// FILE: calc.cxx
// Basic calculator program that takes a fully parenthesized arithmetic
// expression as input, and evaluates the arithmetic expression.
// The purpose is to illustrate a fundamental use of stacks.
#include // Provides isdigit
#include // Provides EXIT_SUCCESS
#include // Provides strchr
#include // Provides cout, cin, peek, ignore
#include // Provides the stack template class
using namespace std;
// PROTOTYPES for functions used by this demonstration program:
double read_and_evaluate(istream& ins);
// Precondition: The next line of characters in the istream ins is a fully
// parenthesized arithmetic expression formed from non-negative numbers and
// the four operations + - * /.
// Postcondition: A line has been read from the istream ins, and this line has
// been evaluated as an arithmetic expression. The value of the expression has
// been returned by the function.
void evaluate_stack_tops(stack& numbers, stack& operations);
// Precondition: The top of the operations stack contains + - * or /, and the
// numbers stack contains at least two numbers.
// Postcondition: The top two numbers have been popped from the number stack,
// and the top operation has been popped from the operation stack. The two
// numbers have been combined using the operation (with the second number
// popped as the left operand). The result of the operation has then been
// pushed back onto the numbers stack.
int main( )
{
double answer;
cout << "Type a fully parenthesized arithmetic expression:" << endl;
answer = read_and_evaluate(cin);
cout << "That evaluates to " << answer << endl;
return EXIT_SUCCESS;
}
double read_and_evaluate(istream& ins)
// Library facilities used: cstring, iostream, stack
{
const char DECIMAL = '.';
const char RIGHT_PARENTHESIS = ')';
stack numbers;
stack operations;
double number;
char symbol;
// Loop continues while istream is not “bad” (tested by ins) and next character isn’t newline.
while (ins && ins.peek( ) != '\n')
{
if (isdigit(ins.peek( )) || (ins.peek( ) == DECIMAL))
{
ins >> number;
numbers.push(number);
}
else if (strchr("+-*/", ins.peek( )) != NULL)
{
ins >> symbol;
operations.push(symbol);
}
else if (ins.peek( ) == RIGHT_PARENTHESIS)
{
ins.ignore( );
evaluate_stack_tops(numbers, operations);
}
else
ins.ignore( );
}
return numbers.top( );
}
void evaluate_stack_tops(stack& numbers, stack& operations)
// Library facilities used: stack
{
double operand1, operand2;
operand2 = numbers.top( );
numbers.pop( );
operand1 = numbers.top( );
numbers.pop( );
switch (operations.top( ))
{
case '+': numbers.push(operand1 + operand2);
break;
case '-': numbers.push(operand1 - operand2);
break;
case '*': numbers.push(operand1 * operand2);
break;
case '/': numbers.push(operand1 / operand2);
break;
}
operations.pop( );
}