Day 3: From string to math

Details

• Difficulty: Hard
• Programming language: Kotlin

Problem

Giving a string consisting of parentheses, single digits, positive and negative signs,
convert the string into a mathematical expression to obtain the answer.

Don't use eval or a similar built-in parser.

For example:

• Given -1+(2+3), you should return 4

Solution

Before looking at my solution, which is probably not the best, why don't you give it a try? 😁

I decide to simply use a simple switch case (when in Kotlin)

/**
 * Parse and calculate the string passed in param
 *
 * @param calculation
 * @return result of the calculation
 */
fun parseAndCalculation(calculation: String): Int {
    var result = 0

    var multiplier = 1
    var subProblem = -1

    for ((index, c) in calculation.withIndex()) {
        if(subProblem == -1 || c != ')')
            when(c) {
                '+' -> multiplier = 1
                '-' -> multiplier = -1
                '(' -> subProblem = index
                ')' -> {
                    result += multiplier * parseAndCalcul(calculation.substring(subProblem + 1, index))
                    subProblem = -1
                }
                else -> result += multiplier * c.toString().toInt()
            }
    }

    return result
}

Comments

[Nick Holmes]

Nice solution to the problem, but you can simplify. Addition is associative, e.g.

a+(b+c) = (a+b)+c = a+b+c

(Subtraction is trivially changed to addition 2 - 1 = 2 + -1. I see you have understood this).

As the parenthesis don't do anything, you can just ignore them. (I would set up a range a characters to ignore, and add whitespace to it as well)

I've got to go to a meeting now, but I'll post an F# version later - might be able to get it down to a single expression.

[Xavier Chretien]

Oh! Really good idea, thank you 😃 I will try this. I don't have really looked into the different types of class there is in Kotlin. Thank you !

[Nick Holmes]

Indeed. :-(