Interview Cake

Array Slicing

Array slicing involves taking a subset from an array and allocating a new array with those elements.

In Python 2.7 you can create a new list of the elements in my_list, from start_index to end_index (exclusive), like this:

my_list[start_index:end_index]

You can also get everything from start_index onwards by just omitting end_index:

my_list[start_index:]

Careful: there's a hidden time and space cost here! It's tempting to think of slicing as just "getting elements," but in reality you are:

  1. allocating a new list
  2. copying the elements from the original list to the new list

This takes time and space, where n is the number of elements in the resulting list.

That's a bit easier to see when you save the result of the slice to a variable:

tail_of_list = my_list[1:]

But a bit harder to see when you don't save the result of the slice to a variable:

return my_list[1:] # Whoops, I just spent O(n) time and space!
for item in my_list[1:]: # Whoops, I just spent O(n) time and space! pass

So keep an eye out. Slice wisely.

See also:

What's next?

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