Lately, one user of my mood tracking app Emo requested that I add percentage values to my Statistics screen. Since displaying percentage values is a little harder than displaying numbers, I thought I would share a few ways of doing this in this article.
Let's say we have two Doubles and a total value equal to the sum of them.
let value1: Double = 1
let value2: Double = 2
let total = value1 + value2
Option 1
Now, if we want to check what percentage of the total is value1 and display this percentage in SwiftUI with 0 fraction digits, we could do something like this:
Text("\(String(format: "%.0f", (value1 / total * 100)))%")
Option 2
Since iOS 15, there is another way, though, and you may find it more readable:
Text((value1 / total).formatted(
.percent.precision(.fractionLength(0)))
)
Both options will result in the same text:
33% (for value1)
67% (for value2)
As you can probably guess, to change the precision to display 2 fraction digits in your percentages, you can modify both versions of the code as follows:
Text("\(String(format: "%.2f", (value1 / total * 100)))%")
Text((value1 / total).formatted(
.percent.precision(.fractionLength(2)))
)
When you display fraction digits, you will notice another difference between both methods. The first one will always display percentage values the same way, as it's a hard-coded format (for example, 33.33%).
The second one, using the formatted() method, takes the user's locale into consideration. So the percentage value may be displayed as 33.33% or 33,33%, depending on the locale. The same applies to other locale-specific rules, like grouping separators (e.g., "10,000" in one country and "10 000" in another).
This means that the second option is not only more readable but also a better choice if the app is localized, as with the formatted() method, all the values will be properly formatted based on the user's locale.
UPDATE: Option 3
After I published this article, Łukasz Rutkowski reminded me on Mastodon about another similar method (which also formats numbers properly based on locale):
Text((value1 / total),
format: .percent.precision(.fractionLength(0))
)
If you know another way to display percentages in SwiftUI, feel free to share it in the comments.
Thank you for reading!
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https://apps.apple.com/developer/next-planet/id1495155532
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Credits
Thank you to Arnaud Joubay and Avery Vine for your input on app localization, and to Łukasz Rutkowski for suggesting Option 3.
