Arithmetic operations

arithmetic.cpp

#include <bits/stdc++.h>

char to_char(int i)
{
	switch (i)
	{
		case 0: return '0'; case 1: return '1'; case 2: return '2';
		case 3: return '3'; case 4: return '4'; case 5: return '5';
		case 6: return '6'; case 7: return '7'; case 8: return '8'; case 9: return '9';
	}
}

int to_digit(char c)
{
	switch (c)
	{
		case '0': return 0; case '1': return 1; case '2': return 2;
		case '3': return 3; case '4': return 4; case '5': return 5;
		case '6': return 6; case '7': return 7; case '8': return 8; case '9': return 9;
	}
}

using namespace std;

typedef long long int llint;

void standardify(string &str1, string &str2)	//pad strings with zeroes
{
	if (str1.size() == str2.size()) return;
    if (str1.size() < str2.size()) str1.insert(str1.begin(),str2.length() - str1.length(), '0');
    else str2.insert(str2.begin(), str1.length() - str2.length(), '0');
}

string add(string str1, string str2)
{
    standardify(str1, str2);
    string answer = "";
    int carry = 0;
    for (llint i = str1.size() - 1 ; i >= 0 ; i--)
    {
        int currentSum = to_digit(str1[i]) + to_digit(str2[i]) + carry;
        carry = (currentSum >> 1) / 5;
        answer.push_back(to_char(currentSum % 10));
    };
    if (carry) answer.push_back('1');
    reverse(answer.begin(), answer.end()); return answer;
}

string sub(string str1, string str2)
{
    standardify(str1, str2);
    
	bool neg = (str1 < str2) ? true : false; if (neg) swap(str1, str2);
	int carry = 0;
	string answer = "";
	for (llint i = str1.length() - 1; i >= 0 ; i--)
	{
		int current = to_digit(str1[i]) - to_digit(str2[i]) - carry; carry = 0;	//take the carry and clear the carry
		if (current < 0)
		{
			carry = 1;	//if borrowing needed, raise the carry
			current += 10;
		};
		answer.push_back(to_char(current % 10));
	}
	while (answer.back() == '0') answer.pop_back();
	reverse(begin(answer), end(answer)); return neg ? ("-" + answer) : answer;
}

string mul(string str1, string str2)
{
	string answer = "0";
	for (llint i = str2.size() - 1 ; i >= 0 ; i--)
	{
		string digit_mul = "0";
		for (llint n = str1.size() - 1 ; n >= 0 ; n--) digit_mul = add(digit_mul, to_string(to_digit(str1[n]) * to_digit(str2[i])) + string(str1.size() - 1 - n, '0'));
		answer = add(answer, digit_mul + string(str2.size() - 1 - i, '0'));
	};
	return answer;
}

string decrement(string str)
{
	if (str == "0") return "-1";
	int carry = 1;
	for (llint i = str.size() - 1 ; i >= 0 ; i--)
	{
		int replace = to_digit(str[i]) - carry;
		carry = 0;
		if (replace < 0) 
		{
			replace += 10; carry = 1;
		};
		str[i] = to_char(replace);
	};
	return str;
}

int main()
{
    string str1;//, str2;
    //string str2;
    //cout << "Enter first number: "; cin >> str1;
    cin >> str1;
    //cout << "Enter second number: "; cin >> str2;
    cout << decrement(str1);
}