Carefull with processing array index, ruby. (reference : Eloquent ruby)

rb

#The easiest way to screw up one of these iterating methods is to change the collection out from underneath the method.
#Here, for example, is a seriously misguided attempt to remove all of the negative numbers from an array:

array = [ 0, -10, -9, 5, 9 ]
array.each_index {|i| array.delete_at(i) if array[i] < 0}
pp array

#The trouble with this code is that it will tend to leave some negative numbers behind: Removing the first one (the -10 ) from 
#the array messes up the internal indexing of the each method, so much that it will miss the second negative number, leaving 
#us with a result of:

[0, -9, 5, 9]