# Lab; Merge Sort

## Exercises

### Exercise 0: Preparation

Make a copy of mergesort.ss, my implementation of merge sort. Scan through the code and make sure that you understand all the procedures.

### Exercise 1: Merging

a. Write an expresssion to merge the lists `(1 2 3)` and `(1 1.5 2.3)`.

b. Write an expression to merge two lists that contain the same values.

c. Write an expression to merge two lists of names. (You may choose the names yourself. Each list should have at least three elements.)

### Exercise 2: Reflecting on Merging

a. What will happen if you call `merge` with unsorted lists as the first two parameters?

c. What will happen if you call `merge` with sorted lists of very different lengths as the first two parameters?

### Exercise 3: Splitting

Use `split` to split:

a. A list of numbers of length 6

b. A list of numbers of length 5

c. A list of strings of length 6

d. A list of lists of length 4 (each sublist should have length 2 or more).

### Exercsie 4: Splitting, Revisited

One of my colleagues prefers to define `split` something like the following

```(define split
(lambda (ls)
(let kernel ((rest ls)
(left null)
(right null))
(if (null? rest)
(list left right)
(kernel (cdr rest) (cons (car rest) right) left)))))
```

a. How does this procedure split the list?

b. Why might you prefer one version of split over the other?

### Exercise 5: Sorting

a. Run merge sort on a list you design of fifteen integers.

b. Run merge sort on a list you design of thirteen strings.

### Exercise 6: Special Cases

a. Run merge sort on the empty list.

b. Run merge sort on a one-element list.

c. Run merge sort on a list with duplicate elements.

### Exercise 7: Sorting Students

Assume that we represent students with a list of the form

```(lastname firstname id major)
```

a. Create a list of ten or more students.

b. Write an expression to sort that list by first name.

c. Write an expression to sort that list by id number.

d. Write an expression to sort that list so that students are arranged alphabetically by major and also alphabetically by last name within each major.

### Exercise 8: Verifying Sorts

a. Write a procedure, `verify-sort`, that verifies the postconditions of `merge-sort`.

b. Use that procedure to verify that `merge-sort` correctly sorts lists of 1000 "random" numbers.

### Exercise 9: Comparing Sorts

a. Using DrScheme's built-in timing mechanism (you may have to look through the online help to find information about that mechanism), make a table of the running time of insertion sort and merge sort on inputs of size 0, 1, 10, 100, 1000, 5000 10000, 50000, 100000, and 1000000.

c. Based on your data, what can you say about the relative speeds of the two sorting methods?

## History

Wednesday, 22 November 2000

• Created. All new!

Disclaimer Often, these pages were created "on the fly" with little, if any, proofreading. Any or all of the information on the pages may be incorrect. Please contact me if you notice errors.