Swift String exprs

Abdullah · April 25, 2021, 2:50am

Hello, How can I calculate the exprs in the uploaded pic to get the output.
The idea is taking the operator to calculate the numbers before.

Chris_Parker · April 25, 2021, 3:46am

The expressions are what I refer to as RPN or Reverse Polish Notation.

In the first case it is 2 + 3 which is pretty straight forward so if you were using a swift playground you would code that as

let output = 2 + 3
print(output)

and you should see 5 printed in the console.

The second expression is pretty straight forward too.

Don’t forget the order in which math operations are done BEMDAS (Brackets, Exponential, Multiplication, Division, Addition, Subtraction)

Expresions 3 and 4 are tricky but if you are familiar with an RPN calculator you should be able to figure out how to write a swift expression to achieve the result.

If you can’t figure it out then let me know and I’ll provide you with a solution.

Abdullah · April 25, 2021, 4:06am

Thank you for your response. I tried to solve it, but my solution is not correct. The point is to write a function to deal with this kind of expressions. One function takes the expressions as a String input and return the solution as an Int.

Chris_Parker · April 25, 2021, 4:18am

Ah, now that puts a completely different light on the requirement compared to what you posted initially.

Abdullah · April 25, 2021, 4:38am

Yes! I’m sorry, it supposed to be explained more

Chris_Parker · April 25, 2021, 4:56am

This algorithm works nicely:

github.com

soapyigu/LeetCode-Swift/blob/master/Stack/EvaluateReversePolishNotation.swift

/**
 * Question Link: https://leetcode.com/problems/evaluate-reverse-polish-notation/
 * Primary idea: Push a number to a stack and pop two for operation when encounters a operator
 * Time Complexity: O(n), Space Complexity: O(n)
 */

class EvaluateReversePolishNotation {
    func evalRPN(_ tokens: [String]) -> Int {
        var stack = [Int]()
        
        for token in tokens {
            if let num = Int(token) {
                stack.append(num)
            } else {
                guard let postNum = stack.popLast(), let prevNum = stack.popLast() else {
                    fatalError("Invalid Input")
                }
                
                stack.append(operate(token, prevNum, postNum))
            }

This file has been truncated. show original

What you need to do is put the expression in an array. For example take the first one:

let expression1 = ["2", "3", "+"]

Paste the code from Github into your playground (or project).
Declare an instance of that class, ie:

let evaluate = EvaluateReversePolishNotation()

then call the function and assign the output to a variable like this:

let result = evaluate.evalRPN(expression1)

then print the result to the console:

print(result)

For the example above you should see 5 printed in the console.

Abdullah · April 25, 2021, 5:29am

Fantastic, Thank you.

Chris_Parker · April 25, 2021, 5:42am

If you wanted to you could change all the Int references to Double in that class and it will work with floating point values.