[英]How to mutate an array of integers in-place in swift through filtering
可以快速过滤这样的数组:
var numbers = Array(1...1000000)
numbers = numbers.filter( { return $0 % 2 == 0 } )
是否可以过滤并避免在过滤完成时发生的复制操作,例如改变原始数组。
与此伪代码类似: numbers.MutablefilterOperation({ return $0 % 2 == 0})
在 C++ 中,相当于上面 Swift 中发生的事情是:
std::vector<int> originalNumbers(1000000);
std::vector<int> newNumbers;
std::copy_if (originalNumbers.begin(), originalNumbers.end(), std::back_inserter(newNumbers), [](int i) { return i % 2 == 0 } );
出于性能原因,我想实现的目标:
std::vector<int> originalNumbers(1000000);
auto pos = std::remove_if(originalNumbers.begin(), originalNumbers.end(), [](int x) { return x % 2 == 0; });
originalNumbers.erase(pos, originalNumbers.end());
此实现应该进行过滤,而不必在进程中制作整个数组的临时副本(除非它的副本被另一个变量引用,请参阅“写入时复制”)
extension Array {
mutating func filterInPlace(isIncluded: (Element) throws -> Bool) rethrows {
var writeIndex = self.startIndex
for readIndex in self.indices {
let element = self[readIndex]
let include = try isIncluded(element)
if include {
if writeIndex != readIndex {
self[writeIndex] = element
}
writeIndex = self.index(after: writeIndex)
}
}
self.removeLast(self.distance(from: writeIndex, to: self.endIndex))
}
}
// example:
var arr = [6,2,6,5,2,5,6,2,2,1,6,7,3]
arr.filterInPlace { $0 % 2 == 1 }
print(arr) // [5, 5, 1, 7, 3]
任何Array
都是一个值类型的struct
。 它不能变异,只能复制或替换。
如果你想要一个引用类型,你将不得不使用一个class
来实现你自己的可变类型的数组 - 或者使用Foundation的NSMutableArray
。
如果您想简化代码以便将其称为变异函数,则可以使用extension
:
extension Array {
// the mutating function should be named "filter" but
// this is already taken by the non-mutating function
mutating func filtering(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element] {
try self = self.filter(isIncluded)
return self
}
}
var numbers = Array(1...10)
numbers.filtering( { return $0 % 2 == 0 } )
print(numbers) // "[2, 4, 6, 8, 10]\n"
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