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COLLECTION操作の罠
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M F O B S ?
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Profile
Takuhiro Muta
iOS Enginner@iRidge inc.
Twitter/Qiita: @417_72ki
GitHub: 417-72KI
[ GitPitch @ github/417-72KI/Slides4LT/trap-of-collections ]
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本題
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Objective-CからSwift
JavaからKotlin(またはJava8)
になるに従って
コレクション操作が大幅に楽になりました
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昔
Objective-C
Java(~7)
NSArray *array = ...
NSArray *evenArray = [NSMutableArray new];
for (NSNumber *num in array) {
if(num.integerValue % 2 == 0) {
[evenArray addObject:num];
}
}
List list = ...
List evenList = new ArrayList<>();
for(int num: list) {
if(num % 2 == 0) {
evenList.add(num);
}
}
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今
Swift
Kotlin
let list = [1,2,3,4,...]
let evenList = list.filter { $0 % 2 == 0 }
val array = arrayOf(1,2,3,4,...)
val evenList = array.filter { it % 2 == 0 }.toTypedArray()
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ちょっと複雑な例
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let range = 1...10
let newList = range.filter { $0 % 2 == 0 }
.map { $0 / 2 }
.map { $0 * 5 }
.filter { $0 > 10 }
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map と filter の定義
public func map(_ transform: (Element) throws -> T) rethrows ->
public func filter(_ isIncluded: (Element) throws -> Bool) rethrow
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どちらも配列を返している...
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分かりやすいように画面出力してみましょう
func multiply(_ i: Int) -> (Int) -> Int {
return {
print($0, "*" , i, "=", $0 * i)
return $0 * i
}
}
func divide(_ i: Int) -> (Int) -> Int {
return {
print($0, "/" , i, "=", $0 / i)
return $0 / i
}
}
func isOdd() -> (Int) -> Bool {
return {
print($0, "odd?" , $0 % 2 == 0)
return $0 % 2 == 0
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実行結果
1 odd? false
2 odd? true
3 odd? false
4 odd? true
5 odd? false
6 odd? true
7 odd? false
8 odd? true
9 odd? false
10 odd? true
2 / 2 = 1
4 / 2 = 2
6 / 2 = 3
8 / 2 = 4
10 / 2 = 5
1 * 5 = 5
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イメージ
List l = new ArrayList<>();
for (int n: array) if (isOdd(n)) l.add(n);
List l2 = new ArrayList<>();
for (int n: l) l2.add(divide(n, 2));
List l3 = new ArrayList<>();
for (int n: l2) l3.add(multiply(n, 5));
List result = new ArrayList<>();
for (int n: l3) if (isMoreThan(n, 10)) result.add(n);
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ループ回しすぎぃ！
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これが要素数100万とかの配列になったら...
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((((；ﾟДﾟ))))
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そこで
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Swift → LazyCollection
Kotlin → Sequence
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どちらも遅延評価で実行してくれる
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さっきの例
これを
var list = 1...10
let newList = list.filter(isOdd())
.map(divide(2))
.map(multiply(5))
.filter(isMoreThan(10))
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こうじゃ
var list = 1...10
let newList = list.lazy
.filter(isOdd())
.map(divide(2))
.map(multiply(5))
.filter(isMoreThan(10))
.reduce(into: [Int] ()) { (array: inout [Int], i: Int) in arr
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実行結果
1 odd? false
2 odd? true
2 / 2 = 1
1 * 5 = 5
5 > 10 : false
3 odd? false
4 odd? true
4 / 2 = 2
2 * 5 = 10
10 > 10 : false
5 odd? false
6 odd? true
6 / 2 = 3
3 * 5 = 15
15 > 10 : true
7 odd? false
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イメージ
1ループで回す感じに
List result = new ArrayList<>();
for (int n: array) {
if (!isOdd(n)) continue;
int n2 = divide(n, 2);
int n3 = multiply(n2, 5)
if (isMoreThan(n3, 10)) result.add(n3);
}
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ちなみにKotlinの場合は
array.asSequence() をmapとかfilterとかの前に
呼ぶだけ
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どういう時に有効？
大きいデータから複数の条件に合致する数件だけを
抽出する
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Before
実行時間: 0.7900744676589966
let range = 1...1000
let result1 = range
.filter(isOdd())
.map(divide(2))
.map(multiply(5))
.filter(isMoreThan(10))
.prefix(5)
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After
実行時間: 0.00704646110534668
let range = 1...1000
let result2 = range.lazy
.filter(isOdd())
.map(divide(2))
.map(multiply(5))
.filter(isMoreThan(10))
.prefix(5)
.reduce(into: [Int] ()) { (array: inout [Int], i: Int) in arr
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まとめ
膨大なコレクションを操作する際は遅延評価の検討
を！
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