Slicing Lab

Goal

Get the basics of sequence slicing down.

Tasks

Write some functions that take a sequence as an argument, and return a copy of that sequence:

  • with the first and last items exchanged

  • with every other item removed

  • with the first 4 and the last 4 items removed, and then every other item in the remaining sequence

  • with the elements reversed (just with slicing)

  • with the last third, then first third, then the middle third in the new order

For example: (1,2,3,4,5,6) should return: (5,6,1,2,3,4)

Start with a length that’s a multiple of three, but make sure it doesn’t crash for other lengths.

NOTE: These should work with ANY sequence. You can even use strings to test, if you like.

Your functions should look like:

def exchange_first_last(seq):
    return a_new_sequence

Hint: your functions should work with ALL sequences. That means that you cannot use list methods, like .append, because that won’t work with strings and tuples. But all sequences support concatenation with the + operator.

Item or Sequence?

A key difference between using a single index – seq[i] – and using a slice – seq[i:j], seq[:i], seq[i:] – is that using an index returns a single item, whereas a slice always returns a sequence, even if that sequence is of length one or even empty. And concatenation requires a sequence. So, make sure you use slicing if you want to concatenate the results.

Note

There is a Python “gotcha” with strings. Python does not have a character type. Instead of a character, you get a string of length one. This can cause confusion sometimes, as other sequences return a single item when you index, so when you index into a list of numbers, you get number which is not a list or any type of sequence. But with strings, when you index into or loop through a string, you get a length-one string, which is itself a string, and therefor a valid sequence. So: a_string[i] + another_string works, but a_list[i] + another_list does not work.

Tests

a_string = "this is a string"
a_tuple = (2, 54, 13, 12, 5, 32)

assert exchange_first_last(a_string) == "ghis is a strint"
assert exchange_first_last(a_tuple) == (32, 54, 13, 12, 5, 2)

Write a test or two like that for each of the above functions.