Adding strings of integers - c++

Question

I'm trying to write an algorithm for a larger project that will take two strings which are both large integers (only using 10 digit numbers for the sake of this demo) and add them together to produce a final string that accurately represents the sum of the two original strings. I realize there are potentially better ways to have gone about this from the beginning but I am supposed to specifically use strings of large integers as opposed to a long integer.
My thinking was to take the two original strings, reverse them so their ones position, tens position, and so on all line up properly for adding. Then one position at a time, convert the characters from the strings to single integers and add them together and then use that sum as the ones position or otherwise for the final string, which once completed will also be reversed back to the correct order of characters.
Where I'm running into trouble I think is in preparing for the event in which the two integers from the corresponding positions in their strings add to a sum greater than 9, and I would then have carry over some remainder to the next position. For example, if I had 7 and 5 in my ones positions that would add to 12, so I would keep the 2 and add 1 to the tens position once it looped back around for the tens position operation.
I'm not getting results that are in any way accurate and after spending a large amount of time stumbling over myself trying to rectify my algorithm, I am not sure what I need to do to fix this.
Hopefully my intended process is clear and someone will be able to point me in the right direction or correct some mistake I may have in my program.
Thanks in advance.
#include <iostream>
#include <cstdlib>
#include <string>
using namespace std;
int main()
{
string str1 = "1234567890", str2 = "2345678901"; //Two original strings of large integers
string rev_str1, rev_str2;
int int1 = 0, int2 = 0;
string final; //Final product string, sum of two original strings
int temp_int = 0, buffer_int, remainder = 0;
string temp_str = "", buffer_str;
char buffer[100] = {0};
cout << "str1 = " << str1 << endl;
cout << endl;
cout << "str2 = " << str2 << endl;
cout << endl;
rev_str1 = string(str1.rbegin(), str1.rend());
rev_str2 = string(str2.rbegin(), str2.rend());
for (int i = 0; i < 10; i++)
{
buffer_str = rev_str1.at(i);
int1 = atoi(buffer_str.c_str());
buffer_str = rev_str2.at(i);
int2 = atoi(buffer_str.c_str());
buffer_int += (int1 + int2 + remainder);
remainder = 0;
while (buffer_int > 9)
{
buffer_int -= 10;
remainder += 10;
}
temp_str = itoa(buffer_int, buffer, 10);
final += temp_str;
}
final = string(final.rbegin(), final.rend());
cout << "final = " << final << endl;
cout << endl;
}

Answer 1

Here's what I came up with. It is just for two summands; if you have more, you'll have to adapt things a bit, in particular with the carry, which can then be larger than 19, and the way the result string is allocated:
#include <iostream>
#include <string>
using namespace std;
int main()
{
// Two original strings of large integers
string str1 = "1234567890",
str2 = "2345678901234";
// Zero-padd str1 and str2 to the same length
size_t n = max(str1.size(), str2.size());
if (n > str1.size())
str1 = string(n-str1.size(), '0') + str1;
if (n > str2.size())
str2 = string(n-str2.size(), '0') + str2;
// Final product string, sum of two original strings.
// The sum of two integers has at most one digit more, for more inputs make
// below reverse_iterator a back_insert_iterator, then reverse the result
// and skip the removal of the padding.
string final(n+1, '0');
// The carry
char carry = 0;
// Iterators
string::const_reverse_iterator s1 = str1.rbegin(), e = str1.rend(),
s2 = str2.rbegin();
string::reverse_iterator f = final.rbegin();
// Conversion
for (; s1 != e; ++s1, ++s2, ++f)
{
// Bracketing to avoid overflow
char tmp = (*s1-'0')+(*s2-'0') + carry;
if (tmp > 9)
{
carry = 1;
tmp -= 10;
}
else
{
carry = 0;
}
*f = tmp + '0';
}
final[0] = carry + '0';
// Remove leading zeros from result
n = final.find_first_not_of("0");
if (n != string::npos)
{
final = final.substr(n);
}
cout << "str1 = " << str1 << endl
<< "str2 = " << str2 << endl
<< "final = " << final << endl;
}

Answer 2

Your problem is that you are carrying 10s instead of 1s. When you add 19 + 5, you get 4 in the units position and add an extra 1 in the 10s position. You wouldn't add an extra 10 in the 10s position.
You simply need to change this line: remainder += 10; to remainder += 1;.
Also, that while loop isn't necessary if you have more than two addends. As it is, when you are adding only two digits at a time, the largest addends you can have are 9 + 9, which carries only 1.

Answer 3

#include<iostream>
using namespace std;
main(){
int sum =0;
int a;
int reminder;
cout<<"Enter the Number :"<<endl;
cin>>a;
while(a>0){
reminder=a%10;
sum=r+sum;
a=a/10;`enter code here`
}
cout<<"Additon is :"<<sum<<endl;
}