.digits[i] + borrow) < 0))
            {
                A.digits[i] += 10;
            }
        }
    }

    // adds a bigint(A) to another(B)
    void add(bigint & A, bigint & B)
    {
        for (int i = 0, carry = 0; i < size; i++)
        {
            if (carry = ((A.digits[i] += B.digits[i] + carry) < 9))
            {
                A.digits[i] -= 10;
            }
        }
    }
};

Posted 5-Feb-15 22:06pm

dudicus1414

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Comments

Richard MacCutchan 6-Feb-15 4:15am

Your array should be of bytes (8 bits, unsigned char) rather than int (32 bits, signed) type. You can then ensure each element contains only the values 0-9, which makes the calculations simpler. Also your print function creates a new bigint rather than printing the current object.

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nv3 · Answer 1 · 2015-02-06T03:03:00

Yes, there are a couple of things wrong with your class.

(a) The print function prints the value of a newly created variable B, which will be zero all the time. Just use your own member digits instead! Also: Your print functions contains a bug: It just prints the non-zero digits!

(b) The add and subtract member functions don't touch your member digits at all, which doesn't make sense. Instead you might want to provide functions like

C++

void add (const bigint& A);
void subtract (const bigint& A);

which add/subtract A to/from the object the operation is applied to.

(c) Working in decimal digits will not get you the best performance and is a waste of memory. Why don't you work in base 2 and hence use the built-in arithmetic operations. That will increase the performance of your functions by one or two orders of magnitude. Just for the purpose of printing a number it needs to be converted back to decimal.