bigint multiplication

[dudicus14]

Here I've got a sad multiplication function that is part of a large bigint calculator class. the arr is a pointer to a resizable array class(get and set are members of that class). When I use this I can mulitply large numbers only by one digit, if more digits )example 12*214 i get 428 cause its only multiplying 214 by the 2 in twelve. I understand why that is happening, I'm just not sure that I can fix it before the assignment is due tomorrow. Any help? Thanks!


void multiply(const bigint &A)
{

int carry=0;
for(int i=size-1;i>=0;i--)
{

int result = arr->get(i)*A.arr->get(size-1)+carry;
arr->set(i,result%10);
carry=result/10;


}

}

int main()
{
a.assign("214");
b.assign("12");
a.multiply(b);
a.print();

return 0;
}

[Gamer2015]

Do you have an add method?

if yes, you could do:
214 * 2 + 2140 * 1 (just add a 0 after the last digit)

It would be nice to know a bit more about what you allready have so we can help you

[dudicus14]

I do have an ann method

void add_pos(const bigint &A) //add big ints
{
int carry=0;
for(int i=size-1;i>=0;i--)
{
int result = arr->get(i)+A.arr->get(i)+carry;
arr->set(i,result%10);
carry=result/10;
}
}
I understand what you are saying but I'm just not sure how to code it.
thanks

[Gamer2015]

What else do you have?
Could you please post the Function declerations of all methods?

Also, you might want to copy a bit of data.
And you have to resize your data a little bit. a multiplication of 999*999 results in a 6-digit value, not a 3-digit one.

And please use the code Tag! :D

I mean like this

void multiply(const bigint &A)
{
    bigint temp1; // bigint with value 0

    int carry = 0;
    int shift = 0;

    for(int j = a.size-1; j >= 0; j--)
    {
        for(int i=size-1; i>=0; i--)
        {
            bigint temp2; // bigint with value 0 and size: size + A.size
            int result = (arr->get(i)*A.arr->get(j)+carry);
            temp2->set(temp2.size - shift - (i - size-1), result%10);
            carry=result/10;
        }
        shift++;
        temp1 += temp2;
    } 
    *this = temp1;
}

[dudicus14]

Not sure what all you would like so I heres the whole thing. the first class is a SafeArray and the second is the calculator

#include <iostream>
using namespace std;


template<typename Element>
class SafeArray
{
    int size;
    Element*Array;
    Element def;
    
public:
    SafeArray()                         //default constructor(with no parameter)
    {
                size = 10;
        Array = new Element[size];

        
    }
    SafeArray(int value)         //constructor with one int
    {
        size = value;
        Array = new Element[value];
       
    }
    ~SafeArray() {
        delete [] Array;
    }                                       //destructor
    Element get(int pos)                    //get method
        { if (pos<0)
        {cout<<"error";}
    
            if(pos>=size)
            { set_default(def);}
            return Array[pos]; }

    void set(int pos, Element val)
    {
        if (pos<0)
        {
            cout<<"error";
            return;
        }
        if(pos>=size)
        {
            resize(pos+1);
        }
        Array[pos] = val;
    }
    void resize(int new_size)
    {
        Element*temp=new Element[new_size];
        for(int i = 0; i<size;i++)
        {temp[i]=Array[i];}
        delete[]Array;
        Array = temp;
        size=new_size;
    }
    void set_default(Element d)        //set_default
    {
        def=d;
    }
        int get_size()                       //get size
    {
        return size;
    }
};

int size = 100;

class bigint
{
    SafeArray<int> *arr;
public:
    bool sign;
bigint()                                                   //initializes to zero
    {
        arr = new SafeArray<int>;
        for(int i =0;i < size; i++)
            arr->set(i,0);
    }
    
void print()                                               //prints numbers without zeroes in front
    {
        bool start_num=false;
        for(int i = 0;i <arr->get_size() ;i++)
        {
            if(arr->get(i)!=0 && start_num==false )
            {start_num=true;
                cout << arr->get(i);}
         else if(start_num==true)
             cout<<arr->get(i);
            
        }
        
       cout<<endl;
    }
    
void assign(const bigint &A)                             //
    {
        for(int i=0;i<arr->get_size();i++)
        {                                                            //Ways to initialize stuff
            arr->set(i,A.arr->get(i));
        }
    
    }
    
void assign(int num)                                     //
    {
        for(int i = arr->get_size()- 1; i >= 0; i--)
        {
            arr->set(i,num%10);
            num /=10;
        }
       
        
    }
    
void assign(string num)                                  //
    {
        long len = num.length();
        int j=arr->get_size()-1;
        for(long i=len-1;i>=0;i--)
        {
            arr->set(j,num[i]-48);
            j--;
        }
    }
   
void add_pos(const bigint &A)                                //add big ints
    {
        int carry=0;
        for(int i=size-1;i>=0;i--)
           {
               int result = arr->get(i)+A.arr->get(i)+carry;
               arr->set(i,result%10);
               carry=result/10;
           }
    }
    void multiply(const bigint &A)
{

int carry=0;
for(int i=size-1;i>=0;i--)
{

int result = arr->get(i)*A.arr->get(size-1)+carry;
arr->set(i,result%10);
carry=result/10;


}

}
};

[Gamer2015]

first off, Did my multiply method work?

