7.4字符串操作

用来熟悉stl里string的用法，当程序中有to_string和stoi时，使用g++编译时会出现如下编译错误

error: 'stoi' is not a member of 'std'

，相关问题见https://stackoverflow.com/questions/38034197/compile-error-stoi-is-not-a-member-of-std

使用vs或更新版本的gnu(?)可以解决该问题

#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
string str[21];
int myfind();
string mycopy();
string add();
int n;
string mystring()//返回字符
{
	string o;
	cin >> o;
	if (o == "copy")
		return mycopy();
	else if (o == "add")
		return add();
	else
		return o;
}

int myfind()//返回数字，仅有可能是find和rfind的返回值
{
	string order;
	while (cin.peek() == ' ')
		cin.get();
	if (cin.peek() == 'f')
	{
		cin >> order;
		string x = mystring();
		int y = myfind();
		return str[y].find(x);
	}
	else if (cin.peek() == 'r')
	{
		cin >> order;
		string x = mystring();
		int y = myfind();
		return str[y].rfind(x);
	}
	else
	{
		int n;
		cin >> n;
		return n;
	}
}

string mycopy()
{
	int N = myfind(), X = myfind(), L = myfind();
	return str[N].substr(X, L);
}

bool isnum(const string &s)
{
	if (s.length() > 5)
		return false;
	else
	{
		for (int i = 0; i < s.length(); ++i)
		{
			if ('0' <= s[i] && s[i] <= '9');
			else
				return false;
		}
	}
	return true;
}

string add()
{
	string s1 = mystring(), s2 = mystring();
	if (isnum(s1) && isnum(s2))
		return to_string(stoi(s1, nullptr) + stoi(s2, nullptr));
	else
		return s1 + s2;
}

string& myinsert()
{
	string S = mystring();
	int N = myfind(), X = myfind();
	str[N].insert(X, S);
	return str[N];
}

void reset()
{
	string s = mystring();
	int n = myfind();
	str[n] = s;
	return;
}

void prints()
{
	int n;
	cin >> n;
	cout << str[n];
	return;
}

void printall()
{
	for (int i = 1; i <= n; ++i)
		cout << str[i] << endl;
	return;
}
int main()
{
	cin >> n;
	for (int i = 1; i <= n; ++i)
	{
		cin >> str[i];
	}
	string order;
	cin >> order;
	while (order != "over")
	{
		if (order == "insert")
			myinsert();
		else if (order == "add")
			add();
		else if (order == "print")
			prints();
		else if (order == "reset")
			reset();
		else if (order == "copy")
			mycopy();
	}
	return 0;
}