Solutions

List re-order

The correct order of commands is as follows:

In [17]:
values = []
values.append(1)
values.append(3)
values.append(5)
print('first time:', values)
values = values[1:]
print('second time:', values)

first time: [1, 3, 5]
second time: [3, 5]

How Large is a Slice?

The list values[low:high] has high - low elements. For example, values[1:4] has the 3 elements values[1], values[2], and values[3]. Note that the expression will only work if high is less than the total length of the list values.

Working With the End

  1. The program prints m.
  2. Python interprets a negative index as starting from the end (as opposed to starting from the beginning). The last element is -1.
  3. The last index that can safely be used with a list of N elements is element -N, which represents the first element.
  4. del values[-N] removes the first (zeroth) element from the list.
  5. values[:-1]

Slice Bounds

First we assign the string lithium to the variable element, which displays nothing.

In [18]:
element = 'lithium'

Next, we ask to print the slice element[-1:3]. But the index -1 for element corresponds to index 6, because the length of element is 7, and 7-1 is famously 6. So we are effectively asking for element[6:3], which is an empty slice because the "low" value of 6 is higher than the "high" value of 3.

In [19]:
print(element[-1:3])



















Lastly we ask to print element[0:20].
Knowing that elements only has length 7, so the final index is 6, you might expect an error to occur here (as there is no item at indices 7 through 19 = 20-1).
However we are asking for a slice of the list element, and when a slice includes an index that is not present, we get an empty list back.
As such, the slice element[0:20] is identical to element[0:7] or even just element[0:] - the "extra indices" we've asked for above 6 simply give us back an empty list, so the output is identical to when we just ask for the whole list.












In [20]:



    


print(element[0:20])
# for clarity, here's element[0:7]:
print('element[0:7]: ', element[0:7])
# and here's what happens when we start our slice beyond the range of indices in our list
print('elemnt[7:20]: ', element[7:20])


    











    




lithium
element[0:7]:  lithium
elemnt[7:20]: