Hello, I have a good thing here.

This is the C + + programming language Textbook of the desktop Calculator source code.
It took me a day to fix it. Can be calculated using, such as input:
a=2
B=4
2*b+34* (A+7)
Program can run the results
The meta program is as follows:


Desktop Calculator Application Example
#include <iostream>
#include <string>
#include <map>
#include <cctype>
using namespace Std;

Enum token_value{
NAME, number, end,
Plus= ' + ', minus= '-', mul= ' *, div= '/',
Print= '; ', assign= ' = ', lp= ' (', rp= ') '
};
Token_value Curr_tok=print;

Double expr (bool get);
Double term (bool get);
Double Prim (bool get);
Token_value Get_token ();
Double error (const string&s);
map<string,double> table;

Double expr (bool get)
{
Double Left=term (get); Add and Substrack
for (;;)
Switch (Curr_tok) {
Case PLUS:
Left+=term (TRUE);
Break
Case minus:
Left-=term (TRUE);
Break
Default
return to left;
}
}

Double term (bool get)
{
Double Left=prim (get);
for (;;)
Switch (Curr_tok) {
Case MUL:
Left*=prim (TRUE);
Break
Case DIV:
if (double D=prim (true)) {
Left/=d;
Break
}
return error ("divide by 0");
Default
return to left;
}
}
Double Number_value;
String string_value;
Double Prim (bool get)//handle primaries
{
if (get) get_token ();
Switch (Curr_tok) {
Case Number://floating-point constant
{Double v=number_value;
Get_token ();
return v;
}
Case NAME:
{double&v=table[string_value];
if (Get_token () ==assign) v=expr (true);
return v;
}
Case minus://unary minus
Return-prim (TRUE);
Case LP:
{Double e=expr (true);
if (CURR_TOK!=RP) return error (") epected");
Get_token (); Eat ') '
return e;
}
Default
Return error ("Primry expected");
}
}

Token_value Get_token ()
{
Char ch=0;
cin>>ch;
Switch (CH) {
Case 0:
return curr_tok=end;
Case '; ':
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=number;
Default://name,name=,or Error
if (Isalpha (CH)) {
Cin.putback (CH);
cin>>string_value;
return curr_tok=name;
}
Error ("Bad token");
return curr_tok=print;
}
}

int no_of_errors;
Double error (const string &s)
{
no_of_errors++;
cerr<< "error:" <<s<< ' n ';
return 1;
}

int main ()
{

table["PI"]=3.1415926535897932385;
table["E"]=2.7182818284590452354;
while (CIN)
{
Get_token ();
if (curr_tok==end) break;
if (curr_tok==print) continue;
Cout<<expr (false) << '/n ';
}

return no_of_errors;
}