Question

I need to implement a program that evaluates an arithmetic expression in postfix notation. The program must receive a postfix expression as the first command line argument and will output the final value after evaluating the input expression. We can say that all input expressions are syntactically correct.

Example:
$ ./eval "-10 20 30 2 / * +"
290
This can be implemented using a stack of numbers, start with an initially empty stack. Follow these steps:
if number
push onto the stack
if operator
pop two numbers from the stack, evaluate the operator on the numbers and push the result back
if end of the string
pop the number from the stack and return it
Thank you guys in advance!

Answer 1

To evaluate an arithmetic expression in postfix notation, use a stack data structure. Follow the step-by-step process described above to push numbers and evaluate operators. The final value is the number left on the stack.

Step-by-step explanation:

In postfix notation, operators are placed after their operands. To evaluate an arithmetic expression in postfix notation, we can use a stack data structure. Here is a step-by-step process:

Let's understand this with an example:

If we have the postfix expression:
-10 20 30 2 / * +

We start with an empty stack. We encounter -10, which is a number, so we push it onto the stack.
Next, we encounter 20, 30, and 2, which are also numbers, so we push them onto the stack as well.
Now, we encounter the division operator (/). We pop 2 and 30 from the stack, evaluate 30 / 2 (= 15), and push the result, 15, back onto the stack.
Next, we encounter the multiplication operator (*). We pop 15 and 20 from the stack, evaluate 20 * 15 (= 300), and push the result, 300, back onto the stack.
Finally, we encounter the addition operator (+). We pop -10 and 300 from the stack, evaluate -10 + 300 (= 290), and push the result, 290, back onto the stack.
At the end, the only number left on the stack is 290, which is our final value.