Evaluating Postfix expressions in JavaScript

 

Question: Postfix expression evaluation

A string containing the postfix expression is given to you. Evaluate it and print the single integer giving the answer.

Input Description:
The first line of the input is a string N, containing operators and numbers separated by a single space which forms a postfix expression.

Output Description:
Evaluate the post expression and print the result.

Sample Input:
5 3 1 * + 9 -

Sample Output:
-1

Answer: 

Steps to evaluate a prefix expression:

1. Split the prefix expression based on space (" ") or blank string ("") as per the given input.
2. Store the split characters in an array.
3. Create an evaluation array with no elements (blank array).
4. Iterate through the elements of the characters array.
5. If the character is a number, simply push the character into the evaluation array.
6. if the character is an operator (+, -, *, /, %), then pop 2 elements from the evaluation array. Store the first popped number as the right Operand and the second as the left Operand. 
7. Calculate the result using the operands and current operator. [left operand (operator) right operand]. You may use switch-case or if-else statements to recognize the operator.
8. After calculating the result push the result into the evaluation array.
9. After iterating through all the elements of the characters array, the evaluation Array will have only one number, resulting from the evaluation of the prefix expression.
10. Pop the element and return it as the output.