Swift相当于Python切片赋值

>>> li=[0,1,2,3,4,5]

并在列表的任何/所有列表上执行切片分配：

>>> li[2:]=[99]         # note then end index is not needed if you mean 'to the end'
>>> li
[0,99]

Swift有一个类似的切片赋值(这是在swift交互式shell中)：

1> var arr=[0,5]
arr: [Int] = 6 values {
  [0] = 0
  [1] = 1
  [2] = 2
  [3] = 3
  [4] = 4
  [5] = 5
}
  2> arr[2...arr.endIndex-1]=[99]
  3> arr
$R0: [Int] = 3 values {
  [0] = 0
  [1] = 1
  [2] = 99
}

到现在为止还挺好.但是,有几个问题.

首先,swift不适用于空列表或索引位于endIndex之后.如果切片索引在结束索引之后,则Python附加：

>>> li=[]             # empty
>>> li[2:]=[6,7,8]
>>> li
[6,8]
>>> li=[0,2]
>>> li[999:]=[999]
>>> li
[0,999]

swift中的等价物是一个错误：

4> var arr=[Int]()
arr: [Int] = 0 values
  5> arr[2...arr.endIndex-1]=[99]
fatal error: Can't form Range with end < start

这很容易测试和编码.

第二个问题是杀手：它的速度非常慢.考虑这个Python代码来执行浮点列表的精确求和：

def msum(iterable):
    "Full precision summation using multiple floats for intermediate values"
    # Rounded x+y stored in hi with the round-off stored in lo.  Together
    # hi+lo are exactly equal to x+y.  The inner loop applies hi/lo summation
    # to each partial so that the list of partial sums remains exact.
    # Depends on IEEE-754 arithmetic guarantees.  See proof of correctness at:
    # www-2.cs.cmu.edu/afs/cs/project/quake/public/papers/robust-arithmetic.ps

    partials = []               # sorted,non-overlapping partial sums
    for x in iterable:
        i = 0
        for y in partials:
            if abs(x) < abs(y):
                x,y = y,x
            hi = x + y
            lo = y - (hi - x)
            if lo:
                partials[i] = lo
                i += 1
            x = hi
        partials[i:] = [x]
    return sum(partials,0.0)

它的工作原理是保持hi / lo部分求和,使msum([.1] * 10)精确地产生1.0而不是0.9999999999999999. ms的等效msum是Python中math library的一部分.

我试图在swift中复制：

func msum(it:[Double])->Double {
    // Full precision summation using multiple floats for intermediate values 
    var partials=[Double]()
    for var x in it {
        var i=0
        for var y in partials{
            if abs(x) < abs(y){
                (x,y)=(y,x)
            }
            let hi=x+y
            let lo=y-(hi-x)
            if abs(lo)>0.0 {
                partials[i]=lo
                i+=1
            }
            x=hi
        }
        // slow part trying to replicate Python's slice assignment partials[i:]=[x]
        if partials.endIndex>i {
            partials[i...partials.endIndex-1]=[x]
        }
        else {
            partials.append(x)
        }    
    } 
    return partials.reduce(0.0,combine: +)
}

测试功能和速度：

import Foundation
var arr=[Double]()
for _ in 1...1000000 {
    arr+=[10,1e100,10,-1e100]
    }

print(arr.reduce(0,combine: +))    // will be 0.0
var startTime: CFAbsoluteTime!
startTime = CFAbsoluteTimeGetCurrent()
print(msum(arr),arr.count*5)          // should be arr.count * 5
print(CFAbsoluteTimeGetCurrent() - startTime)

在我的机器上,需要7秒才能完成. Python native msum需要2.2秒(大约快4倍),库fsum函数需要0.09秒(几乎快90倍)

我试图用arr.removeRange(i ..< arr.endIndex)替换partials [i ... partials.endIndex-1] = [x]然后追加.快一点但不多. 题：
>这是惯用的swift：partials [i … partials.endIndex-1] = [x]
>有更快/更好的方式吗？