Question

I wrote the following code to judge whether an expression entered by user has correct sequence of brackets or not, e.g. if user enters [a*(b+c)] its ok. But if he enters [a*(b+c)[ its not correct.

Stacklist.cpp is a file which contains the linked list implementation of stacks and definitions of functions of push and pop. Display is the function which just shows the top entry.

#include<iostream>
#include<exception>
using namespace std;
#include"stacklist.cpp"

int main()
{
string s;
cin>>s;//user inputs the string
stacklist<int> stack1;//the class in stacklist.cpp...int because all bracket's ascii values are ints
char c;

while((c=cin.get())!=EOF)
{

   switch('c')
   {
       case '(': case '{': case '[':
           stack1.push('c');
           break;

       case ')':
           {char s=stack1.display();
              try
              {
                if(s=='(')
                  { stack1.pop();
                   continue;}
                else
                  throw 5;
              }//try block    

              catch(5) //.......(a)
              {
              cout<<"unmatched bracket error";
              exit(-1);
              }//catch over    
           }//')' case 
           break;

        case '}': //.......(b)
           {char s=stack1.display();
              try
               {
               if(s=='{')
                  { stack1.pop();
                  continue;}
               else
                  throw 6;
               }//try block    

              catch(6) //......(a)
               {
                cout<<"unmatched bracket error";
                exit(-1);
               }//catch over    
           }//'}' case
           break;

         case ']': ........(c)
           {char s=stack1.display();
             try
              {
               if(s==']')
                  { stack1.pop();
                  continue;}
               else
                  throw 7;
              }//try block    

             catch(7) //.............(a)
              {
              cout<<"unmatched bracket error";
              exit(-1);
              }//catch over    
           }//']' case  
           break;//..........(d)

         default:
             break;     
    }   //switch 

} //while  

 if(stack1.display==0)//0 is displayed if stack is empty
    cout<<"string is correct"<<endl;
 else
    cout<<"unequal number of brackets"<<endl;

  system("pause"); //........(e)
  return 0; 
}    //main

Now the problem is that when I compiled the code there were various errors:

syntax error before numeric constant.........in all (a)
case label '}' not within switch statement........(b)
case label ']' not within switch statement........(c)
syntax error before break.................(d)
ISO forbids declaration of 'system' with no type...........(e)

Please tell me how to plug these errors?

Answer 1

• system is found in the header cstdio, but you did not #include that header.

  Anyway, it’s best not to use “tricks” like system("pause") to keep the console window after your program ends: if your console environment is not hanging around after your program has finished doing its meaningful work, then that’s your console environment’s fault/problem and you should configure it properly so that that does not happen. Pausing is not part of your program’s job.

• Catching looks like this:

  catch (Type object-name) { code }
  

  object-name is optional, but Type is not.

  Therefore catch (6) is ill-formed. The other errors are a result of this one: parsing of your program goes all wonky when you write stuff that is not valid C++!

And your indentation is pretty horrendous.