(Latest Revision: 10/28/2001)
Directions for Lab #6
Read about while-loops in our text book before starting to work
on this lab.
Your assignment for this lab is to write a program that works
this way:
- The program asks the user to enter the number of fence posts
in a fence.
- The user enters a number.
- The program then "draws" a fence having that same number of
posts.
Have a look at the file named
fence.script
for sample runs of my version of this program.
DISCUSSION:
The heart of this assignment is to write some code that inputs a number of
fence posts and draws one layer of a fence. The fence is composed of two
layers: an upper layer and a lower layer. Each layer looks like this
example: |---|---|
You will need to write a loop for this program. The job of the loop will be
to draw (most of) one layer of a fence. When the program executes, the
number of posts input by the user will determine how many times the loop
body will execute. You have to write the loop so that the loop body is
guaranteed to execute the correct number of times, according to the input of
the user.
This problem is an example of a fence post problem. It is very
common to run into a fence post problem when one is trying to write a loop.
The fence post part of the problem has to do with the fact that the
number of posts in a fence is one more than the number of horizontal
sections. Suppose that your loop writes one post and one horizonal section
each time through the loop body. In that case, the loop by itself cannot
possibly draw a whole layer of a fence correctly. It would be capable only
of drawing something that looks like
|---|---|---
or
---|---|---|
SUGGESTIONS:
Write a function called "makeOneLayer" whose job is to draw one layer of a
fence. Let the number of posts be a parameter of makeOneLayer.
Here is one way to take care of the fence post problem:
- Write "|" one time.
- Enter a loop that writes "---|" as many times as is
necessary.
Use a local variable to control the loop. For example, you could use a
local variable named "sections" to keep track of how many of these strings:
---| have been written. You could initialize sections = 0 before
entering the loop. The loop could add 1 to sections each time through the
loop body, and the loop could be rigged to stop when sections reaches some
limit (determined by how many posts there are supposed to be).
In your main program, you can prompt the user for the number of posts. You
can have an if-statement that causes makeOneLayer to be called twice if the
number of posts given is reasonable. You can use an else-statment with the
if-statement to make the program write a message like "Sorry, no can do." if
the number of posts is too low or too high.
GETTING IT RIGHT:
Plan your lab program during the week. You don't have to write everything
out in perfect C++
before the lab starts. However, you could write a specific step-by-step
description of what your program will do. Write this plan so that it will
be easy to translate it into C++ when the time comes. Write some of the C++
statements in advance if you anticipate having difficulty with them. In
lab, compare plans with your partner and execute a good compromise plan.
Compile, test, and debug repeatedly until the program is working correctly.
You need to run the program on a lot of different inputs. Try numbers like
-10, -1, 0, 1, 2, 3, 5, 10, 17, 18, 19, and 20.
If you make an error in how you write the loop code, there is a possibility
that your loop will become a "runaway" or "infinite" loop. If your program
writes excessive output or takes excessive time to finish, you can force it
to abort by typing a control-c.
Be very careful about letting your loop run when the number of posts "does
not make sense." Sometimes this can cause a loop to become a runaway. Look
at your loop and think about what it may do if the number of posts is 1, 0,
or some negative number. Would it become an infinite loop? Is there
something simple you can do to prevent this from happening, even if the user
decides to enter a "foolish" number? Try to write the program in a way that
defends against such a problem. Test your defenses by inputting numbers of
posts such as -1, 0 and 40. Remember: use control-c if you have to
abort.
SENDING IT IN:
Send me a listing (source file) and script by e-mail. Make the script so it
shows you running these inputs: -10, -1, 0, 1, 2, 3, 5, 10, 17, 18, 19, and
20.
Please use the following subject lines:
Source Code: "CS1500_Lab6_Source"
Script: "CS1500_Lab6_Script"
You can refer to
the
Hello World! example
for details on how to make and filter a script file, and on how
to e-mail files to me.