SECTION(14, Tutorial 14)
m5_question(Dr Seuss' story ``Yurtle the Turtle'' describes how a
turtle called Yurtle sits at the top of a pile of other turtles. In
this example, the pile of turtles is represented by a linked list of
CLSS(Turtle) objects, with the TT(up) property serving to connect one
CLSS(Turtle) object to another. If a CLSS(Turtle) object has a non-NULL
TT(up) property, then this represents a turtle that is sitting on top
of the current one. The last turtle in the linked list is the turtle
that is at the top of the pile, which has a NULL value for its TT(up)
property.)
m5_study(14.1)
m5_question(
m4_begin_indent
NU()m4_include(sjs-tutorials/TurtleTest.m4)
m4_end_indent
The code in the MAIN function after the TT(BO(***)) sets up the
following relationships between the three CLSS(Turtle) objects
(Bungle, Zippy and Yurtle). The following diagram shows the
relationship between the different turtles. When you traverse the
list of turtles you must always start at the bottom turtle (known as
the EM(head of the linked list)). If you give a different value for
the bottom turtle, your code will think that the given turtle is the
one at the bottom of the pile and you will get the wrong result.
PCLOSE
m4_begin_center
m4_image(Turtles)
m4_end_center
POPEN
)
m5_question(BO(Question 14.2:) Move the code for calculating the
total weight of the turtles from the MAIN function to a function
called TT(FUNCTION VOID FUNC(printTotalWeight)(CLSS(Turtle)
VARI(bottom))) in the CLSS(Turtle) class that prints out the total
weight of the turtles. Then call that function from the MAIN function
to get the same result as before. Note that that if
TT(printTotalWeight) was a method then calling that method using NULL
(representing an empty list) like so: TT(NULL.printTotalWeight())
would be an error, whereas TT(CLSS(Turtle).printTotalWeight(NULL))
wouldn't be and therefore is better. This is one example of how
methods and functions differ.)
m5_question(BO(Question 14.3: Revision question for getters.) By
copying the pattern established by the TT(getName) method, add two
getter methods to the CLSS(Turtle) class: TT(getAge) which returns the
current turtle's age and TT(getWeight) which returns the current
turtle's weight. Then call these methods on the Yurtle object in the
MAIN function. Note that the TT(toString) method would be more
appropriate as it handles nulls better but you known that the
TT(yurtle) reference is not NULL so you know it is safe to call the
TT(getAge) and TT(getWeight) methods on the TT(yurtle) reference.)
m5_question(BO(Question 14.4:) Write a function
TT(FUNC(findTopTurtle)(CLSS(Turtle) VARI(bottom))) that returns the
CLSS(Turtle) object that is at the top of the pile, and returns NULL
if there isn't one.)
m5_question(BO(Question 14.5:) Then call this function from the
MAIN function using SYSTEM_OUT_PRINTLN and the bottom turtle
TT(bungle).)
m5_question(BO(Question 14.6:) Write a function
TT(FUNC(findOldestTurtle)(CLSS(Turtle) VARI(bottom))) that returns the
oldest turtle or NULL if there isn't one.)
m5_question(BO(Question 14.7:)Then call this function from the
MAIN function using SYSTEM_OUT_PRINTLN and the bottom turtle
TT(bungle).)
m5_question(BO(Question 14.8:) Write a function
TT(FUNC(findHeaviestTurtle)(CLSS(Turtle) VARI(bottom))) returns the
heaviest turtle, or NULL if there isn't one.)
m5_question(BO(Question 14.9:) Then call this function from the
MAIN function using SYSTEM_OUT_PRINTLN and the bottom turtle
TT(bungle).)
m5_question(BO(Question 14.10:) Write a function
TT(FUNC(sayPile)(CLSS(Turtle) VARI(bottom))) that prints the names of the
turtles in the pile starting from the bottom turtle and finishing at
the top turtle. Then call this function from the MAIN function.)
m5_question(BO(Question 14.11:) Under what circumstances would it
be okay to change the visibility of the TT(up) property to private,
like the TT(name), TT(age) and TT(weight) properties?)
m5_question(BO(Question 14.11:) Add an extra parameter to the
constructor which is a reference the to the turtle on top of the
current one. Then remove all occurences of the TT(up) property from
the MAIN function. The advantage of this is that it enables you to
change the visibility of the TT(up) property to private.)
