Question

Valid Expression = single digit

AND

Valid Expression = ( Valid Expression + Valid Expression )

Which means that the boolean function that I’m trying to create will only accept as valid, expressions of the following type:

5
(5+3)
(6+(3+2))
((7+1)+(5+1))
............... etc.

I want my function’s parameters to stay as they are (istringstream& is) and I want to use a get(ch) in my function.

I also want to do it recursively. However, I’m doing something wrong and it only validates expressions of type : single digit

Where’s the problem in my recursion? And I know it’s not even a proper recursion, I’m sure a proper recursion could do the job even in fewer lines and with no nested ifs..

Thank you for any helpful suggestions!

#include <iostream>
#include <string>
#include <sstream>
#include <cctype>

using namespace std;

bool isvalid(istringstream& is)
{
    char ch;
    is.get(ch);
    if(ch-'0'>=0 || ch-'0'<=9) return true;
    if(ch=='(' && isvalid(is))
    {   
        is.get(ch);
        if(ch=='+' && isvalid(is))
        {
            is.get(ch);
            if(ch==')') return true;
        }
     }
     return false; 
}
bool empty(istringstream& is)
{
    char ch;
    is.get(ch);
    return is.fail(); 
}

int main() 
{    
    string s;    
    while(getline(cin,s))    
    {   
        istringstream is(s);
        cout<<(isvalid(is) && empty(is) ? "Expression OK" : "Not OK")<<endl;    
    } 
 }

Answer 1

if(ch-'0'>=0 || ch-'0'<=9) return true;

This needs to be an && expression, doesn’t it?