Eiffel - How do I make my classes readable? - linked-list

Question

I'm new to Eiffel and I'm trying to use the LINKED_LIST class for organizing instances of other class "MONOMIO" I've made. I added a function for ordering this elements and I use the remove and the cursor movement features and when I try to execute the code it raises an exception saying that the objects contained should be readable and writable. I would like to know how to do it, this is my class:
class
MONOMIO
feature --Initialization
make (coef:INTEGER; expX:INTEGER; expY:INTEGER)
do
coeficiente := coef
exponenteX := expX
exponenteY := expY
end
feature
evaluar(valX: INTEGER; valY: INTEGER): REAL_64
do
Result := coeficiente*(valX^exponenteX)*(valY^exponenteY)
end;
coeficiente: INTEGER;
exponenteX: INTEGER;
exponenteY: INTEGER;
feature --setter
set_coeficiente(val: INTEGER)
do
coeficiente := val
end;
end
I think the exception raises because of this feature I've made for a class that has as a feature the LINKED_LIST[MONOMIO] and it's called "contenido":
simplificar
local
tamanio_polinomio: INTEGER -- Número de monomios que tiene el polinomio
contador: INTEGER
monomio_a_comparar: MONOMIO -- Auxiliar
coeficiente_total:INTEGER -- Auxiliar
indice_monomio_en_revision:INTEGER
do
from
contenido.start
indice_monomio_en_revision := 0
tamanio_polinomio := contenido.count
until
indice_monomio_en_revision = tamanio_polinomio
loop
contenido.start
contenido.move (indice_monomio_en_revision)
monomio_a_comparar := contenido.item
from
contador := indice_monomio_en_revision
coeficiente_total := monomio_a_comparar.coeficiente
contenido.forth
until
contador = tamanio_polinomio
loop
if
(monomio_a_comparar.exponentex = contenido.item.exponentex) and
(monomio_a_comparar.exponentey = contenido.item.exponentey)
then
coeficiente_total := coeficiente_total + contenido.item.coeficiente
contenido.remove -- Mueve el cursor a la derecha
tamanio_polinomio := tamanio_polinomio - 1
contador := contador - 1
else
if
not contenido.islast
then
contenido.forth
end
end
contador := contador + 1
end
contenido.start
contenido.move (indice_monomio_en_revision)
contenido.item.set_coeficiente (coeficiente_total)
indice_monomio_en_revision := indice_monomio_en_revision + 1
end
end;
I hope anyone can help me with this problem. Thanks.

Answer 1

Suppose you have a list with 1 element. Then we enter the outer loop and move to the first element. Then we execute contador := indice_monomio_en_revision that is still 0 at this point and do contenido.forth. Now we are beyond the list because there is only one element. However contador = tamanio_polinomio is false (0 = 1), so we enter the inner loop and try to retrieve the second (non-existing) item. BOOM!
Other issues include:
There are multiple calls like contenido.start followed by contenido.move. You could use a single call to go_i_th instead.
Instead of counting number of items in the list I would look at the feature after. It tells when you reach an end of the list. It would simplify the logic of your loop (e.g. the call to islast would be removed) and let you to remove some local variables.
Taking the last point into account I would write the inner loop condition as
contenido.after
At least this would avoid the crash you experience. As to the logic, you may need to check features start, after, forth and remove to see what effect they have. The usual way to write loops in such cases is like
from
l.start
until
l.after
loop
... -- Use l.item
l.forth
end
In case of remove probably you do not need to call forth.