Stack Question (Balanced Expression problem set) - c++

Question

I have solved a problem set for a CPSC class where we have to implement a stack to check if an expression is balanced. For example, when a user enters {a}, the program would check if it has been properly closed and return true or false. My program is running accordingly, but only for char values such as. {, ( & and [. When I enter an input of (2(1+2)) it returns false. The algorithm makes sense and works, but I cannot seem to find the issue with my isBalanced function.
I could really use some help
#ifndef BALANCED_H
#define BALANCED_H
#include<string>
class Balanced {
public:
Balanced(std::string);
bool isBalanced();
bool isMatch(char c, char d);
private:
std::string expression;
};
#endif // BALANCED_H
#include "Balanced.h"
#include <string>
#include<iostream>
#include <stack>
Balanced::Balanced(std::string s) : expression(s)
{
}
bool Balanced::isBalanced()
{
std::stack<char> b;
for(unsigned int i=0; i < expression.size(); i++)
{
if(expression[i]=='{'|| expression[i] == '[' || expression[i] == '(')
{
b.push(expression[i]);
continue;
}
if(b.empty() || !isMatch(b.top(), expression[i]))
{
return false;
}
else{
b.pop();
}
}
return b.empty();
}
bool Balanced::isMatch(char c, char d)
{
if(c == '{' && d == '}')
{
return true;
}
else if(c == '[' && d == ']')
{
return true;
}
else if(c == '(' && d == ')')
{
return true;
}
else
{
return false;
}
}
int main()
{
std::string s;
std::string expression;
std::cout<<"Welcome to balance expression program" <<std::endl;
do{
std::cout<<"Enter any key to continue or type 'Help' to display a help menu ";
std::cout<<"You may also type 'Exit' to exit the program: ";
std::cin>>s;
if(s=="Help")
{
displayHelp();
continue;
}
else if(s=="Exit")
{
break;
}
else{
std::cout<<"Enter an expression: ";
std::cin>>expression;
}
Balanced d(expression);
if(d.isBalanced()!=true){
std::cout<<"The expressions is not balanced";
std::cout<<std::endl;
}
else{
std::cout<<"The expression is balanced";
std::cout<<std::endl;
}
}while(s!="Exit");
return 0;
}
void displayHelp()
{
std::cout<<std::endl;
std::cout<<"The purpose of this program is to check ";
std::cout<<"if an expression is balanced ";
std::cout<<"You will enter an expression and it will check to see if there is
closing ";
std::cout<<"brackets. If the expression is balanced, then it will return true if
not ";
std::cout<<"then the program will return false ";
std::cout<<"You can enter as many expression as you like. If you like to exit
the program, ";
std::cout<<"type 'Exit'"<<std::endl;
std::cout<<std::endl;
}

Answer 1

It would seem that your isMatch function will return false if the current character is not an open or close bracket, thus causing the whole thing to return false.