Page 252 - Learn To Program With Scratch

5. We’ll repeat the bubble sort process until no numbers are swapped dur-
ing a pass, meaning that our list is in order. The final three passes of
the algorithm are shown below:

No
Change No
Change Swap
Pass 3
No
Change No
Change

No
Swap
Change
No
Pass 4
Change No
Change No
Change

No
Change No
Change No
Pass 5
Change No
Change No
Change

Now that you’ve seen how bubble sort works, let’s implement it in
Scratch. The script, shown in Figure 9-24, has two loops. The inner loop
cycles through a list, comparing and swapping as needed, and sets a flag
(named done) to 0 when another pass is needed. The outer loop repeats as
long as the done flag is 0, because that value means we aren’t done sorting.
If the inner loop completes one pass without swapping elements, the outer
loop will exit, ending the procedure.
Let’s explore this procedure in more detail. Since we haven’t done any
sorting yet, it sets done to 0 u. The outer loop uses a repeat until block to
pass through the list until it is sorted (that is, until done becomes 1) v. At
the beginning of every pass, this loop sets done to 1 w (that is, it assumes
that we won’t make any swaps). It also sets pos to 1 to start the sort with the
first number.
The inner loop then compares each pair of elements in the list. The
loop needs to perform N – 1 comparisons x, where N is the number of
items in the list.
If the item at index pos+1 is greater than the item at pos y, the two
need to be swapped. Otherwise, the procedure adds 1 to pos so it can com-
pare the next pair of items. If we do need to swap, this procedure does so
with the aid of a temporary variable named temp z.
Once the current pass through the list ends, the inner loop sets done
back to 0 if it swapped numbers or leaves done=1 if it made no changes {.
The outer loop will continue until the list is sorted.

230 Chapter 9
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