View difference between Paste ID: PJ9w3WR1 and QN0JXUjT

-#include <iostream> //((a+b)*((-d)+(-e)))
+#include <algorithm>
-#include <fstream>
+#include <cctype>
 #include <string>
 
-using namespace std;
+namespace parser
 {
   class parse_symbol
-int fun(string &str)
+  {
     constexpr bool is_letter(const char & C) const { return (C >= 'a') && (C <= 'z'); }
 
-    while(str.size())
+    constexpr bool is_number(const char & C) const { return (C >= '0') && (C <= '9'); }
 
-        cout<<str<<endl;
+    constexpr bool is_operator(const char & C) const  { return (C == '-') || (C == '+') || (C == '*'); }
 
-        if(str[0]=='(')
+  public:
     parse_symbol() = default;
-            str=str.substr(1,str.size()-1);
+
-            int k=fun(str);
+    constexpr int operator<<(const char & C) const
-            str=str.substr(k,str.size()-k);
+
-            if(str[0]==')')
+      if (is_letter(C))
-                continue;
+        return 1;
-            if(str[0]=='\0')
+      else if (is_operator(C))
-                return 0;
+        return 2;
-            if((str[0]=='-'||str[0]=='*'||str[0]=='+')&&((str[1]>=97&&str[1]<=122)||str[1]=='('))
+      else if (C == '(')
-            {
+        return 3;
-                str=str.substr(2,str.size()-2);
+      else if (C == ')')
-                continue;
+        return 4;
-            }
+      return 0;
     }
-            if(str[0]!='*'&&str[0]!='+'&&str[0]!='-')
+  }; // End of 'parse_symbol' class
-            {
+
   static parse_symbol Parser;
 
-                cout<<"Error1"<<endl;
+  bool parse_string(std::string & Str)
-                exit(0);
+  {
-            }
+    if (Str.size() <= 0)
       return false;
 
-        else
+    // remove spaces
     Str.erase(std::remove(Str.begin(), Str.end(), ' '), Str.end());
     // switch to lowercase
-            if(str[0]==')')
+    std::transform(Str.begin(), Str.end(), Str.begin(), [](unsigned char c) { return std::tolower(c); });
-            {
+
-                if(str[1]>=97&&str[1])
+    int state = 0; // 1 - letter, 2 - operation, 3 - end, 4 - ), 5 - btw
-                {
+
-                    cout<<"Error2"<<endl;
+    for (auto & a : Str)
-                    exit(0);
+
-                }
+      switch (Parser << a)
-/*
+      {
-                if(str[1]!=')'&&(str.size()-1))
+      case 3: // (
-                    str=str.substr(1,str.size()-1);
+        return ((Parser << Str.back()) == 4) ? parse_string(Str.substr(1, Str.size() - 2)) : false;
-                    */
+        break;
-                str=str.substr(1,str.size()-1);
+      case 1: // a
-                if((str[0]=='-'||str[0]=='*'||str[0]=='+')&&((str[1]>=97&&str[1]<=122)||str[1]=='('))
+        switch (state)
-                {
+
-                    str=str.substr(2,str.size()-2);
+        case 0: // no
-                    continue;
+        case 2: // after op
-                }
+          state++;
-                continue;
+          break;
-            }
+        default: // end or after a
-            if(str[0]>=97&&str[0]<=122&&str[2]>=97&&str[2]<=122&&(str[1]>='*'||str[1]<='+'||str[1]>='-'))
+          return false;
-                return 3;
+          break;
-            cout<<"Error3"<<endl;
+
-            cout<<str<<endl;
+        break;
-            exit(0);
+      case 2: // +
         switch (state)
         {
         case 1: // after a
           state = 2;
           break;
         default:
           return false;
           break;
-    return 0;
+
-}
+        break;
       case 4: // )
-int main()
+      case 0: // odd symbols
         return false;
-    string str, command;
+        break;
-    cout << R"(Would u like to get input from "input.txt"? Y/N)" << endl;
+      }
-    cin>>str;
+
-    if (str[0] == 'Y') {
+    return true;
-        cout << R"(Searching for it...)" << endl;
+  }
-        ifstream input ("input.txt", ifstream::in);
+