我以为我会发布这个，因为对于一个相对的菜鸟来说这是相对棘手的，我没有看到任何 关于这次演习的具体帖子。
另外，当我输入：'sqrt x）;'我收到正确的错误消息， 但是当我输入：'sqrtx）;'我收到错误：'get：undefined name sqrtx' 显然程序认为它是一个变量的声明， 有关更好的错误处理的任何建议???
这是第7章第7课的解决方案，为calc程序的用户创建sqrt（）：
/*
calculator08buggy.cpp
This program implements a basic expression calculator.
Input from cin; output to cout.
The grammar for input is :
Statement:
Expression
Print
Quit
Print:
;
Quit:
q
Expression:
Term
Expression + Term
Expression - Term
Term:
Primary
Term * Primary
Term / Primary
Term % Primary
Secondary:
primary
sqrt ( Expression )
Primary:
Number
( Expression )
-Primary
+Primary
Number:
floating point literal
Input comes from cin through the Token_stream called ts.
We have inserted 3 bugs that the compiler will catch and 3 that it won't.
*/
//------------------------------------------------------------------------------
#include "std_lib_facilities.h"
//------------------------------------------------------------------------------
struct Token {
char kind;
double value;
string name;
Token() {}//SQRT OVERHAULD CODE HERE!!!
Token(char ch) :kind(ch), value(0) { }
Token(char ch, double val) :kind(ch), value(val) { }
Token(char ch, string n) :kind(ch), name(n) { } //contructor for variable names
};
//------------------------------------------------------------------------------
class Token_stream {
bool full;
Token buffer;
public:
Token_stream() :full(0), buffer(0) { }
Token get();
void unget(Token t) { buffer=t; full=true; }
void ignore(char); //is called by clean_up_mess() and ignores erroneous statement until it reads a ';'
};
//------------------------------------------------------------------------------
const char let = 'L'; //this the t.kind for declartion
const string declkey = "let"; //this is string used to declar a variable
const char quit = 'Q';
const char print = ';';
const char number = '8'; //this the t.kind for a number
const char name = 'a'; //this the t.kind for a variable name
const char square_root = 'S'; //SQRT OVERHAULD CODE HERE!!!
//------------------------------------------------------------------------------
Token Token_stream::get()
{
if (full) { full=false; return buffer; }
char ch;
cin >> ch;
switch (ch) {
case '(':
case ')':
case '+':
case '-':
case '*':
case '/':
case '%':
case ';':
case '=':
return Token(ch);
case '.':
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
{ cin.unget();
double val;
cin >> val;
return Token(number,val);
}
default:
if (isalpha(ch)) {
string s;
s += ch;
while(cin.get(ch) && (isalpha(ch) || isdigit(ch))) s+=ch;
cin.unget();
if (s == "let") return Token(let);
if (s == "q") return Token(quit);
if (s == "sqrt") return Token(square_root); //SQRT OVERHAULD CODE HERE!!!
return Token(name,s);
}
error("Bad token");
}
}
//------------------------------------------------------------------------------
void Token_stream::ignore(char c)// argument is 'print' or ';'
{
if (full && c==buffer.kind) {
full = false;
return;
}
full = false;
char ch;
while (cin>>ch)
if (ch==c) return;
}
//------------------------------------------------------------------------------
struct Variable {
string name;
double value;
Variable(string n, double v) :name(n), value(v) { }
};
//------------------------------------------------------------------------------
vector<Variable> names;
//------------------------------------------------------------------------------
double get_value(string s) //returns variable value if 's' matches name already in vector 'names'
{
for (int i = 0; i<names.size(); ++i)
if (names[i].name == s) return names[i].value;
error("get: undefined name ",s);
}
//------------------------------------------------------------------------------
void set_value(string s, double d)//if 's' matches name in 'names' resets value to 'd'
{
for (int i = 0; i<=names.size(); ++i)
if (names[i].name == s) {
names[i].value = d;
return;
}
error("set: undefined name ",s);
}
//------------------------------------------------------------------------------
bool is_declared(string s)// if 's' is already in 'names' return true/else return false
{
for (int i = 0; i<names.size(); ++i)
if (names[i].name == s) return true;
return false;
}
//------------------------------------------------------------------------------
void define_name(string name, double val)
{
names.push_back(Variable(name,val));
}
//------------------------------------------------------------------------------
Token_stream ts; //declare ts here so following functions can use
//------------------------------------------------------------------------------
double expression();//declare here so following primary() & term () can use
//------------------------------------------------------------------------------
double primary()
{
Token t = ts.get();
switch (t.kind) {
case '(':
{ double d = expression();
t = ts.get();
if (t.kind != ')') error("')' expected");
return d;
}
case '-':
return - primary();
case number:
return t.value;
case name:
return get_value(t.name);
case square_root:
{ ts.unget(t);
return NULL; } //SQRT OVERHAULD CODE HERE!!!
