Easiest way to truncate float to 2 decimal places?

问题

In Swift, is there a way to truncate a float to 2 decimals, such that you can perform further calculations with it? All of the threads I've seen deal with casting to a string, which I can't figure out how to then use mathematically.

I tried using an extension (found on this forum), figuring I could cast back to float after the truncation, but I end up where I started, with another, non-truncated float. I need my return value to be in quarter steps (i.e. 6.50, 6.75, 5.25, etc), and what I'm ending up with, are results like 6.990022....

There has to be a simple way to do this, but I'm hitting a wall. Thanks in advance...

Here's the issue:

func roundToNearestQuarter(#power : Float) -> String {

     var errorToLowerQuarterRaw : Float = power % 0.25  // 0.210000038146973

     var errorToLowerQuarterString = errorToLowerQuarterStepRaw.string(2)  // "0.21"

     var errorToLowerQuarter = NSString(string: errorToLowerQuaterStepString).floatValue  // 0.209999993443489

// more code

}

roundToNearestQuater(6.71)

回答1:

You cannot round a Float or Double to 2 decimal digits exactly. The reason is that these data types use a binary floating point representation, and cannot represent numbers like 0.1 or 0.01 exactly. See for example

Why Are Floating Point Numbers Inaccurate?
What Every Computer Scientist Should Know About Floating-Point Arithmetic

But you said:

I need my return value to be in quarter steps (i.e. 6.50, 6.75, 5.25, etc),

and that is exactly possible because 0.25 = 2^-2 can be represented exactly as a floating point number.

The round() function rounds a floating point number to the nearest integral value. To round to the nearest quarter, you just have to "scale" the calculation with the factor 4:

func roundToNearestQuarter(num : Float) -> Float {
    return round(num * 4.0)/4.0
}

roundToNearestQuarter(6.71) // 6.75
roundToNearestQuarter(6.6)  // 6.5

回答2:

If you need to work with true precision (for currency-related applications, for example), you will probably want to use NSDecimalNumber instead of floating point.

The above approach can be applied to NSDecimalNumbers as shown below. In this example, the "step" that you are rounding to can be anything you choose, just set "increment" accordingly.

let number: NSDecimalNumber = 100.52
let increment: NSDecimalNumber = 0.25

let handler = NSDecimalNumberHandler(roundingMode: NSRoundingMode.RoundBankers, scale: 0, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: false)     // Rounds to the nearest whole number
let result = number.decimalNumberByDividingBy(increment).decimalNumberByRoundingAccordingToBehavior(handler).decimalNumberByMultiplyingBy(increment)

For more on rounding with NSDecimalNumber see here: How to round an NSDecimalNumber in swift?

And yes, working with NSDecimalNumber is a terribly verbose way to do math, but it's not complicated. If you find yourself doing a project involving them frequently, I recommend you consider setting up Swift operator extensions so you can manipulate them in a more elegant way. Check out here for a nice example: https://gist.github.com/mattt/1ed12090d7c89f36fd28

来源：https://stackoverflow.com/questions/27827742/easiest-way-to-truncate-float-to-2-decimal-places

标签

swift

decimal

truncation