Easiest way to truncate float to 2 decimal places?

Question

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)

Answer 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