计算器——C++实现2

//在C++1的基础上增加了括号功能

#include <iostream>
#include <cctype>
#include <cstdlib>
#include <cstring>
using namespace std;

void eatspaces(char* str);
double expr(char* str);
double term(char* str,int & index);
double number(char* str,int & index);
char* extract(char* str,int & index);
const int MAX=80;

int main(void)
{
char buffer[MAX]={0};
cout<<endl
<<"Welcome to your friendly calculator"
<<endl
<<"enter an expression,or an empty line to quit"
<<endl;

for(;;)
{
cin.getline(buffer,sizeof buffer);
eatspaces(buffer);

if(!buffer[0])
return 0;

cout<<"\t="<<expr(buffer)<<endl;

}
}

void eatspaces(char* str)
{
int i=0;
int j=0;
while((*(str+i)=*(str+j++))!='\0')
if(*(str+i)!=' ')
i++;
return ;
}

double expr(char* str)
{
double value=0.0;
int index=0;

value=term(str,index);

for(;;)
{
switch(*(str+index++))
{
case '\0':
return value;
case '+':
value=value+term(str,index);
break;
case '-':
value=value-term(str,index);
break;
default:
cout<<endl
<<"Baby,There is an error"
<<endl;
exit(1);
}
}
}

double term(char* str,int & index)
{
double value=0.0;
value=number(str,index);
while(true)
{
if(*(str + index)=='*')
value*=number(str,++index);
else if(*(str + index)=='/')
value/=number(str,++index);
else
break;
}
return value;
}

double number(char* str,int & index)
{
double value=0.0;

if(*(str + index)=='(')
{
char* psubstr(nullptr);
psubstr=extract(str,++index);
value=expr(psubstr);
delete []psubstr;
return value;
}

if(!isdigit(*(str+index)))
{
cout<<endl
<<"Baby!!! There is an error"
<<endl;
exit(1);
}
while(isdigit(*(str + index)))
value=10*value + (*(str + index++)-'0');
if(*(str + index)!='.')
return value;

double factor=1.0;
while(isdigit(*(str + (++index))))
{
factor*=0.1;
value=value + (*(str + index)-'0')*factor;
}
return value;
}

char* extract(char* str,int& index)
{
char buffer[MAX];
char* pstr(nullptr);
int numL=0;
int bufindex=index;

do
{
buffer[index-bufindex]=*(str + index);
switch(buffer[index -bufindex])
{
case ')':
if(0==numL)
{
size_t size = index - bufindex;
buffer[index-bufindex]='\0';
++index;
pstr=new char[index-bufindex];
if(!pstr)
{
cout<<"Memory allocation failed,"
<<"program terminated.";
exit(1);
}
strcpy_s(pstr,index-bufindex,buffer);
return pstr;
}
else
numL--;
break;

case '(':
numL++;
break;
}
}while(*(str + index++) !='\0');

cout<<"Ran off the end of the expression,must be bad input"<<endl;
exit(1);
}