Notes

using System.Collections.Generic;
using System;

public static class GlobalMembersCalculator
{

public static int no_of_errors;

public static double error(string s)
{
no_of_errors++;
cerr << "error: " << s << 'n';
return 1;
}

public static Token_value curr_tok = Token_value.PRINT;
public static double number_value;
public static string string_value;


public static Token_value get_token()
{
sbyte ch;

do
{
if (!cin.get(ch))
return curr_tok = Token_value.END;
} while (ch != 'n' && char.IsWhiteSpace(ch));

switch (ch)
{
case ';':
case 'n':
return curr_tok = Token_value.PRINT;
case '*':
case '/':
case '+':
case '-':
case '(':
case ')':
case '=':
return curr_tok = Token_value(ch);
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
case '.':
cin.putback(ch);
cin >> number_value;
return curr_tok = Token_value.NUMBER;
default:
if (char.IsLetter(ch))
{
string_value = ch;
while (cin.get(ch) && char.IsLetterOrDigit(ch))
string_value += ch;

cin.putback(ch);
return curr_tok = Token_value.NAME;
}
GlobalMembersCalculator.error("bad token");
return curr_tok = Token_value.PRINT;
}
}

public static Dictionary<string,double> table = new Dictionary<string,double>();

public static double expr(bool get)
{
double left = GlobalMembersCalculator.term(get);

for (;;)
switch (curr_tok)
{
case Token_value.PLUS:
left += GlobalMembersCalculator.term(true);
break;
case Token_value.MINUS:
left -= GlobalMembersCalculator.term(true);
break;
default:
return left;
}
}


public static double prim(bool get)
{
if (get)
GlobalMembersCalculator.get_token();

switch (curr_tok)
{
case Token_value.NUMBER:
{
double v = number_value;
GlobalMembersCalculator.get_token();
return v;
}
case Token_value.NAME:
{
double & v = table[string_value];
if (GlobalMembersCalculator.get_token() == Token_value.ASSIGN)
v = GlobalMembersCalculator.expr(true);
return v;
}
case Token_value.MINUS:
return -GlobalMembersCalculator.prim(true);
case Token_value.LP:
{
double e = GlobalMembersCalculator.expr(true);
if (curr_tok != Token_value.RP)
return GlobalMembersCalculator.error(") expected");
GlobalMembersCalculator.get_token();
return e;
}
default:
return GlobalMembersCalculator.error("primary expected");
}
}

public static double term(bool get)
{
double left = GlobalMembersCalculator.prim(get);

for (;;)
switch (curr_tok)
{
case Token_value.MUL:
left *= GlobalMembersCalculator.prim(true);
break;
case Token_value.DIV:
if (double d = GlobalMembersCalculator.prim(true))
{
left /= d;
break;
}
return GlobalMembersCalculator.error("divide by 0");
default:
return left;
}
}

static int Main()
{

table.Add("pi", 3.1415926535897932385);
table.Add("e", 2.7182818284590452354);

while (cin)
{
GlobalMembersCalculator.get_token();
if (curr_tok == Token_value.END)
break;
if (curr_tok == Token_value.PRINT)
continue;
Console.Write(GlobalMembersCalculator.expr(false));
Console.Write('n');
}

return no_of_errors;
}
}

public enum Token_value
{
NAME,
NUMBER,
END,
PLUS = '+',
MINUS = '-',
MUL = '*',
DIV = '/',
PRINT = ';',
ASSIGN = '=',
LP = '(',
RP = ')'
}