Round Half Down in Swift - swift

Question

Is there a rounding mode in Swift that behaves same as ROUND_HALF_DOWN in Java?
Rounding mode to round towards "nearest neighbor" unless both neighbors are equidistant, in which case round down. Behaves as for RoundingMode.UP if the discarded fraction is > 0.5; otherwise, behaves as for RoundingMode.DOWN.
Example:
2.5 rounds down to 2.0
2.6 rounds up to 3.0
2.4 rounds down to 2.0
For a negative number:
-2.5 rounds up to -2.0
-2.6 rounds down to -3.0
-2.4 rounds up to -2.0

Answer 1

There is – as far as I can tell – no FloatingPointRoundingRule with the same behavior as Java's ROUND_HALF_DOWN, but you can get the result with a combination of rounded() and nextDown or nextUp:
func roundHalfDown(_ x: Double) -> Double {
if x >= 0 {
return x.nextDown.rounded()
} else {
return x.nextUp.rounded()
}
}
Examples:
print(roundHalfDown(2.4)) // 2.0
print(roundHalfDown(2.5)) // 2.0
print(roundHalfDown(2.6)) // 3.0
print(roundHalfDown(-2.4)) // -2.0
print(roundHalfDown(-2.5)) // -2.0
print(roundHalfDown(-2.6)) // -3.0
Or as a generic extension method, so that it can be used with all floating point types (Float, Double, CGFloat):
extension FloatingPoint {
func roundedHalfDown() -> Self {
return self >= 0 ? nextDown.rounded() : nextUp.rounded()
}
}
Examples:
print((2.4).roundedHalfDown()) // 2.0
print((2.5).roundedHalfDown()) // 2.0
print((2.6).roundedHalfDown()) // 3.0
print((-2.4).roundedHalfDown()) // -2.0
print((-2.5).roundedHalfDown()) // -2.0
print((-2.6).roundedHalfDown()) // -3.0

Answer 2

Swift implements .round() function with rules, According to Apple
FloatingPointRoundingRule
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.

Answer 3

Yes, You can do the similar things using NSNumberFormatter and RoundingMode
Read them here
NSNumberFormatter
RoundingMode

Answer 4

var a = 6.54
a.round(.toNearestOrAwayFromZero)
// a == 7.0
var b = 6.54
b.round(.towardZero)
// b == 6.0
var c = 6.54
c.round(.up)
// c == 7.0
var d = 6.54
d.round(.down)
// d == 6.0
You can do like this as well but need to take values after decimal as well.

Answer 5

As #MohmmadS said those are built in methods for rounding.
You can implement custom rounding like this:
func round(_ value: Double, toNearest: Double) -> Double {
return round(value / toNearest) * toNearest
}
func roundDown(_ value: Double, toNearest: Double) -> Double {
return floor(value / toNearest) * toNearest
}
func roundUp(_ value: Double, toNearest: Double) -> Double {
return ceil(value / toNearest) * toNearest
}
Example:
round(52.376, toNearest: 0.01) // 52.38
round(52.376, toNearest: 0.1) // 52.4
round(52.376, toNearest: 0.25) // 52.5
round(52.376, toNearest: 0.5) // 52.5
round(52.376, toNearest: 1) // 52