[dudicus14]

No I got alot of errors with the referencing of the methods of the other class for temp 1 and 2.My fault though, I should have sent you the full code first

[Gamer2015]

could you please post those errors?

[dudicus14]

Here are the errors, I commented them


void multiply(const bigint &A)
{
bigint temp1; // bigint with value 0

int carry = 0;
int shift = 0;

for(int j = A.size-1; j >= 0; j--) //no member size in bigint
{
for(int i=size-1; i>=0; i--)
{
bigint temp2; // bigint with value 0 and size: size + A.size
int result = (arr->get(i)*A.arr->get(j)+carry);
temp2->set(temp2.size - shift - (i - size-1), result%10); //no member size or set in bigint
carry=result/10;
}
shift++;
temp1 += temp2; //no viable overloaded+=
}
*this = temp1;
}

[Gamer2015]

Use the code-tag! (on the right under Format the top left symbol)

well, all of them are quite obvious

void multiply(const bigint &A)
{  
  bigint temp1; // bigint with value 0

  int carry = 0;
  int shift = 0;

  for(int j = size-1; j >= 0; j--) //no member size in bigint
  {  
    for(int i=size-1; i>=0; i--)
    {
      bigint temp2; // bigint with value 0 and size: size + A.size
      int result = (arr->get(i)*A.arr->get(j)+carry);
      temp2->set(size - shift - (i - size-1), result%10);
      carry=result/10;
    }
    shift++;
    temp1.add_pos(temp2);
  }
  *this.assign(temp1);
}

changed all *.size to size
and the operator+ to add_pos
and *this = temp1 to *this assign

[dudicus14]

Sorry, I'll use it from now on.
I'm still getting errors on lines 14,18, and 20
14 says ( member reference type bigint is not a pointer)
18 says ( use of undeclared identifier temp2)
20 says ( member reference type bigint is a pointer

[Gamer2015]

These are up to you to clear up, they are all quite obvious :)

[dudicus14]

I changed it to this in order to get rid of the errors but it doesnt run so I may have just screwed it up haha.

 void multiply(const bigint &A)
    {
        bigint temp1; // bigint with value 0
        
        int carry = 0;
        int shift = 0;
         bigint temp2;
        for(int j = size-1; j >= 0; j--) //no member size in bigint
        {
            for(int i=size-1; i>=0; i--)
            {
                // bigint with value 0 and size: size + A.size
                int result = (arr->get(i)*A.arr->get(j)+carry);
                temp2.arr->set(size - shift - (i - size-1), result%10);
                carry=result/10;
            }
            shift++;
            temp1.add_pos(temp2);
        }
        this->assign(temp1);
    }

[Gamer2015]

Well, looks fine to me now ^^
Seems like my algorithm does not work.

I have a question.
Why is your arr a pointer?

[dudicus14]

I'm doing this for a class, and our teacher showed us how to get started and he made it a pointer, so I just went with it.

[Gamer2015]

Well, it makes sense if you want to assign data... but did he also write the rest of the SaveArray class?
Do you have to use that SaveArray class?

[dudicus14]

I wrote the safe array class (he checked it and said it'll work fine for my bigint class) yeah we have to use it for the bigint.

[Gamer2015]

I added that range check:

if(size - shift - (i - size-1) >= 0)
    temp2.arr->set(size - shift - (i - size-1), result%10);
else
    break;

It looks like this now...
I have a hard time using your SaveArray class for that example because we loop from the end to the beginning and so the resizing mechanism does not work

void multiply(const bigint &A)
{  
  bigint temp1; // bigint with value 0

  int carry = 0;
  int shift = 0;

  for(int j = size-1; j >= 0; j--) //no member size in bigint
  {  
    for(int i=size-1; i>=0; i--)
    {
      bigint temp2; // bigint with value 0 and size: size + A.size
      int result = (arr->get(i)*A.arr->get(j)+carry);

void multiply(const bigint &A)
    {
        bigint temp1; // bigint with value 0
        
        int carry = 0;
        int shift = 0;
         bigint temp2;
        for(int j = size-1; j >= 0; j--) //no member size in bigint
        {
            for(int i=size-1; i>=0; i--)
            {
                // bigint with value 0 and size: size + A.size
                int result = (arr->get(i)*A.arr->get(j)+carry);
                
                if(size - shift - (i - size-1) >= 0)
                    temp2.arr->set(size - shift - (i - size-1), result%10);
                else
                    break;

                carry=result/10;
            }
            shift++;
            temp1.add_pos(temp2);
        }
        this->assign(temp1);
    }