只需将round替换为floor！

关于Swift 3.0：

myDouble.rounded(.down)

SWIFT4+

不同情况下的舍入方法

var w = 6.5
w.round(.toNearestOrAwayFromZero)
// w == 7.0

// Equivalent to the C 'trunc' function:
var x = 6.5
x.round(.towardZero)
// x == 6.0

// Equivalent to the C 'ceil' function:
var y = 6.5
y.round(.up)
// y == 7.0

// Equivalent to the C 'floor' function:
var z = 6.5
z.round(.down)
// z == 6.0

关于Swift 5.0

var x = 6.5
x.round(.nextDown)
// x == 6.0