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// FILE: parens.cxx
// A small demonstration program for a stack.
#include <cstdlib> // Provides EXIT_SUCCESS
#include <iostream> // Provides cin, cout
#include <stack> // Provides stack
#include <string> // Provides string
using namespace std;
// PROTOTYPE for a function used by this demonstration program
bool is_balanced(const string& expression);
// Postcondition: A true return value indicates that the parentheses in the
// given expression are balanced. Otherwise, the return value is false.
int main( )
{
string user_input;
cout << "Type a string with some parentheses:\n";
getline(cin, user_input);
if (is_balanced(user_input))
cout << "Those parentheses are balanced.\n";
else
cout << "Those parentheses are not balanced.\n";
cout << "That ends this balancing act.\n";
return EXIT_SUCCESS;
}
bool is_balanced(const string& expression)
// Library facilities used: stack, string
{
// Meaningful names for constants
const char LEFT_PARENTHESIS = '(';
const char RIGHT_PARENTHESIS = ')';
stack<char> store; // Stack to store the left parentheses as they occur
string::size_type i; // An index into the string
char next; // The next character from the string
bool failed = false; // Becomes true if a needed parenthesis is not found
for (i = 0; !failed && (i < expression.length( )); ++i)
{
next = expression[i];
if (next == LEFT_PARENTHESIS)
store.push(next);
else if ((next == RIGHT_PARENTHESIS) && (!store.empty( )))
store.pop( ); // Pops the corresponding left parenthesis.
else if ((next == RIGHT_PARENTHESIS) && (store.empty( )))
failed = true;
}
return (store.empty( ) && !failed);
}
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