xcode: need to convert strings to double and back to string - xcode

Question

this is my line of code.
budgetLabel.text = String((budgetLabel.text)!.toInt()! - (budgetItemTextBox.text)!.toInt()!)
the code works, but when I try to input a floating value into the textbox the program crashes. I am assuming the strings need to be converted to a float/double data type. I keep getting errors when i try to do that.

Answer 1

In Swift 2 there are new failable initializers that allow you to do this in more safe way, the Double("") returns an optional in cases like passing in "abc" string the failable initializer will return nil, so then you can use optional-binding to handle it like in the following way:
let s1 = "4.55"
let s2 = "3.15"
if let n1 = Double(s1), let n2 = Double(s2) {
let newString = String( n1 - n2)
print(newString)
}
else {
print("Some string is not a double value")
}
If you're using a version of Swift < 2, then old way was:
var n1 = ("9.99" as NSString).doubleValue // invalid returns 0, not an optional. (not recommended)
// invalid returns an optional value (recommended)
var pi = NSNumberFormatter().numberFromString("3.14")?.doubleValue

Answer 2

Fixed: Added Proper Handling for Optionals
let budgetLabel:UILabel = UILabel()
let budgetItemTextBox:UITextField = UITextField()
budgetLabel.text = ({
var value = ""
if let budgetString = budgetLabel.text, let budgetItemString = budgetItemTextBox.text
{
if let budgetValue = Float(budgetString), let budgetItemValue = Float(budgetItemString)
{
value = String(budgetValue - budgetItemValue)
}
}
return value
})()

Answer 3

You need to be using if let. In swift 2.0 it would look something like this:
if let
budgetString:String = budgetLabel.text,
budgetItemString:String = budgetItemTextBox.text,
budget:Double = Double(budgetString),
budgetItem:Double = Double(budgetItemString) {
budgetLabel.text = String(budget - budgetItem)
} else {
// If a number was not found, what should it do here?
}