Comp 271 lab 2 - iterative mergesort (and C++ lists)
A Race of Sorts
Part 1 Overview: bottom-up mergesort
Bailey implements a recursive version of mergesort. You
are to implement the non-recursive version.
I've provided you with an IntelliJ project named sorters (as sorters.zip).
In the Sorters.java class in this project, I implement most of the sorting
methods of Bailey chapter 6, each applied to a class IntList which is my own
implementation of an ArrayList<int>.
In particular, recursive mergesort appears as IntList.rmsort(). This would
sort an array of length 100 by:
- dividing it into two pieces of size 50 each
- dividing each of those into two pieces of size 25 each
- dividing each of those into two pieces of sizes 13 and 12
- (down to pieces of size 2)
- divide those pieces into two pieces of size 1, which are
- Now do all the merges on the way back up
Iterative mergesort works differently. At any one time, you visualize the
array as a sequence of "blocks", each of size a perfect power of two
except for the final block. At the start, the blocksize is 1, and each
block of length 1 is, of course, sorted. The process repeats with
successive block sizes 2, 4, 8, 16, 32, ... until the blocksize becomes
larger than the entire array size. On an array of size 100 you would now
do the following:
- merge each pair of consecutive blocks of length 1 into a block of
- merge each pair of consecutive blocks of length 2 into a block of
- merge each pair of consecutive blocks of length 4 into a block of
- as the last merge, we have one block of length 64, and a second block
of length 100-64 = 36. Merge them.
I've supplied you with the following merging methods:
- mergeblocks (int A, int T, int start, int blocksize)
- mergeblocks2(int A, int T, int start, int block1size, int
T is the temp space, to be allocated at the start of imsort(). The first
merger method, mergeblocks(), merges two equal-sized blocks; the second
allows for the block sizes to be unequal. Use whichever you prefer. Both should
handle having an undersized block as the last block of the
array, but let me know if you have problems.
In class Sorters.java I've started the implementation of iterative
mergesort in method imsort(). The code there is this:
int temp = null; //
allocate temp; finish this!
int blockLength = 1;
while (blockLength <
swap elements and temp as follows:
 a = elements; // this is just a pointer, not an
//elements = temp;
//temp = a;
blockLength *= 2;
You will have to edit this as necessary, and also edit the mergepass()
code. You should not change the mergeblocks() methods.
Make sure you allocate temp!
My class SortTester runs several versions of sorting algorithms on the
same data, and records the time. Currently it runs InsertionSort,
recursive MergeSort (Sorters.rmsort()), and Java's built-in
Collections.sort(). You are to complete the definition of iterative
mergesort, imsort(), and test it for speed against recursive mergesort and
Collections.sort(). You will need to comment out the InsertionSort test
once you've got imsort() working, as you'll need to test it with sizes
much too large for InsertionSort to handle in a reasonable time.
In my definition of class IntList, I've left out the expand() method (the
topic of the previous lab), but you don't need that as you're not using
add(). Instead, you will create IntLists of the desired size using the
IntList A = new IntList(100000);
and then use A.RandomFill() to create random entries. My code does that, and
then makes copies of the data, so that different sorts all
start with the same initial data. Currently the code runs InsertionSort on nums
(this is the one you will comment out), recursive mergesort on nums1,
and is set up to run iterative mergesort on nums2. After
you run your sort method, you can check that it worked with
IntList.checksort(). Right now, this fails for nums2, because imsort() isn't
We've already compared selection sort and insertion sort.
For block sizes of 1,000,000 up to 10,000,000 or so (doubling each time; say
1,000,000; 2,000,000; 5,000,000; 10,000,000), tell me the relative
times of iterative and recursive mergesort. You could add
qsort2() if it works, or the built-in Java sort, but that's optional. Do the
timing numbers seem consistent with O(n log(n))?
Second, find out how large an array you can sort with iterative mergesort in
Make sure you use checksort() to verify that your sort implementation works.
Part 2: implement StrList (from lab 1) in C++
Start with StrList.cpp (one file). Do the same
thing to the add() method that you did in lab 1 in Java. Don't worry about
There are two catches. First, you need to call delete
 oldelements (or whatever the name of the variable is) after
you're done with it. C++ does not have garbage collection.
Second, what is the type of elements?
It's declared to be an array of string. That's the same as pointer to
string, or string*. A string*
is the same as a pointer to an array of strings. If newelements
and elements are two string*,
then the assignment
causes elements to point to
the same object as newelements;
that is, to the new array. It is safe to call delete
 elements before this, as that simply erases the
current contents of elements,
as a pointer.
To submit your Java project, create a zipfile and email it to me at
firstname.lastname@example.org, or use sakai. Alternatively, you can upload
SortTester.java and Sorters.java individually (for this project I will
For the C++ project, all I will need is the StrList.cpp file.