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C++程序設(shè)計語言6.1節(jié)計算器。 命令行計算器，可以進(jìn)行加減乘除運算，支持變量定義。 有單文件和多文件兩種

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代碼片段和文件信息 

#include?//I/O
#include?//strings
#include??//map
#include?//isalpha（）?etc.
#include

using?namespace?std;

istream?*?input;?//pointer?to?input?stream
int?no_of_errors;
double?number_value;
string?string_value;
enum?Token_value{
????NAME?NUMBER?END
????PLUS=‘+‘?MINUS=‘-‘?MUL=‘*‘?DIV=‘/‘
????PRINT=‘;‘?ASSIGN=‘=‘?LP=‘（‘?RP=‘）‘
};
Token_value?curr_tok?=?PRINT;
map?table;

double?expr（bool?get）;

double?error（const?string&?s）
{
????no_of_errors++;
????cerr?<????return?1;
}

Token_value?get_token（）
{
????char?ch?=?0;
????do?{?//?skip?whitespace?except?’\n’
????????if（!（*input）.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?‘.‘:
????????（*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.push_back（ch）;
????????????（*input）.putback（ch）;
????????????return?curr_tok=?NAME;
????????}
????error（“bad?token“）?;
????return?curr_tok=?PRINT;
????}
}

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;
?????

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