1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 | // Code derived from Stroustrup's PPP2 book
// § 6.9 Program structure
// -and beginning on p 215
#include <iostream>
#include <stdexcept>
#include <string>
using namespace std;
void error(const char* s) { throw runtime_error(s); }
//------------------------------------------------------------------------------
class Token {
public:
Token(char ch) : kind{ch} {}
Token(char ch, double val) : kind{ch}, value{val} {}
char kind = '0';
double value = 0.0;
};
//------------------------------------------------------------------------------
class Token_stream {
public:
Token_stream(); // make a Token_stream that reads from cin
Token get(); // get a Token
void putback(Token t); // put a Token back
private:
bool full; // is there a Token in the buffer?
Token buffer; // here is where we keep a Token put back using putback()
};
//------------------------------------------------------------------------------
Token_stream::Token_stream() : full{false}, buffer{0} // no Token in buffer
{
}
//------------------------------------------------------------------------------
// read a token from cin
Token Token_stream::get()
{
if (full) { // check if we already have a Token ready
full = false;
return buffer;
}
char ch;
cin >> ch;
// clang-format off
switch (ch) {
case ';':
case 'q':
case '(': case ')': case '+': case '-': case '*': case '/':
return Token{ch};
break;
case '.':
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9': {
cin.putback(ch); // put digit back into the input stream
double val;
cin >> val; // read a floating-point number
return Token{'8', val}; // let '8' represent "a number"
break;
}
default:
error("Bad token");
return Token{'K'}; // invalid, shouldn't reach here
}
// clang-format on
}
//------------------------------------------------------------------------------
// put token back into stream
void Token_stream::putback(Token t)
{
if (full)
error("putback() into a full buffer");
buffer = t; // copy t to buffer
full = true; // buffer is now full
}
//------------------------------------------------------------------------------
Token_stream ts;
double expression();
//------------------------------------------------------------------------------
// deal with numbers and parentheses
double primary()
{
Token t = ts.get();
switch (t.kind) {
case '(': {
double d = expression();
t = ts.get();
if (t.kind != ')')
error("')' expected");
return d;
break;
}
case '8': // we use '8' to represent a number
return t.value;
break;
default:
error("primary expected");
// shouldn't reach here
return 0.0;
}
}
//------------------------------------------------------------------------------
// deal with *, /
double term()
{
double left = primary();
Token t = ts.get();
while (true) {
switch (t.kind) {
case '*': left *= primary(); break;
case '/': {
double d = primary();
if (d == 0)
error("divide by zero");
left /= d;
break;
}
default: ts.putback(t); return left;
}
t = ts.get();
}
}
//------------------------------------------------------------------------------
// deal with + and -
double expression()
{
double left = term();
Token t = ts.get();
while (true) {
switch (t.kind) {
case '+': left += term(); break;
case '-': left -= term(); break;
default: ts.putback(t); return left;
}
t = ts.get();
}
}
//------------------------------------------------------------------------------
int main()
try {
double val = 0.0;
while (cin) {
Token t = ts.get();
if (t.kind == 'q') // 'q' for quit
break;
if (t.kind == ';') // ';' for "print now"
cout << "= " << val << '\n';
else
ts.putback(t);
val = expression();
}
} catch (exception& e) {
cerr << e.what() << '\n';
return 1;
} catch (...) {
cerr << "exception \n";
return 2;
}
|
build & run:
g++ -std=c++20 -O2 -Wall -pedantic ./ch_06/main_p215.cpp && ./a.out