drop_while and take_while go through the collection’s items in order and stop as soon as the condition in the block is no longer met, whereas reject and select don’t operate under that constraint.

Here’s an example showing the difference:

temperatures = [28, 25, 30, 22, 27]
=> [28, 25, 30, 22, 27]

# "reject" returns an array that is made by using the
# original collection and discarding all of its elements
# that meet the condition in the block
temperatures.reject {|temperature| temperature < 28}
=> [28, 30]

# "drop_while" is like "reject", but when working
# through the given collection in order, it stops discarding
# elements as soon as it encounters an element that
# *doesn't* meet the condition in the block
temperatures.drop_while {|temperature| temperature < 28}
=> [28, 25, 30, 22, 27]

In the drop_while example above, the first element, 28, causes the condition in the block to return false. drop_while stops checking the remaining elements and returns the result array. So in the case of the temperatures array above, drop_while’s result is equal to the original array.

drop_while will return the same results as reject if the array is sorted in such a way that all the elements that meet the condition in the block are moved to the beginning of the collection. In the case of the temperatures array, that’s easy:

temperatures.sort.drop_while {|temperature| temperature < 28}
=> [28, 30]

The practical upshot of all this is that you can think of drop_while (and its evil twin take_while) as “lazy evaluation” versions of reject and select where the order of the collection matters.

As for practical uses for drop_while? I’m sure there are some, but I’m feeling a little lazy today and will use the standard cop-out: “I’ll leave that as an exercise for the reader.”