|COMP1011 Exercises for Week 03|
|Computing 1A 05s2||
Sat 06 Aug 2005 19:53
Lab1 covered some basic features of XEmacs, such as file loading and saving. These features were activated using the mouse (ie, using point-and-click1 operations). The disadvantage of using a mouse is that it requires you to take one hand from the keyboard, move it to the mouse, and back to the keyboard to continue typing. This can become annoying when you become a proficient computer user.
Therefore, any halfway decent program can also be operated using the
keyboard only - without touching the mouse (drawing programs and the
like excepted). To allow the computer to distinguish between keystrokes
meant to input text and those that are used to control the program
(i.e., those replacing mouse operations), there are a couple of
modifier keys on the keyboard. The most important of them is
the control key (the one labelled
Ctrl). In the
following, when I write
C-x, I mean pressing the control
key simultaneously with the key labelled
x. Moreover, I
write sequences of keystrokes as
C-x b or
C-b. Note that these two sequences are different. The former
specifies to hold down the control key only while pressing
x, whereas the latter means to press down the control key
x and while pressing
XEmacs allows all operations to be performed by keyboard only - in fact,
many of the advanced features are only available via the keyboard.
Therefore, let us have a look at some basic XEmacs keyboard wizardry,
which you will find useful when writing Haskell programs. Start up
xemacs by typing
xemacs & into an terminal window (and
don't forget to
leave the mouse cursor over the XEmacs window while typing into XEmacs).
C-x C-f(so, control is used with both
f); then, the cursor will move to the minibuffer (that is, the line at the very bottom of the XEmacs window) and you can enter the file name
Lab02.hs(which you should have used last week).
C-c C-lto run GHCi within XEmacs and to load
Lab02.hsinto GHCi in one go.
C-x oto jump from one window into the other.
C-x 1. It will remove the other window (the one displaying GHCi). By typing
C-c C-lonce again, you can make GHCi appear again (and actually reload you code into GHCi).
Here is a summary of the commands. I recommend compiling a list of XEmacs commands as you learn them, for easy reference when you forget something. (There will be other useful commands in coming lab exercises.)
Observe how all commands manipulating buffers and windows start with
C-x and the second key in the key sequence was chosen to be
As both the XEmacs and GHCi installation on CSE machines is very new, you may experience problems when attempting to use
C-c C-l. In this case, ask your tutor for help.
Start a new file called
Lab03.hs into which you add your
solutions to the this week's Haskell exercises. Before beginning with
the exercises, don't forget to add comments at the beginning of this
file, which describe the purpose of this Haskell module and who wrote
it. (Remember that comment lines start with
introduce the new module with the two lines
module Lab03 where
Your function definitions follow this module header.
When implementing the functions outlined below, make sure that you follow the Five Steps to Designing and Implementing a Function from the lecture. Your tutor will check that you added a description and examples to the comment preceding each function and that you have tested it properly. You will only get lab marks for functions that a properly implemented using the five steps.
Implement a function
isPositive :: Int -> Bool that
returns True if the input value is greater than zero,
False otherwise. The use of the funtion would look as follows:
Lab03> isPositive 3 True Lab03> isPositive (-3) False
Note the parenthesis around the literal
-3. They are
necessary to avoid a syntax error.
Write a function
sort3 that has type
sort3 :: Int -> Int -> Int -> (Int, Int, Int)
I.e., it gets three integer arguments and also returns three integer values. In fact, it should return exactly the same values, but ordered such that in the resulting triple, the smallest number is first and the greatest last. The following examplifies the use of the function:
Lab03> sort3 5 7 8 (5,7,8) Lab03> sort3 7 5 8 (5,7,8) Lab03> sort3 7 8 5 (5,7,8) Lab03> sort3 8 7 5 (5,7,8) Lab03> sort3 8 5 7 (5,7,8) Lab03> sort3 5 8 7 (5,7,8)
Write a function
rangeWithInc that gets an interval - i.e.,
a pair of numbers specifying the interval's start and end value -
together with an increment value as arguments. It yields all numbers in
the interval that can be obtained by adding the increment onto the start
value. The function has type
type Interval = (Int, Int) rangeWithInc :: Interval -> Int -> [Int]
and it behaves as follows:
Lab03> rangeWithInc (1, 10) 1 [1,2,3,4,5,6,7,8,9,10] Lab03> rangeWithInc (1, 10) 3 [1,4,7,10] Lab03> rangeWithInc (8, 20) 2 [8,10,12,14,16,18,20]
Hint: Use the
[x, y..z] operation.
When you have completed the above exercises show them to your tutor for this week's core mark.
What is the minimal number of comparisons needed to implement
sort3? Less than three comparisons will generally not be
sufficient, but in fact it is possible to restrict the implementation to
not using more than three comparisons. Write a second version of
sort3 - call it
sort3' (i.e., a prime
character added after the previous name of the function) - but this
time, the function is not allowed to use more than three comparisons to
compute the result.
Hints: You will have to use a
where clause. In
fact, there are two possible solutions - one of them makes use of
sort2 from this week's tutorial exercises (both
solutions are ok).
When you have completed the above exercise show them to your tutor for this week's advanced mark.
The following is an additional exercise to help you getting practice in programming. It is not part of the lab mark, but it should help you understand some of the material covered in the lecture.
Write a function
bothTrue:: Bool -> Bool ->
Bool that returns True if and only if both its inputs are
Lab03> bothTrue True False False Lab03> bothTrue True True True Lab03> bothTrue False False False Lab03> bothTrue False True False Lab03> bothTrue (6 < 100) ('a' < 'b') True Lab03> bothTrue ("Haskell" == "uncool") (4*5 == 20) False
Can you do this without using pattern matching or the && function?
1Hackers2 sometimes teasingly call these interfaces point-and-drool interfaces.
2I am using the term hacker in its original sense, where it describes a computer expert who enjoys exploring the details of programmable systems. This is in contrast to the misuse in main stream media who use the term wrongly for a person breaking into computer systems with a malicious intention. The correct term for such a person is cracker.3
3These are recursive footnotes3 and if you find them funny, you have already understood the essence of recursion. (If you don't find them funny, it doesn't mean that you didn't understand recursion, but only that you don't have such a twisted sense of humour as I have ;-)