How to round a Double to the nearest Int in swift

Question

I m trying to make a calculator of growth rate  Double  that will round the result to the nearest Integer and recalculate from there  as such   let firstUsers   10 0 let growth   0 1 var users   firstUsers var week   0   while users  lt  14       println  week   week  has   users  users       users    users   growth     week    1     but I ve been unable so far   EDIT I kinda did it like so   var firstUsers   10 0 let growth   0 1 var users Int   Int firstUsers  var week   0   while users  lt   14       println  week   week  has   users  users       firstUsers    firstUsers   growth     users   Int firstUsers      week    1     Although I don t mind that it is always rounding down  I don t like it because firstUsers had to become a variable and change throughout the program  in order to make the next calculation   which I don t want it to happen

User · Answer

To round a double to the nearest integer, just use round().

var x = 3.7
x.round() // x = 4.0

If you don't want to modify the original value, then use rounded():

let x = 3.7
let y = x.rounded() // y = 4.0. x = 3.7

As one might expect (or might not), a number like 3.5 is rounded up and a number like -3.5 is rounded down. If you need different rounding behavior than that, you can use one of the rounding rules. For example:

var x = 3.7
x.round(.towardZero) // 3.0

If you need an actual Int then just cast it to one (but only if you are certain that the Double won't be greater than Int.max):

let myInt = Int(myDouble.rounded())

Notes

This answer is completely rewritten. My old answer dealt with the C math functions like round, lround, floor, and ceil. However, now that Swift has this functionality built in, I can no longer recommend using those functions. Thanks to @dfri for pointing this out to me. Check out @dfri's excellent answer here. I also did something similar for rounding a CGFloat.

User · Answer

You may also want to check whether the double is higher than the max Int value before trying to convert the value to an Int    let number   Double infinity if number  gt   Double integerLiteral  Int64 max      let rounded   Int max   else     let rounded   Int number rounded

User · Answer

In Swift    var a   14 123456789 var b   14 123456789 var c   14 123456789 var d   14 123456789 var e   14 123456789 var f   14 123456789  a rounded  up                         15 b rounded  down                       14 c rounded  awayFromZero               15 d rounded  towardZero                 14 e rounded  toNearestOrAwayFromZero    14 f rounded  toNearestOrEven            14

User · Answer

You can also extend FloatingPoint in Swift 3 as follow   extension FloatingPoint       func rounded to n  Int  - gt  Self           let n   Self n          return  self   n  rounded     n           324 0 rounded to  5       325

