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swiftswift4

Round a double down to one decimal place (dropping decimal places)

I want to round a double down to 1 decimal place. For example if I have a double let val = 3.1915 I want to round this down to 3.1. Normal rounding functions will round it to 3.2 but I want to basically just drop the remaining decimal places. What is the best way to do this? Is there a native function for this? I know this is pretty straight forward to do but I want to know what the best way to do this would be where I am not using any kind of workaround or bad practices. This is not a duplicate of other rounding questions because I am not asking about about rounding, I am asking how to drop decimal places.

Similarly, if the value was 3.1215, it would also round to 3.1

Solution

  • Use the function trunc() (which stands for truncate) which will chop away the decimal portion without rounding. Specifically, multiply the Double value by 10, truncate it, then divide by 10 again. Then, to display using 1 decimal place, use String(format:):

    let aDouble = 1.15
let truncated = trunc(aDouble * 10) / 10 
let string = String(format: "%.1f", truncated
print(string)

(displays "1.1")

    or, to process an entire array of sample values:

    let floats = stride(from: 1.099, to: 2.0, by: 0.1)

    let truncs = floats
  .map { trunc($0 * 10) / 10 }
  .map { String(format: "%.1f", $0) }

let beforeAndAfter = zip(floats, truncs)
    .map { (float: $0.0, truncString: $0.1)}

beforeAndAfter.forEach { print(String(format: "%.3f truncated to 1 place is %@", $0.0, $0.1)) }

    Outputs:

    1.099 truncated to 1 place is 1.0
1.199 truncated to 1 place is 1.1
1.299 truncated to 1 place is 1.2
1.399 truncated to 1 place is 1.3
1.499 truncated to 1 place is 1.4
1.599 truncated to 1 place is 1.5
1.699 truncated to 1 place is 1.6
1.799 truncated to 1 place is 1.7
1.899 truncated to 1 place is 1.8
1.999 truncated to 1 place is 1.9