NSExpresseion，从左结合判断操作

Question

func postFix(_ expr: String) -> Int {
    // write your code here
    let nums = expr.components(separatedBy:CharacterSet.decimalDigits.inverted)
        .joined()
    let set = CharacterSet(charactersIn: "+*/%-")
    var ch = expr.components(separatedBy: set.inverted).joined()
    ch.append(" ")
    var rslt = String()


    for i in 0..<nums.count {
        let index = str.index(str.startIndex, offsetBy: i)
        rslt.append(nums[index])
        rslt.append(ch[index])
    }
    let theSet = CharacterSet(charactersIn: "+*/%-0123456789")
    let final = rslt.components(separatedBy: theSet.inverted).joined()
    let expn = NSExpression(format: final)
    let chal = expn.expressionValue(with: nil, context: nil)

    return  chal as! Int
}
print(postFix("4 1 - 2 *"))

我正在制作一种方法来从未排序的字符串中解析数学运算我面临的唯一问题是我无法确定从 left-associative 操作。而不是 4 - (1 * 2) = 2 我希望它从左侧开始并忽略是否先以 * 或 / 开始所以 (4 - 1 ) * 2 = 6

Answer 1

我会稍微改变一下你的逻辑。首先获取所有数字和所有运算符，然后获取第一个元素并将其用作 lhs，迭代数字和运算符删除第一个数字。使用 lhs + 运算符 + 当前数字 (rhs) 构造表达式。将结果分配给 var，下一个 lhs 将是转换为字符串的结果。类似于：

func postFix(_ expr: String) -> Int {
    let numbers = expr.split(whereSeparator: {!"0123456789".contains([=10=])})
    let operators = expr.split(whereSeparator: {!"+*/%-".contains([=10=])} )
    guard var lhs = numbers.first else { return .zero }
    var result = 0
    for (rhs, oprtr) in zip(numbers.dropFirst(),operators)  {
        let format = String([lhs,rhs].joined(separator: " " + oprtr + " "))
        result = NSExpression(format: format)
            .expressionValue(with: nil, context: nil) as? Int ?? 0
        lhs = String(result)[...]
    }
    return  result
}

print(postFix("4 1 - 2 *"))  // "6\n"

NSExpresseion，从左结合判断操作

NSExpresseion, determine operating from left-associative

swift

string

math

postfix-operator

nsexpression