问题:字符串“4.000”不能转换成整数使用Int(“4.000”)?

答案:Int()检查字符串是否是整数，如果是，然后给你整数，否则为nil。但是Float或Double可以将任何数字字符串转换为各自的Float或Double而不给nil。例如，如果你有“45”整数字符串，但使用Float("45")会给你45.0浮点值，或使用Double("4567")会给你45.0。

解决方案:NSString(string: "45.000").integerValue或Int(Float("45.000")!)!才能得到正确的结果。

编辑/更新:Xcode 11.4•Swift 5.2

请检查代码中的注释

IntegerField.swift文件内容:

import UIKit

class IntegerField: UITextField {

    // returns the textfield contents, removes non digit characters and converts the result to an integer value
    var value: Int { string.digits.integer ?? 0 }

    var maxValue: Int = 999_999_999
    private var lastValue: Int = 0

    override func willMove(toSuperview newSuperview: UIView?) {
        // adds a target to the textfield to monitor when the text changes
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
        // sets the keyboard type to digits only
        keyboardType = .numberPad
        // set the text alignment to right
        textAlignment = .right
        // sends an editingChanged action to force the textfield to be updated
        sendActions(for: .editingChanged)
    }
    // deletes the last digit of the text field
    override func deleteBackward() {
        // note that the field text property default value is an empty string so force unwrap its value is safe
        // note also that collection remove at requires a non empty collection which is true as well in this case so no need to check if the collection is not empty.
        text!.remove(at: text!.index(before: text!.endIndex))
        // sends an editingChanged action to force the textfield to be updated
        sendActions(for: .editingChanged)
    }
    @objc func editingChanged() {
        guard value <= maxValue else {
            text = Formatter.decimal.string(for: lastValue)
            return
        }
        // This will format the textfield respecting the user device locale and settings
        text = Formatter.decimal.string(for: value)
        print("Value:", value)
        lastValue = value
    }
}

您还需要将这些扩展添加到您的项目中:

扩展UITextField.swift文件内容:

import UIKit
extension UITextField {
    var string: String { text ?? "" }
}

Formatter.swift文件内容:

import Foundation
extension Formatter {
    static let decimal = NumberFormatter(numberStyle: .decimal)
}

扩展NumberFormatter.swift文件内容:

import Foundation
extension NumberFormatter {
    convenience init(numberStyle: Style) {
        self.init()
        self.numberStyle = numberStyle
    }
}

StringProtocol.swift文件内容:

extension StringProtocol where Self: RangeReplaceableCollection {
    var digits: Self { filter(\.isWholeNumber) }
    var integer: Int? { Int(self) }
}

示例项目

基本想法，注意这只适用于Swift 1。(查看ParaSara的回答，看看它是如何在Swift 2.x中工作的):

    // toInt returns optional that's why we used a:Int?
    let a:Int? = firstText.text.toInt() // firstText is UITextField
    let b:Int? = secondText.text.toInt() // secondText is UITextField

    // check a and b before unwrapping using !
    if a && b {
        var ans = a! + b!
        answerLabel.text = "Answer is \(ans)" // answerLabel ie UILabel
    } else {
        answerLabel.text = "Input values are not numeric"
    }

Swift 4更新

...
let a:Int? = Int(firstText.text) // firstText is UITextField
let b:Int? = Int(secondText.text) // secondText is UITextField
...

你可以使用NSNumberFormatter(). numberfromstring (yourNumberString)。这很好，因为它返回一个可选的，然后你可以用if let测试，以确定转换是否成功。 如。

var myString = "\(10)"
if let myNumber = NSNumberFormatter().numberFromString(myString) {
    var myInt = myNumber.integerValue
    // do what you need to do with myInt
} else {
    // what ever error code you need to write
}

斯威夫特5

var myString = "\(10)"
if let myNumber = NumberFormatter().number(from: myString) {
    var myInt = myNumber.intValue
    // do what you need to do with myInt
  } else {
    // what ever error code you need to write
  }

在Swift 4中:

extension String {            
    var numberValue:NSNumber? {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        return formatter.number(from: self)
    }
}
let someFloat = "12".numberValue

在Swift 4.2和Xcode 10.1中

let string = "789"
if let intValue = Int(string) {
    print(intValue)
}

let integerValue = 789
let stringValue = String(integerValue)

OR

let stringValue = "\(integerValue)"
print(stringValue)