C++ program Booths Algorithm 2s Complement using array

Question

I have included the program in which I am having a logical problem. The program is based on booth's algorithm and I have put a snippet of it. In this 'working' snippet decimal number is accepted from the user converted to decimal form with the help of array (a[0]=1 LSB) and lastly the 2s complement is calculated of array b[]. Now, when I run the program:

    #include<iostream>
    using namespace std;
    class booth
    {
    public:
    int n;
    int b[3];
    int comb[3], q[3];              //b=multiplicand q=multiplier
    int bb, qq;                     //bb and qq store actual decimal no.s    
    booth()
    {
        for(int i=0; i<4; i++)
        {
            b[i]=0;                 //b array stores multiplicand in binary
            q[i]=0;                 //q array stores multiplier in binary
            comb[i]=0;              //will store calculated 2s complement in      
                                      binary
        }
        n=4;
        bb=0;
        qq=0;
    }
void acceptMm();
void display();
};

void booth :: acceptMm()     //function to accept value from user and                     
                             //converting it into binary in the form of 
                             //array and then calculating its 2s complement
{
    cout<<"Enter Multiplicand: ";
    cin>>bb;
    cout<<"Enter Multiplier: ";
    cin>>qq;
    //decimal to binary
    int rem1, rem2, i=0, j=0; //rem1 and rem2 are remainders
    while(qq!=0)
    {
        rem2=qq%2;
        qq/=2;
        q[i]=rem2;
        i++;
    }
    cout<<q[3]<<q[2]<<q[1]<<q[0]<<endl;  // to display binary no.
    //again decimal to binary
    while(bb!=0)
    {
        rem1=bb%2;
        bb/=2;
        b[j]=rem1;
        j++;
    }
    cout<<b[3]<<b[2]<<b[1]<<b[0]<<endl;  //to display binary no.
    // 2s complement:
    int ii=0;
    int jj=4;    //4 bit binary number
        while(b[ii]==0 && jj!=0)
        {
            comb[ii]=b[ii];
            ii++;
            jj--;
        }
        comb[ii]=b[ii];
        cout<<b[3]<<b[2]<<b[1]<<b[0]<<endl;   //displayed value (problem)
        ii++;
        jj--;
        if(jj==0)
        {
            return;
        }
        while(jj!=0)
        {
        if(b[ii]==0)
        {
            comb[ii]=1;
            ii++;
            jj--;   }
        else
        {
            comb[ii]=0;
            ii++;
            jj--;
        }
        }
}

void booth :: display()
{
    cout<<"multiplicand\n";
    for(int x=3; x>=0; x--)
        {
            cout<<b[x]<<" ";
        }
    cout<<endl;
    cout<<"multiplier\n";
    for(int j=3; j>(-1); j--)
    {
        cout<<q[j]<<" ";
    }
    cout<<endl;
    cout<<"compliment of multiplicand\n";
    for(int y=3; y>(-1); y--)
    {
        cout<<comb[y]<<" ";
    }
}

int main()
{
booth obj;
cout<<"Booths Algorithm\n";
    obj.acceptMm();
    obj.display();
return 0;
}

Output

Booths Algorithm
Enter Multiplicand: 5
Enter Multiplier: 4
0100
0101
1101
multiplicand
1 1 0 1 
multiplier
0 1 0 0 
compliment of multiplicand
0 0 1 1 

Here in the output I expect the 6th line as 0101 but get 1101. Why are the values of array b[] changing? The value of array b[] at line 5 is correct so why does it change? According to the code the value shouldnt be changing right? I am stuck.. Please help!! Any suggestion will be appreciated!!

Answer 1

b, q and comb are array of 3 elements so b[3] is an array overflow (whose value is unknown). In fact comb being allocated right after b it is likely that b[3] equals comb[0].

Answer 2

You are doing an overflow . Do it like this. Simple .

class booth {
public:
    int n;
    int b[4];
    int comb[4], q[4];              //b=multiplicand q=multiplier
    int bb, qq;                     //bb and qq store actual decimal no.s
    booth() {
        for (int i = 0; i < 4; i++) {
            b[i] = 0;                 //b array stores multiplicand in binary
            q[i] = 0;                 //q array stores multiplier in binary
            comb[i] = 0;       //will store calculated 2s complement in        }
            n = 4;
            bb = 0;
            qq = 0;
        }
    }