Why does using keep_if in Ruby skip over the first element in an array? -

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def unique(arr)   return arr.keep_if { |x| arr.count(x) == 1 } end  print unique([2, 5, 5, 4, 22, 8, 2, 8]) #=> [4, 22, 2]

the value 2 appears twice in array, using following method incorrectly returns it. why happen, , can fix it?

unfortunately, due hidden behavior in how keep_if works. illustrate behavior, can make use of lowest-hanging fruit in our debugging orchard, ol' puts:

def unique(arr)   return arr.keep_if { |x|      puts x, arr.join(',')     arr.count(x) == 1   } end  print unique([2, 5, 5, 4, 22, 8, 2, 8])

this gives following output:

2 2,5,5,4,22,8,2,8 5 2,5,5,4,22,8,2,8 5 2,5,5,4,22,8,2,8 4 2,5,5,4,22,8,2,8 22 4,5,5,4,22,8,2,8 8 4,22,5,4,22,8,2,8 2 4,22,5,4,22,8,2,8 8 4,22,2,4,22,8,2,8 [4, 22, 2]

look @ happens whenever method discovers new value wants keep: stores value in 1 of indexes in array, overwriting what's there. next time finds value wants keep, places in next spot, , on.

this means first time keep_if looks @ 2, sees 2 of them , decides skip it. sees 4 wants keep, , overwrites first 2. thus, second time sees 2, decides keep it.


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