痘痘反复长怎么治疗:用c++设计一个计算器程序

// The desk calulator

// includes character-level input (sec6.1.3) and
// command line input (sec6.1.7),
// but no namespaces and
// no exceptions

// pp 107-119, sec 6.1, A Desk Calculator

// uses += rather than push_back() for string
// to work around standard library bug

// uses istrstream from <strstream> rather than istringstream from <sstream>
// to work around standard library bug

// No guarantees offered. Constructive comments to bs@research.att.com

/*
program:
END // END is end-of-input
expr_list END

expr_list:
expression PRINT // PRINT is semicolon
expression PRINT expr_list

expression:
expression + term
expression - term
term

term:
term / primary
term * primary
primary

primary:
NUMBER
NAME
NAME = expression
- primary
( expression )
*/

#include <string>
#include <cctype>
#include<iostream>
#include<map>

//#include<sstream> // string streams
#include<strstream> // C-style string streams

using namespace std;

istream* input; // pointer to input stream

int no_of_errors; // note: default initialized to 0

double error(const char* s)
{
no_of_errors++;
cerr << "error: " << s << '\n';
return 1;
}

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

Token_value curr_tok = PRINT;
double number_value;
string string_value;

Token_value get_token()
{
char ch;

do { // skip whitespace except '\en'
if(!input->get(ch)) return curr_tok = END;
} while (ch!='\n' && isspace(ch));

switch (ch) {
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 '.':
input->putback(ch);
*input >> number_value;
return curr_tok=NUMBER;
default: // NAME, NAME=, or error
if (isalpha(ch)) {
string_value = ch;
while (input->get(ch) && isalnum(ch))
string_value += ch; // string_value.push_back(ch);
// to work around library bug
input->putback(ch);
return curr_tok=NAME;
}
error("bad token");
return curr_tok=PRINT;
}
}

map<string,double> table;

double expr(bool); // cannot do without

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(") expected");
get_token(); // eat ')'
return e;
}
default:
return error("primary expected");
}
}

double term(bool get) // multiply and divide
{
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 left;
}
}

double expr(bool get) // add and subtract
{
double left = term(get);

for (;;) // ``forever''
switch (curr_tok) {
case PLUS:
left += term(true);
break;
case MINUS:
left -= term(true);
break;
default:
return left;
}
}

int main(int argc, char* argv[])
{

switch (argc) {
case 1: // read from standard input
input = &cin;
break;
case 2: // read argument string
// input = new istringstream(argv[1]);
input = new istrstream(argv[1]);
break;
default:
error("too many arguments");
return 1;
}

table["pi"] = 3.1415926535897932385; // insert predefined names
table["e"] = 2.7182818284590452354;

while (*input) {
get_token();
if (curr_tok == END) break;
if (curr_tok == PRINT) continue;
cout << expr(false) << '\n';
}

if (input != &cin) delete input;
return no_of_errors;d
}