default:
error("primary expected");
}
}
//------------------------------------------------------------------------------
// deal with 'sqrt()'
double secondary() //SQRT OVERHAULD CODE HERE!!!
{
double left = primary(); //is null if sqrt called
Token t = ts.get(); // get the next token from token stream
Token t2;
double d;
while(true) {
switch (t.kind) {
case square_root:
t2 = ts.get(); //make sure '(' is next token
if (t2.kind != '(')
error("'(' missing in declaration sqrt ");
d = expression();
left = sqrt(d);
cout << endl << "new output works: " << left << endl;
t = ts.get();
if (t.kind != ')') //make sure ')' is last token in call to sqrt
error("')' expected");
t = ts.get();
break;
default:
ts.unget(t); // put t back into the token stream
return left;
}
}
}
//------------------------------------------------------------------------------
double term()
{
double left = secondary();
while(true) {
Token t = ts.get();
switch(t.kind) {
case '*':
left *= primary();
break;
case '/':
{ double d = primary();
if (d == 0) error("divide by zero");
left /= d;
break;
}
default:
ts.unget(t);
return left;
}
}
}
//------------------------------------------------------------------------------
double expression()
{
double left = term();
while(true) {
Token t = ts.get();
switch(t.kind) {
case '+':
left += term();
break;
case '-':
left -= term();
break;
default:
ts.unget(t);
return left;
}
}
}
//------------------------------------------------------------------------------
double declaration()
{
Token t = ts.get();
if (t.kind != 'a') error ("name expected in declaration");
string name = t.name;
if (is_declared(name)) error(name, " declared twice");
Token t2 = ts.get();
if (t2.kind != '=') error("= missing in declaration of " ,name);
double d = expression();
names.push_back(Variable(name,d));
return d;
}
//------------------------------------------------------------------------------
double statement()
{
Token t = ts.get();
switch(t.kind) {
case let:
return declaration();
default:
ts.unget(t);
return expression();
}
}
//------------------------------------------------------------------------------
void clean_up_mess()
{
ts.ignore(print);
}
//------------------------------------------------------------------------------
const string prompt = "> ";
const string result = "= ";
//------------------------------------------------------------------------------
void calculate()
{
while(true) try {
cout << prompt;
Token t = ts.get();
while (t.kind == print) t=ts.get();
if (t.kind == quit) return;
ts.unget(t);
cout << result << statement() << endl;
}
catch(runtime_error& e) {
cerr << e.what() << endl;
clean_up_mess();
}
}
//------------------------------------------------------------------------------
int main()
{
define_name("k", 1000);
try {
calculate();
return 0;
}
catch (exception& e) {
cerr << "exception: " << e.what() << endl;
char c;
while (cin >>c&& c!=';') ;
return 1;
}
catch (...) {
cerr << "exception\n";
char c;
while (cin>>c && c!=';');
return 2;
}
}
//------------------------------------------------------------------------------
答案 0 :(得分：0)
'get: undefined name sqrtx方法
get_value
在Token::get方法中：
在循环while(cin.get(ch) && (isalpha(ch) || isdigit(ch))) s+=ch; ... sqrt x)
s等于"sqrt"，但对于"sqrtx"，s等于"sqrtx"，因此与sqrt不匹配并返回Token(name,s)而不是Token(square_root)