User · Answer

Swift 3  amp  4 - making use of the rounded     method as blueprinted in the FloatingPoint protocol  The FloatingPoint protocol  to which e g  Double and Float conforms  blueprints the rounded     method   func rounded   rule  FloatingPointRoundingRule  - gt  Self    Where FloatingPointRoundingRule is an enum enumerating a number of different rounding rules      case awayFromZero      Round to the closest allowed value whose magnitude is greater than or   equal to that of the source       case down      Round to the closest allowed value that is less than or equal to the   source       case toNearestOrAwayFromZero      Round to the closest allowed value  if two values are equally close    the one with greater magnitude is chosen       case toNearestOrEven      Round to the closest allowed value  if two values are equally close    the even one is chosen       case towardZero      Round to the closest allowed value whose magnitude is less than or   equal to that of the source       case up      Round to the closest allowed value that is greater than or equal to   the source    We make use of similar examples to the ones from  Suragch s excellent answer to show these different rounding options in practice    awayFromZero  Round to the closest allowed value whose magnitude is greater than or equal to that of the source  no direct equivalent among the C functions  as this uses  conditionally on sign of self  ceil or floor  for positive and negative values of self  respectively   3 000 rounded  awayFromZero     3 0 3 001 rounded  awayFromZero     4 0 3 999 rounded  awayFromZero     4 0   -3 000  rounded  awayFromZero     -3 0  -3 001  rounded  awayFromZero     -4 0  -3 999  rounded  awayFromZero     -4 0    down  Equivalent to the C floor function   3 000 rounded  down     3 0 3 001 rounded  down     3 0 3 999 rounded  down     3 0   -3 000  rounded  down     -3 0  -3 001  rounded  down     -4 0  -3 999  rounded  down     -4 0    toNearestOrAwayFromZero  Equivalent to the C round function   3 000 rounded  toNearestOrAwayFromZero     3 0 3 001 rounded  toNearestOrAwayFromZero     3 0 3 499 rounded  toNearestOrAwayFromZero     3 0 3 500 rounded  toNearestOrAwayFromZero     4 0 3 999 rounded  toNearestOrAwayFromZero     4 0   -3 000  rounded  toNearestOrAwayFromZero     -3 0  -3 001  rounded  toNearestOrAwayFromZero     -3 0  -3 499  rounded  toNearestOrAwayFromZero     -3 0  -3 500  rounded  toNearestOrAwayFromZero     -4 0  -3 999  rounded  toNearestOrAwayFromZero     -4 0   This rounding rule can also be accessed using the zero argument rounded   method   3 000 rounded      3 0          -3 000  rounded      -3 0           toNearestOrEven  Round to the closest allowed value  if two values are equally close  the even one is chosen  equivalent to the C rint   very similar to nearbyint  function   3 499 rounded  toNearestOrEven     3 0 3 500 rounded  toNearestOrEven     4 0  up to even  3 501 rounded  toNearestOrEven     4 0  4 499 rounded  toNearestOrEven     4 0 4 500 rounded  toNearestOrEven     4 0  down to even  4 501 rounded  toNearestOrEven     5 0  up to nearest     towardZero  Equivalent to the C trunc function   3 000 rounded  towardZero     3 0 3 001 rounded  towardZero     3 0 3 999 rounded  towardZero     3 0   -3 000  rounded  towardZero     3 0  -3 001  rounded  towardZero     3 0  -3 999  rounded  towardZero     3 0   If the purpose of the rounding is to prepare to work with an integer  e g  using Int by FloatPoint initialization after rounding   we might simply make use of the fact that when initializing an Int using a Double  or Float etc   the decimal part will be truncated away   Int 3 000     3 Int 3 001     3 Int 3 999     3  Int -3 000     -3 Int -3 001     -3 Int -3 999     -3    up  Equivalent to the C ceil function   3 000 rounded  up     3 0 3 001 rounded  up     4 0 3 999 rounded  up     4 0   -3 000  rounded  up     3 0  -3 001  rounded  up     3 0  -3 999  rounded  up     3 0     Addendum  visiting the source code for FloatingPoint to verify the C functions equivalence to the different FloatingPointRoundingRule rules  If we d like  we can take a look at the source code for FloatingPoint protocol to directly see the C function equivalents to the public FloatingPointRoundingRule rules   From swift stdlib public core FloatingPoint swift gyb we see that the default implementation of the rounded     method makes us of the mutating round     method    public func rounded   rule  FloatingPointRoundingRule  - gt  Self       var lhs   self     lhs round rule      return lhs      From swift stdlib public core FloatingPointTypes swift gyb we find the default implementation of round      in which the equivalence between the FloatingPointRoundingRule rules and the C rounding functions is apparent    public mutating func round   rule  FloatingPointRoundingRule        switch rule       case  toNearestOrAwayFromZero           value   Builtin int round FPIEEE  bits   value      case  toNearestOrEven           value   Builtin int rint FPIEEE  bits   value      case  towardZero           value   Builtin int trunc FPIEEE  bits   value      case  awayFromZero          if sign     minus                value   Builtin int floor FPIEEE  bits   value                    else                value   Builtin int ceil FPIEEE  bits   value                case  up           value   Builtin int ceil FPIEEE  bits   value      case  down           value   Builtin int floor FPIEEE  bits   value

User · Answer

A very easy solution worked for me     if  62   50    0          var number   62   50   1    adding 1 is doing the actual  round up        number contains value 2

User · Answer

Swift 3  If you want to round to a certain digit number e g  5 678434 -  5 68 you can just combine the round   or roundf   function with a multiplication   let value Float   5 678434 let roundedValue   roundf value   100    100 print roundedValue    5 68

User · Answer

Swift 3  var myNum   8 09 myNum rounded      result   8 and leaves myNum unmodified

User · Answer

There is a round available in the Foundation library  it s actually in Darwin  but Foundation imports Darwin and most of the time you ll want to use Foundation instead of using Darwin directly    import Foundation  users   round users    Running your code in a playground and then calling   print round users     Outputs      15 0   round   always rounds up when the decimal place is  gt    5 and down when it s  lt   5  standard rounding   You can use floor   to force rounding down  and ceil   to force rounding up    If you need to round to a specific place  then you multiply by pow 10 0  number of places   round  and then divide by pow 10  number of places    Round to 2 decimal places   let numberOfPlaces   2 0 let multiplier   pow 10 0  numberOfPlaces  let num   10 12345 let rounded   round num   multiplier    multiplier print rounded    Outputs      10 12   Note  Due to the way floating point math works  rounded may not always be perfectly accurate  It s best to think of it more of an approximation of rounding  If you re doing this for display purposes  it s better to use string formatting to format the number rather than using math to round it

[swift] How to round a Double to the nearest Int in swift?

Notes

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