From: finer recliner <finerrecliner@...>
Date: 2007-05-26T05:25:04+09:00
Subject: printing nil objects

hey, im pretty new to ruby and im having trouble. this is not my
original code (much longer), but i quickly rewrote the part of the
data structure that was creating the problem so i could post it here:

i have a node class that is used to make a linked list. the node
object has a value, a pointer to the next node in the list, and
another node that is sometimes used to point to another node in the
list somewhere.
when pointer2 is not used it is set to nil. However when i try to
print out my linked list (which will print each node's values and
where its pointers are pointing) it crashes when it reaches a pointer
set to nil.

i tried making an if-then-else check, but it doesnt seem to work. can
anyone help me? thanks in advance.

[code]
class Node

    def initialize(v,p1,p2)
        @value = v
        @pointer1 = p1      #next node
        @pointer2 = p2      #points to another node
    end

    def getP1
        @pointer1
    end

    def setP1(x)
        @pointer1 = x
    end

    def getP2
        @pointer2
    end

    def setP2(x)
        @pointer2 = x
    end

    def getV
        @value
    end

    def setV(x)
        @value = x
    end

end


### code starts here ###
c1 = Node.new(1,nil,nil)
c2 = Node.new(2,nil,nil)
c3 = Node.new(3,nil,nil)

c1.setP1(c2)
c1.setP2(c2)
c2.setP1(c3)


current = c1
while current.getP1 != nil
    print "value = #{current.getV}, pointer1 = #{current.getP1.getV},
"
    if current.getP2.getV != nil then print "pointer2 =
#{current.getP2.getV}\n" else print "null\n" end
    current = current.getP1
end
[/code]


output:
$ ruby node.rb
value = 1, pointer1 = 2, pointer2 = 2
node.rb:49: undefined method `getV' for nil:NilClass (NoMethodError)
value = 2, pointer1 = 3,
$