4

我正在尝试从字符串中进行数学运算。

当我用 NSExpression 把一个字符串变成一个数学问题，然后用 expressionValue 得到结果时，Swift 假设我想要一个整数。考虑以下两个 Playground 示例：

let currentCalculation = "10 / 6"
let currentExpression = NSExpression(format: currentCalculation)
print(currentExpression) // 10 / 6
if let result = currentExpression.expressionValue(with: nil, context: nil) as? Double {
    print(result) // 1
}

let anotherCalculation = "10.0 / 6.0"
let anotherExpression = NSExpression(format: anotherCalculation)
print(anotherExpression) // 10 / 6
if let result = anotherExpression.expressionValue(with: nil, context: nil) as? Double {
    print(result) // 1.666666667
}

我应该怎么做才能让我总是得到一个 Double 结果？我不想提前解析字符串。

非常有趣的是，第二个示例将“anotherExpression”转换为整数，但结果仍然返回 Double。

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2 回答 2

4

使用 3rd 方表达式解析器/评估器可能会更好，例如DDMathParser。 NSExpression非常有限，并且没有强制浮点评估的选项。

如果您想（或必须）坚持NSExpression：这是一种可能的解决方案，可以（递归地）用浮点值替换表达式中的所有常量值：

extension NSExpression {

    func toFloatingPoint() -> NSExpression {
        switch expressionType {
        case .constantValue:
            if let value = constantValue as? NSNumber {
                return NSExpression(forConstantValue: NSNumber(value: value.doubleValue))
            }
        case .function:
           let newArgs = arguments.map { $0.map { $0.toFloatingPoint() } }
           return NSExpression(forFunction: operand, selectorName: function, arguments: newArgs)
        case .conditional:
           return NSExpression(forConditional: predicate, trueExpression: self.true.toFloatingPoint(), falseExpression: self.false.toFloatingPoint())
        case .unionSet:
            return NSExpression(forUnionSet: left.toFloatingPoint(), with: right.toFloatingPoint())
        case .intersectSet:
            return NSExpression(forIntersectSet: left.toFloatingPoint(), with: right.toFloatingPoint())
        case .minusSet:
            return NSExpression(forMinusSet: left.toFloatingPoint(), with: right.toFloatingPoint())
        case .subquery:
            if let subQuery = collection as? NSExpression {
                return NSExpression(forSubquery: subQuery.toFloatingPoint(), usingIteratorVariable: variable, predicate: predicate)
            }
        case .aggregate:
            if let subExpressions = collection as? [NSExpression] {
                return NSExpression(forAggregate: subExpressions.map { $0.toFloatingPoint() })
            }
        case .anyKey:
            fatalError("anyKey not yet implemented")
        case .block:
            fatalError("block not yet implemented")
        case .evaluatedObject, .variable, .keyPath:
            break // Nothing to do here
        }
        return self
    }
}

例子：

let expression = NSExpression(format: "10/6+3/4")
if let result = expression.toFloatingPoint().expressionValue(with: nil, context: nil) as? Double {
    print("result:", result) // 2.41666666666667
}

这适用于使用算术运算符和函数的“简单”表达式以及一些“高级”表达式类型（联合、交集等）。如有必要，可以添加剩余的转换。

于 2017-10-03T19:42:18.290 回答
2

这是 Martin R 的一个很好的答案的变体，它有两个重要的变化：

  • 它仅将参数转换为除法。任何其他函数仍然可以接收整数参数。
  • 它处理诸如count({1,2,3,4,5}) / count({1,2})除法参数不是常量值的表达式。

代码：

import Foundation

extension NSExpression {
    func toFloatingPointDivision() -> NSExpression {
        switch expressionType {
        case .function where function == "divide:by:":
            guard let args = arguments else { break }
            let newArgs = args.map({ arg -> NSExpression in
                if arg.expressionType == .constantValue {
                    if let value = arg.constantValue as? Double {
                        return NSExpression(forConstantValue: value)
                    } else {
                        return arg
                    }
                } else {
                    return NSExpression(block: { (object, arguments, context) in
                        // NB: The type of `+[NSExpression expressionForBlock:arguments]` is incorrect.
                        // It claims the arguments is an array of NSExpressions, but it's not, it's
                        // actually an array of the evaluated values. We can work around this by going
                        // through NSArray.
                        guard let arg = (arguments as NSArray).firstObject else { return NSNull() }
                        return (arg as? Double) ?? arg
                    }, arguments: [arg.toFloatingPointDivision()])
                }
            })
            return NSExpression(forFunction: operand, selectorName: function, arguments: newArgs)
        case .function:
            guard let args = arguments else { break }
            let newArgs = args.map({ $0.toFloatingPointDivision() })
            return NSExpression(forFunction: operand, selectorName: function, arguments: newArgs)
        case .conditional:
            return NSExpression(forConditional: predicate,
                                trueExpression: self.true.toFloatingPointDivision(),
                                falseExpression: self.false.toFloatingPointDivision())
        case .unionSet:
            return NSExpression(forUnionSet: left.toFloatingPointDivision(), with: right.toFloatingPointDivision())
        case .intersectSet:
            return NSExpression(forIntersectSet: left.toFloatingPointDivision(), with: right.toFloatingPointDivision())
        case .minusSet:
            return NSExpression(forMinusSet: left.toFloatingPointDivision(), with: right.toFloatingPointDivision())
        case .subquery:
            if let subQuery = collection as? NSExpression {
                return NSExpression(forSubquery: subQuery.toFloatingPointDivision(), usingIteratorVariable: variable, predicate: predicate)
            }
        case .aggregate:
            if let subExpressions = collection as? [NSExpression] {
                return NSExpression(forAggregate: subExpressions.map({ $0.toFloatingPointDivision() }))
            }
        case .block:
            guard let args = arguments else { break }
            let newArgs = args.map({ $0.toFloatingPointDivision() })
            return NSExpression(block: expressionBlock, arguments: newArgs)
        case .constantValue, .anyKey:
        break // Nothing to do here
        case .evaluatedObject, .variable, .keyPath:
            // FIXME: These should probably be wrapped in blocks like the one
            // used in the `.function` case.
            break
        }
        return self
    }
}
于 2017-10-03T22:26:57.927 回答