迭代過程中的一些調試打印說明爲什麼你的特定結果的發生:
irb(main):005:0> x
=> ["r", "a", "c", "e", "c", "a", "r"]
irb(main):006:0> x.select! { |c| p x; x.count(c).odd? }
["r", "a", "c", "e", "c", "a", "r"]
["r", "a", "c", "e", "c", "a", "r"]
["r", "a", "c", "e", "c", "a", "r"]
["r", "a", "c", "e", "c", "a", "r"] # "e" is kept...
["e", "a", "c", "e", "c", "a", "r"] # ... and moved to the start of the array
["e", "a", "c", "e", "c", "a", "r"]
["e", "a", "c", "e", "c", "a", "r"] # now "r" is kept
=> ["e", "r"]
您可以通過最後的迭代看到,有是只有一個r
,那e
已經移到前面的陣列。推測該算法就地修改陣列,將匹配元素移到前面,覆蓋已經通過測試的元素。它跟蹤有多少元素被匹配和移動,然後將數組截斷爲多個元素。因此,使用select
。
匹配多個元素更長的例子使這個問題更清楚一點:
irb(main):001:0> nums = (1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
irb(main):002:0> nums.select! { |i| p nums; i.even? }
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 4, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 4, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 4, 6, 4, 5, 6, 7, 8, 9, 10]
[2, 4, 6, 4, 5, 6, 7, 8, 9, 10]
[2, 4, 6, 8, 5, 6, 7, 8, 9, 10]
[2, 4, 6, 8, 5, 6, 7, 8, 9, 10]
=> [2, 4, 6, 8, 10]
你可以看到,它確實動匹配的元素添加到數組的前面,覆蓋不匹配元素,然後截斷數組。
檢查被突變的同一對象上的東西('Array#select!'是一個增變器)總是會導致奇怪的行爲。 – avlazarov 2014-12-03 18:39:20