Swift - Negative string to integer conversion not being parsed correctly

Question

This is a fairly simple question but has been causing me a multitude of issues. I have a integer which comes from my API endpoint in a string format, as this: "-43.47" (for example). I need to display the number differently (as they're percentages of change) in my SwiftUI view. The code I have been trying to use for this is below;
if((Int(self.percentageChange) ?? 0) >= 0) {
   Text("+ " + String(self.percentageChange + "%")).foregroundColor(Color.green)
} else {
   Text("- " + String((0 - Int(self.percentageChange)) + "%")).foregroundColor(Color.red)
}

The problem is, whether or not the string number is a negative integer, it gets parsed as a positive integer and appears as so in my actual view (without an actual photo, but take my word for it): "+ -43.47%", and I have no idea why. Obviously -43.47 is less than 0, so it should be caught by the exception, but for some strange reason, perhaps as a result of my application of parenthesis or otherwise, it isn't being caught as such.

Sulthan · Answer

Use NumberFormatter:
let numberFormatter = NumberFormatter()
numberFormatter.numberStyle = .percent
numberFormatter.multiplier = 1
numberFormatter.maximumFractionDigits = 2
numberFormatter.positivePrefix = "+"

---

let percentageChange = Double(self.percentageChange) ?? 0
let color = percentageChange < 0 ? Color.red : Color.green

Text(numberFormatter.string(for: percentageChange)).foregroundColor(color)

Note that this will make your number formatted correctly and properly localized.

vadian · Answer

Isn't it easier to check the string for the leading minus sign
if self.percentageChange.hasPrefix("-") {
   Text(self.percentageChange + "%").foregroundColor(Color.red)
} else {
   Text("+ " + self.percentageChange + "%").foregroundColor(Color.green)
}

Frankenstein · Answer

You just need to use the Float initializer to convert floating-point string to float and then compare.
Replace:
if((Int(self.percentageChange) ?? 0) >= 0) {

With:
if((Float(percentageChange) ?? 0) >= 0) {

The better alternative is to do this:
if !self.percentageChange.hasPrefix("-") {

Swift - Negative string to integer conversion not being parsed correctly

3 Answers

Add your own answers!

Ask a Question