C++ parenthesis matching application

Question

I have to write a function that accepts a string and returns a bool. The string passed in is a series or different parenthesis opened or closed ex.({[]}) and returns whether the 'parens' are well-balanced. This has to be implemented by adding items to a stack. I am getting the following error:

parenMatching_demo.cpp:18:12: error: no match for 'operator==' in 'c == '('

The psudocode is:

matcher(expression) 
    for each character in expression 
        if  the character is an opener
            push on stack
        else if the character is a closr
            if stack is empty return false
            if the (opener at) the top of the stack does not match the closer
                return false
            pop the stack

     if the stack is not empty return false
     return true

This is what I have.

template <typename T>   
bool parenMatching(string c) {
    stack<string> s;
    for (int i = 0; i < s.size(); i++) {
        if (c == '(' || c == '[' || c == '{' || c == '<')
            s.push(c);
        else if (c == ')' || c == ']' || c == '}' || c == '>') {
            if (s.empty()) return false;
            if (s.top() != '(' || s.top() != '[' || s.top() != '{' || s.top() != '<')
                return false;
            s.pop();
        }
    }
}

The error is not crazy. Please tell us exactly what it says. — Joseph Mansfield, Apr 19 '13 at 21:32
i dont really know, it is some long error and when i the beginning part of the error gets cut off. — FJam, Apr 19 '13 at 21:33
What do you use the `typename T` for? It looks like `T` don't appear in your code. Why do you define a `template`? — Basile Starynkevitch, Apr 19 '13 at 21:34
Here i got the error it is. parenMatching_demo.cpp:18:12: error: no match for 'operator==' in 'c == '('' — FJam, Apr 19 '13 at 21:36
Your solution is a bit complex. It can be [made much simpler](http://coliru.stacked-crooked.com/view?id=2b8e21e9978d8b8dad8ac35f4c4fd59b-1f48a68887043b2645051583fb11641d). — Rapptz, Apr 19 '13 at 21:46
@Rapptz Your solution is wrong though. It only checks that the number of opening brackets equal the number of closing brackets, which is not the requirement. Look at the pseudo-code. — JBentley, Apr 19 '13 at 21:51
@Rapptz: For instance, parenMatch(")))(((") returns true, but should return false. — Bill, Apr 19 '13 at 21:58

Daniel Frey · Accepted Answer · 2013-04-19T21:38:57.187

9

One problem is the type of the stack, it should be

stack<char> s;

Then the for loop, the condition should be

i < c.size()

The next problem is

if (c == '(' || c == '[' || c == '{' || c == '<')

does not compare the character of the string c at i, so you need

const char c2 = c[i];
if (c2 == '(' || c2 == '[' || c2 == '{' || c2 == '<')

If you need help to fix the rest, let me/us know :)

edited Apr 19 '13 at 21:38

answered Apr 19 '13 at 21:33

Daniel Frey

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that did not really change anyting – FJam Apr 19 '13 at 21:38
@FJam The first problem I spotted or the current answer? – Daniel Frey Apr 19 '13 at 21:42
ok that got rid of the other error. but no it says, undefined reference to `parenMatching(std::basic_string, std::allocator >)' In the main I called the function by saying parenMathching("({[]})"); – FJam Apr 19 '13 at 21:44
1

@FJam Hm, actually I don't see any reason for the `template`. Does removing it help? – Daniel Frey Apr 19 '13 at 21:45

score 2 · Answer 2 · answered Apr 19 '13 at 21:40

You're comparing a string with a char in your codes that check for the parentheses. You're missing a line where you get the current character you're looking at as you iterate your string - you probably want something like:

char current = c.at(i);

Additionally, you need to fix your stack type as Daniel Frey says in his answer.

edit - he's since changed his to contain this info, so that answer should have everything you need.

C++ parenthesis matching application

2 Answers2