I'm not sure what Bjarne meant with this exercise:
"Convert the desk calculator to use a symbol structure instead of using the static variables number_value and string_value."
Did he mean puting those 2 variables inside a structure and then using them through a structure?
Edit: also one exercise related to the clculator and it says: "Allow a user to define functions in the claculator. Hint: Define a function as a sequence of operations just as a user would ave typed them. Such a sequence can be stored either as a char string or as a list of tokens. Then read and execute those operations when the function is called. If you want a user-defined function to take args, you'll have to invent a notation for that."
Could anyone give ma an example of what it means? What sort of functions should user define, i dont get it. Ability to define for example function that returns a ^2 value from its argument for example?
Here's the code.
#include <iostream>
#include <map>
using namespace std;
double term(bool);
double expr(bool);
double prim(bool);
double error(const string&);
double number_value;
string string_value;
int no_of_errors;
map<string, double> table;
enum Token_value {
NAME, NUMBER, END,
PLUS = '+', MINUS = '-', MUL = '*', DIV = '/',
PRINT = ';', ASSIGN = '=', LP = '(', RP = ')'
};
Token_value curr_tok = PRINT;
Token_value get_token();
double expr(bool get)
{
double left = term(get);
for(;;)
switch(curr_tok) {
case PLUS:
left += term(true);
break;
case MINUS:
left -= term(true);
break;
default:
return left;
}
}
double term(bool get)
{
double left = prim(get);
for(;;)
开发者_开发知识库 switch(curr_tok) {
case MUL:
left *= prim(true);
break;
case DIV:
if(double d = prim(true)) {
left /= d;
break;
}
return error("divide by zero");
default:
return left;
}
}
double prim(bool get)
{
if(get)
get_token();
switch(curr_tok) {
case NUMBER: {
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:
return -prim(true);
case LP: {
double e = expr(true);
if(curr_tok != RP)
return error(") expected");
get_token();
return e;
}
default:
return error("primary expected");
}
}
Token_value get_token()
{
char ch = 0;
do {
if(!cin.get(ch))
return curr_tok = END;
} while(ch != '\n' && isspace(ch));
switch(ch) {
case 0:
return curr_tok = END;
case ';':
case '\n':
return curr_tok = 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 = NUMBER;
default:
if(isalpha(ch)) {
string_value = ch;
while(cin.get(ch) && isalnum(ch))
string_value.push_back(ch);
cin.putback(ch);
return curr_tok = NAME;
}
error("bad token");
return curr_tok = PRINT;
}
}
double error(const string &s)
{
no_of_errors++;
cerr << "error: " << s << '\n';
return 1;
}
int main()
{
table["pi"] = 3.14;
table["e"] = 2.71;
while(cin) {
get_token();
if(curr_tok == END)
break;
if(curr_tok == PRINT)
continue;
cout << expr(false) << endl;
}
return no_of_errors;
}
That's exactly what he means, in my opinion. Also the structure shouldn't be global either, it should be passed as a parameter (by value or reference appropriately).
精彩评论