Creating a postfix calculator in C++? - c++

Question

I have an assignment where I need to create a postfix calculator. I'm running into some difficulties that previous questions regarding the subject on SO didn't cover adequately.
Here's an example of input I want to handle:
12 6 +
2 *
[Carriage return]
Result: 36
The description of the assignment:
The arithmetical expressions we normally use are infix expressions, meaning that the operator appears between its two operands, as in "4 + 5". In postfix expressions, the operator appears after its operands, as in "4 5 +". Here is a slightly more complex postfix expression: "25 12 7 - 2 * /". The equivalent infix expression is: "25 / ((12 - 7) * 2)". The result of that expression should be 2.5 (don't use integer division). Postfix expressions don't require parentheses.
Write a program that uses a stack to evaluate postfix expressions.
Each input expression should be entered on its own line, and the
program should terminate when the user enters a blank line. The only
symbols in an expression will be +, -, *, /, digits and spaces.
Hint: Read a postfix expression from left to right. When you read a
number, push it on the stack. When you read an operand, pop the top
two numbers off the stack, apply the operator to them, and push the
result on top of the stack. At the end, the result of the expression
should be the only number on the stack.
Here's my code so far:
#include <list> /* Linked Lists */
#include <stack> /* Stacks */
#include <iostream> /* cout cin */
int main() {
std::stack< double, std::list<double> > postfixStack;
std::string input;
std::cout << "Enter a postfix expression: ";
std::getline(std::cin, input);
while (input != "") {
std::cout << "Input expression: " << input << std::endl;
for (int i = 0; i<input.length()-1; i++) {
if (input.compare(i, 1, " ")) { // should ignore spaces, but doesn't
std::cout << "Skipping element " << i << " \n";
} else if (static_cast<int>(input[i]) == input[i]) { // push numbers onto the stack
postfixStack.push(static_cast<double>(input[i]));
std::cout << "Pushing " << input[i] << " onto the stack.\n";
} else if (input.compare(i, 1, "+")) { // pop two numbers off the stack (1), apply the operator to them (2), and push that onto the stack (3)
double operand1 = postfixStack.top();
postfixStack.pop();
double operand2 = postfixStack.top();
postfixStack.pop();
postfixStack.push(operand1 + operand2);
std::cout << "Adding " << operand1 << " and " << operand2 << std::endl;
}
}
std::getline(std::cin, input);
}
if (!postfixStack.empty()) {
std::cout << "Result of expression: " << postfixStack.top() << std::endl;
} else {
std::cout << "It appears that you did not enter an expression to evaluate.\n";
}
return 0;
}
Syntax highlighting and line numbers on Gist.
Where I'm struggling:
My while loop should allow more than one line of input per the spec. I was originally using cin instead of getline, but that presented the opposite problem: unlimited lines to enter anything whatsoever, even when checking for /r and the empty string.
I'm trying to skip over spaces in my loop, but instead my conditional appears to skip over anything that isn't a space.
Checking string subscripts means that I can only use numbers with one digit, which definitely isn't what I want. When I encounter a number: I think I could check the next element, if that's a number then I could concatenate them and bump up the loop counter. In theory, at least. I'd like to get basic functionality before I work on that.

Answer 1

My while loop should allow more than one line of input per the spec.
You've already found getline. You can simply use that to read whole lines. That approach you've taken looks fine.
I'm trying to skip over spaces in my loop, but instead my conditional appears to skip over anything that isn't a space.
That's right. You're checking the result of string::compare, but the result isn't a boolean, and non-zero doesn't mean the strings are equal, it means they aren't equal.
The way you've performed the comparison is uncommon, BTW, and more usual would be to simply compare the character to ' ', or to use isspace or maybe isblank.
Checking string subscripts means that I can only use numbers with one digit, which definitely isn't what I want.
Right. When you see a digit, you can enter a nested loop to read all subsequent digits. The base case, a one-digit string, is trivially converted to a number. (But just a tad less trivial than you think, see the note below.) And if you know how to determine the value of an N-digit string (let's say ABC), you can determine the value of an N+1-digit string (let's say ABCD) by multiplying by 10, and adding the value of the next digit.
Note: static_cast<int>(input[i]) == input[i] is always true. This doesn't mean what you think it means. Use your debugger to inspect the value of static_cast<int>(input[i]) for some characters, and try to understand what this cast is doing. When you understand what this does, think about another check you could do instead, and how you can use that, after checking a character is a digit, to determine the numeric value of that digit.