Evaluating arithmetic expressions using recursive descent parser

[karanGrower]

#include <sstream>
#include <iostream>

using namespace std;

using namespace std;

//const char * expressionToParse = "6.5-2.5*10/5+2*5";
const char * expressionToParse = "-5+2";
istringstream parse(expressionToParse) ;

char peek()
{
return static_cast<char>(parse.peek()) ;
}

char get()
{
return static_cast<char>(parse.get()) ;
}

double expression();

double number()
{
double result ;
parse >> result ;
return result;
}

double factor()
{
if ((peek() >= '0' && peek() <= '9') || peek() == '.')
return number();
else if (peek() == '(')
{
get(); // '('
double result = expression();
get(); // ')'
return result;
}
else if (peek() == '-')
{
get();
return -expression();
}
return 0; // error
}

double term()
{
double result = factor();
while (peek() == '*' || peek() == '/')
if (get() == '*')
result *= factor();
else
result /= factor();
return result;
}

double expression()
{
double result = term();
while (peek() == '+' || peek() == '-')
if (get() == '+')
result += term();
else
result -= term();
return result;
}

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

double result = expression();
cout << result << endl;
return 0;
}


// This is working fine for all inputs but when we enter input as -3+2 answer is -5 can someone help

[coder777]

The problem is in factor():

else if (peek() == '-')
{
get();
return -expression();
}

You need to return a negative number not an expression.

[dhayden]

You need to return a negative number not an expression.

But that won't handle a case like -(22+4). I think the code needs to return a negative factor, not a negative number.

[karanGrower]

Hi @coder777 and dhayden.... if you could give the code snippet as in where and what to change . I need it as an urgent code assigment. Please help

[dhayden]

We have given enough information for you to figure it out. Sorry, but the goal of this website is to help you become a better programmer, not to do your work for you.

[coder777]

dhayden wrote:

But that won't handle a case like -(22+4).

I will when using factor() instead of expression().

[dhayden]

coder777 wrote:

It will when using factor() instead of expression().

But that isn't the advice you gave:

coder777 wrote:

You need to return a negative number not an expression

This says that OP should return -number(); instead of return -expression();

In response, I said:

the code needs to return a negative factor.

We seem to agree that return -factor(); is the right code.

[coder777]

dhayden wrote:

But that isn't the advice you gave:

I wasn't refering to the function number(). Actually I did not see that such a function exists. It was not intended as the solution.

In order to avoid such a confusion functions should have a verb like calc_number() or so.