Swift 4.2 round function

Question

I have func in my app which trims numbers, e.x. 9.81->9.8. But after updating swift behavior has changed.

let myNum = 9.7
print(round(myNum / 0.1) * 0.1)

Swift 4.1.2 output - 9.7 
Swift 4.2 output - 9.700000000000001

Please advise how to resolve this issue. May it is swift's bug?

if that is _really_ `4.7` that is a huge bug, indeed – it should be `9.7`. — holex, Sep 19 '18 at 09:34
This is another variation of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken). — Martin R, Sep 19 '18 at 09:35
You cannot round a `Double` if that "rounded" value cannot be represented as a floating-point number. You should either use `DecimalNumber` if you actually need to store decimals or use a `NumberFormatter` or simply a formatted `String` if you just need the "rounding" for displaying the number on a UI — Dávid Pásztor, Sep 19 '18 at 09:35
what is the issue, actually here? e.g. `9.81` will output `9.8` — holex, Sep 19 '18 at 09:52
What's your goal; do you want to round and then get a string with only one decimal digit? — meaning-matters, Sep 19 '18 at 10:04

Yury Imashev · Accepted Answer · 2018-09-19T10:13:59.257

3

If you need the correct value itself, and not just a String representation, you can try this.

let value = 9.71
let roundedValue = round(value * 10) / 10
print(roundedValue)

It prints 9.7 on Swift 4.2, so I guess that the rounded value is correct and you can use it for additional computations.

edited Sep 19 '18 at 10:13

answered Sep 19 '18 at 10:08

Yury Imashev

meaning-matters · Answer 2 · 2018-09-19T17:17:04.880

1

This seems to be what you want:

let trimmedString = String(format: "%@.1f", round(myNum / 0.1) * 0.1)

edited Sep 19 '18 at 17:17

answered Sep 19 '18 at 10:07

meaning-matters

2 Answers2