It is not casting, it is creating a string from a value with a format.
let a: Double = 1.5
let b: String = String(format: "%f", a)
print("b: (b)") // b: 1.500000
With a different format:
let c: String = String(format: "%.1f", a)
print("c: (c)") // c: 1.5
You can also omit the format
property if no formatting is needed.
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