Converting String to Int with Swift

Question

The application basically calculates acceleration by inputting Initial and final velocity and time and then use a formula to calculate acceleration. However, since the value

Answer 1

for Alternative solution. You can use extension a native type. You can test with playground.

extension String {
    func add(a: Int) -> Int? {
        if let b = Int(self) {
            return b + a
        }
        else {
            return nil
        }
    }     
}

"2".add(1)

Answer 2

About int() and Swift 2.x: if you get a nil value after conversion check if you try to convert a string with a big number (for example: 1073741824), in this case try:

let bytesInternet : Int64 = Int64(bytesInternetString)!

Answer 3

for Swift3.x

extension String {
    func toInt(defaultValue: Int) -> Int {
        if let n = Int(self.trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)) {
            return n
        } else {
            return defaultValue
        }
    }
}

Answer 4

edit/update: Xcode 11.4 • Swift 5.2

Please check the comments through the code

---

IntegerField.swift file contents:

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
    }
}

---

You would need to add those extensions to your project as well:

---

Extensions UITextField.swift file contents:

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

---

Extensions Formatter.swift file contents:

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

---

Extensions NumberFormatter.swift file contents:

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

---

Extensions StringProtocol.swift file contents:

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

---

Sample project

Answer 5

Updated answer for Swift 2.0+:

toInt() method gives an error, as it was removed from String in Swift 2.x. Instead, the Int type now has an initializer that accepts a String:

let a: Int? = Int(firstTextField.text)
let b: Int? = Int(secondTextField.text)

Answer 6

Because a string might contain non-numerical characters you should use a guard to protect the operation. Example:

guard let labelInt:Int = Int(labelString) else {
    return
}

useLabelInt()