Assignment #3: Code Transformations and Modules
Fall 1997
(To be completed in your lab period Sept. 24/25)
Objective: To do some small code transformation exercises and start using modules.
Often you are faced with converting the description of an algorithm into a programming language that does not have the constructs (usually looping constructs) that have been used to describe the algorithm. These small exercises will get you to work through some of these situations. In each case, you must produce a program that will implement the same algorithm.
Consider the following program:
var x,y : int
loop
put "x? "..
get x
put "y? "..
get y
exit when x = 0 and y = 0
loop
exit when y = 0
put x*y," "..
y := y-1
end loop
put ""
end loop
put "done",skip
Assume now that you only have the do-while looping construct available. A do-while statement has the form
do
<actions>
while <condition>
Recode the program using do-while loops. Recall that you can simulate a do-while in Turing by putting the exit statement in the appropriate place. For you C programmers, C has this construct.
Consider the following program that has multiple exits:
var x, xsquared, xroot : real
loop
put "x? "..
get x
exit when x < 0
put "x = ",x
xsquared := x*x
put "x**2 = ",xsquared
exit when xsquared > 10000
xroot := sqrt(x)
put "sqrt(",x,") = ",xroot,skip
end loop
put skip,"Good bye!"
Recode the program using while loops. A while statement has the form
while <condition>
<actions>
end while
Recall that you can simulate a while in Turing by putting the exit statement in the appropriate place. For you C programmers, C has this construct.
Replace the for statement in the following program with a do-while style construct as described in Exercise 1.
var x,y : int
loop
put "x? "..
get x
put "y? "..
get y
for Num : x .. y
put Num," "..
end for
put skip
end loop
Procedures and functions are used to improve the organization of your programs by separating individual tasks into ``sub-programs". To illustrate the new concepts associated with procedures and functions, the quadratic equation solver program of Lab 2 has been rewritten and error checking added. The modified program is depicted in Figure 1. Please study it so that you fully understand it.
The getvalue procedure is used to prompt the user for input. The same procedure is used for all three coefficients.
Figure 1: The quadratic equation solver program
The parameters of this procedure are declared as "name:string; var coefficient:real". What is the difference between having var written before a parameter, and not having it? How would the procedure change if var was not written before coefficient?
What is the basic difference between a procedure and a function? In the function discriminant what does the :real at the end of the first line of the function declaration signify?
What are the global variables of the program? Identify some local variables. Could the solvequad procedure be rewritten without parameters? How?
You just won the Super 7 Lotto and purchased two new cars. You would like to keep track of the fuel efficency as measured in litres/100 kilometres, as well as the total amount you have spent for fuel on each car. Write a program that allows you to input the current odometer reading, the number of litres that you put in, and the cost of the fuel after each time you fuel one of your cars. It is assumed that you fill-up the tank each time. For each car, the program will keep track of the initial and current odometer readings, the total amount of fuel consumed, and the total amount spent on fuel since the program was reset.
The program starts by initializing all values to zero and then continuously prompts the user to select an action. The choices are to input data after a fill-up, to request the fuel efficiency for the last tank of gas, the overall efficiency and cost since you started, and to reset the initial values of the odometer, total fuel used, and total cost. Since the actions are the same for either car, you should write only one module for each action.
Note that there is a special boundary case when you start your program because no data has been entered. You must handle this and any other problems reasonably. Assume that the correct type of data will be input, i.e., you will not get a letter when you are asking for a number.
The Input module should check that the new odometer reading is greater than the previous one and that the cost was greater than zero. The Last module will give you the fuel efficiency for the most recent fill-up, and the Average module will compute the overall efficiency and cost of fuel. The Reset module will prompt for an odometer reading, and set total fuel usage and cost to zero.
An example session is shown in Figure 2. The program should accept either upper or lower case letters to specify actions, e.g., either Q or q for (Q)uit.
Figure 2: Fuel Efficiency Program
I assume the input procedure of your fuel efficiency program checked the user input for errors. Well-engineered software will always anticipate problems that might come up during the execution of the program, and will always be prepared to handle them. In your program, you may have used if-then statements to detect improper input. However, some problems that might come up during the execution of the program may not be resolved using such program control statements. For example, if your program prompts the user to enter a real value, but s/he enters a non-numeric character, this will result in the generation of an exception, and if you have not specified in your program how to deal with an exception, the program will terminate.
A formal method of dealing with exceptions is provided in OOT by way of the exceptionHandler mechanism. Full instructions on the use of the exceptionHandler are provided in the OOT reference manual. Extend your fuel efficiency program by adding handler blocks to deal with exceptions that result from improper input. If the user provides a non-numeric input for the odometer, fuel, or cost amounts, the exception handler should post a warning to the user and request for input again.
A more human-readable description of exception handling is described on the home page.