Adding two strings mathematically?

Question

i was looking around the forums and i still couldnt find my answer to my problem. I got two strings, that are just really an array of numbers. for example(i just choose random numbers

    string input1="12345678909876543212";
    string input2="12345";

I want to add these two string together but act them like there integers. My goal is creating a class where i can add bigger numbers than (long long int) so it can exceed the largest long long int variable.

So i revese the string with no problem, so now there

  input1="21234567890987654321" 
  input2="54321"

then i tried adding, let's say input1[0]+input2[0] (2+5) to a new string lets call it newString[0] where that would equal (7); but i cant find a good way to temporally convert the current number in the string so i can add it to the new string? can anyone help. I get sick and tired of atoi,stof,stod. they don't seem to work at all for me. Any way i can make this function work. I don't care about making the class yet, i just care about finding a way to add those two strings mathematically but still maintaining the newString's string format. Thank you for whoever can figure this out for me

Answer 1

Okay, so, assuming your only problem is with the logic, not the class design thing, I came up with this logic

• fill up the inputs with 0s, checking the lengths, match the lengths
• add like normal addition, keeping track of carry
• finally remove leading zeros from result

So using std::transform with a lambda function on reverse iterators :-

char carry = 0;

std::transform(input1.rbegin(),input1.rend(),input2.rbegin(),
              result.rbegin(),[&carry]( char x,  char y){
    char z = (x-'0')+(y-'0') + carry;
    if (z > 9)
    {
        carry = 1;
        z -= 10;
    }
    else
    {
        carry = 0;
    }
    return z + '0';
});

//And finally the last carry
result[0] = carry + '0';

//Remove the leading zero
n = result.find_first_not_of("0");
if (n != string::npos)
{
    result = result.substr(n);
}

See Here

Edit "Can you comment on what your doing here"

                +--------+--------------+------------+-------> Reverse Iterator
                |        |              |            |
std::transform( | input1.rbegin(), input1.rend(),input2.rbegin(),
               result.rbegin(), [&carry]( char x,  char y){
               //This starts a lambda function
    char z = (x-'0')+(y-'0') + carry; // x,y have ASCII value of each digit
    // Substracr ASCII of 0 i.e. 48 to get the "original" number
    // Add them up
    if (z > 9) //If result greater than 9, you have a carry
    {
        carry = 1; // store carry for proceeding sums
        z -= 10; // Obviously 
    }
    else
    {
        carry = 0; //Else no carry was generated
    }
    return z + '0'; // Now you have "correct" number, make it a char, add 48
});

std::transform is present in header <algorithm>, see the ideone posted link.

Answer 2

Here's A Solution for adding two numbers represented as strings .

#include<iostream>
using namespace std;

string add(string a, string b)
{
   int al=a.size()-1;
   int bl=b.size()-1;

   int carry=0;
   string result="";

   while(al>=0 && bl>=0)
    {
    int temp = (int)(a[al] - '0') + (int)(b[bl] - '0') + carry ;
    carry = 0;
    if(temp > 9 )
    {
        carry=1;
        temp=temp-10;
    }

    result+=char(temp + '0');
    al--;
    bl--;
    }

   while(al>=0)
    {
        int temp = (int)(a[al] - '0') + carry ;
        carry = 0;
        if(temp>9)
        {
            carry=1;
            temp=temp%10;
        }

        result+=char(temp + '0');
        al--;
    }

   while(bl>=0)
    {
        int temp = (int)(b[bl] - '0') + carry ;
        carry = 0;
        if(temp>9)
        {
            carry=1;
            temp=temp%10;
        }

        result+=char(temp + '0');
        bl--;
    }

if(carry)
    result+="1";

string addition="";

for(int i=result.size()-1;i>=0;i--)
    addition+=result[i];   // reversing the answer

return addition;
}

string trim(string a)   // for removing leading 0s
{
   string res="";
   int i=0;

   while(a[i]=='0')
      i++;

   for(;i<a.size();i++)
    res+=a[i];

   return res;
}


int main()
{
    string a;
    string b;

    cin>>a>>b;

    cout<<trim(add(a,b))<<endl;
}