Borrow in C++

[zain ahmad]

I am subtracting first number form second by using borrow it is giving right answer from 20th to 15th and wrong values from 15th to 0th
plzzz help me

# include <iostream>
# include <conio.h>
# include <string>
using namespace std;
int main()
{


	int start = 0;
	string sub1[3];
	string sub2[3];
	int bigint1[3];
	int bigint2[3];
	string s1 = "245469345927452978149";

	string s2 = "927987192697643467899";

	for (int i = 0; i < 3; i++)
	{
		sub1[i] = s1.substr(start, 9);
		start = start + 9;

	}
	
	start = 0;
	for (int i = 0; i < 3; i++)
	{
		sub2[i] = s2.substr(start, 9);
		start = start + 9;

	}
	
	int final1[20];
	int final2[20];
	
	for (int i = 0; i < 3; i++)
	{
		bigint1[i] = stoi(sub1[i]);
	}

	for (int i = 0; i < 3; i++)
	{
		bigint2[i] = stoi(sub2[i]);
	}
	
	int j = 0;

	int l = 20;

	for (int i = 2; i >= 0; i--)
	{

		while (bigint1[i] != 0)
		{
			final2[l] = bigint1[i] % 10;

			bigint1[i] = bigint1[i] / 10;
			l--;
			j++;
		}
	}


	int z = 20;
	for (int i = 2; i >= 0; i--)
	{

		while (bigint2[i] != 0)
		{
			final1[z] = bigint2[i] % 10;
			bigint2[i] = bigint2[i] / 10;
			z--;
		}
	}


	cout << "\n\n";
	for (int i = 0; i <= 20; i++)
	{
		cout << final1[i];
	}


	cout << "\n\n";
	for (int i = 0; i <= 20; i++)
	{
		cout << final2[i];
	}
	
	int final3[20];

	for (int i = 20; i >= 0; i--)    //this for loop is used for subtraction by using borrow but its not giving rigth value from 15th to 0th index
	{
		if (final1[i] >= final2[i])
		{
			final3[i] = final1[i] - final2[i];
		}
		else
		{
			for (int j = i - 1; j >= 0; j--)
			{
				
				if (final1[j] > final2[j])
				{
					final1[j] = final1[j] - 1;
					

					for (int z = j - 1; z <= i; z++)
					{
						if (z == i)
						{
							final1[i] = final1[i] + 10;
							final3[i] = final1[i] - final2[i];
						}
						else
						{
							final1[z] = final1[z] + 9;
						}
					}
					
					break;
				}
				
			}
		
		}
	}                                     
	cout << "\n\n";
	for (int i = 0; i <= 20; i++)
	{
		cout << final3[i];
	}                                     //it is giving write value from 20th to 15th 
	
	_getch();
	return 0;
}

[JLBorges]

#include <iostream>
#include <string>
#include <cctype>

// true if every character in num_str is a decimal digit
bool is_number( const std::string& num_str )
{
    for( char c : num_str ) if( !std::isdigit(c) ) return false ;
    return true ;
}

// prefix the shorter string with '0's so that a and b are of the same size
void make_same_size( std::string& a, std::string& b )
{
    while( a.size() < b.size() ) a = '0' + a ;
    while( b.size() < a.size() ) b = '0' + b ;
}

std::string minus( std::string a, std::string b )
{
    if( is_number(a) && is_number(b) ) // if both are numbers
    {
        make_same_size( a, b ) ; // make them to be of the same size

        if( a == b ) return "0" ;

        // note that it is guaranteed that '3' > '2' etc.
        if( b > a ) return "error: negative" ; // can't subtract, return error string

        // convert both strings to hold integer values of the digits
        for( char& c : a ) c -= '0' ; // '7' - '0' == 7 etc
        for( char& c : b ) c -= '0' ;

        // perform the subtraction a -= b
        int borrow = 0 ;
        for( int i = a.size() - 1 ; i >= 0 ; --i )
        {
            const int r = a[i] - b[i] - borrow ;

            if( r < 0 ) { /* negative r, there is a borrow */ a[i] = r+10 ; borrow = 1 ; }
            else { /* non-negative r, no borrow */ a[i] = r ; borrow = 0 ; }
        }

        // convert a from integer values back to characters
        for( char& c : a ) c += '0' ; // 5 + '0' == '5' etc.

        // return a after removing leading zeroes
        int pos = 0 ;
        while( !a.empty() && a[pos] == '0' ) ++pos ;
        return a.substr(pos) ;
    }

    return "not a number" ; // not number, return error string
}

int main()
{
    std::string s1 = "245469345927452978149";
	std::string s2 = "555237987192697643467899";

	std::cout << s2 << " -\n" << std::string( s2.size() - s1.size(), ' ' ) << s1
	          << "\n----------------------------\n" << minus(s2,s1) << '\n' ;
}

http://coliru.stacked-crooked.com/a/5172a3319bdfb226