Repetition and References

Section 10.13 Repetition and References

We have already seen the repetition operator working on strings as well as lists. For example,

Listing 10.13.1.

With a list, the repetition operator creates copies of the references. Although this may seem simple enough, when we allow a list to refer to another list, a subtle problem can arise.

Consider the following extension on the previous example.

Listing 10.13.2.

newlist is a list of three references to origlist that were created by the repetition operator. The reference diagram is shown below.

Now, what happens if we modify a value in origlist .

Listing 10.13.3.

newlist shows the change in three places. This can easily be seen by noting that in the reference diagram, there is only one origlist , so any changes to it appear in all three references from newlist .

Here is the same example in codelens. Step through the code paying particular attention to the result of executing the assignment statement origlist[1] = 99 .

Checkpoint 10.13.4.

Activity: CodeLens 10.13.4 (reprefstep)

Check your understanding

Checkpoint 10.13.5.

What is printed by the following statements?

alist = [4, 2, 8, 6, 5]
blist = alist * 2
blist[3] = 999
print(alist)

[4, 2, 8, 999, 5, 4, 2, 8, 6, 5]
print(alist) not print(blist)
[4, 2, 8, 999, 5]
blist is changed, not alist.
[4, 2, 8, 6, 5]
Yes, alist was unchanged by the assignment statement. blist was a copy of the references in alist.

Checkpoint 10.13.6.

What is printed by the following statements?

alist = [4, 2, 8, 6, 5]
blist = [alist] * 2
alist[3] = 999
print(blist)

[4, 2, 8, 999, 5, 4, 2, 8, 999, 5]
[alist] * 2 creates a list containing alist repeated 2 times
[[4, 2, 8, 999, 5], [4, 2, 8, 999, 5]]
Yes, blist contains two references, both to alist.
[4, 2, 8, 6, 5]
print(blist)
[[4, 2, 8, 999, 5], [4, 2, 8, 6, 5]]
blist contains two references, both to alist so changes to alist appear both times.