I'm a python/programming newb, and I am self-learning python. Unfortunately I'm really stuck! I'm in chapter 19 of "How to Think Like a Computer Scientist". The object here is to write an implementation of a Priority Queue ADT, using a linked list.
I'm getting an error at lines 22 and 32. My understanding of this error, is that one or more of my node objects do not have the attribute "cargo". I don't understand this, as all of my node objects are assigned a cargo of "None", or some number, when they are created. The node object is written as it was given in the book.
I should mention, that LINE 31 works FINE. I only have a problem when I add LINE 32, which calls LINE 22 (which is apparently where the problem is hiding) that I get an error.
Any insight would be helpful. Thank you so much!!!
I reproduced the problem on a smaller scale:
class Node:
def __init__(self, cargo= None, next = None):
self.cargo = cargo
self.next = next
class Priority_Queue_For_Linked():
def __init__ (self):
self.max = None
self.nextmax = None
self.min = None
self.length = 0
def insert (self, cargo):
node = Node(cargo)
if self.length == 0:
node = self.max = self.min
self.length = 1
return
if node.cargo > self.max.cargo: # <--LINE 22
notmaxanymore = self.max
notmaxanymore.next = node
node = self.max
self.nextmax = notmaxanymore
self.length += 1
return
testlist = Priority_Queue_For_Linked()
testlist.insert(12)
testlist.insert(22) # <--LINE 32
And here is the error:
=== RESTART: /Users/Desktop/Programming Career/Untitled6.py ===
Traceback (most recent call last):
File "/Users/Desktop/Programming Career/Untitled6.py", line 31, in <module>
testlist.insert(22)# <--line 32
File "/Users/Desktop/Programming Career/Untitled6.py", line 21, in insert
if node.cargo > self.max.cargo:# <--line 22
AttributeError: 'NoneType' object has no attribute 'cargo'
I doubt you need more context, but in case it helps, here is all of the code:
class Node:
def __init__(self, cargo= None, next = None):
self.cargo = cargo
self.next = next
def __str__(self):
return str(self.cargo)
def print_backward (self):
if self.next != None:
tail = self.next
tail.print_backward()
print self.cargo,
def print_list(node):
while node:
print node,
node = node.next
print
class Priority_Queue_For_Linked():
def __init__ (self):
self.max = None
self.nextmax = None
self.min = None
self.length = 0
def is_empty (self):
if self.length == 0:
print "List is empty."
def insert (self, cargo):
node = Node(cargo)
if self.length == 0:
node = self.max = self.min
self.length = 1
return
if node.cargo > self.max.cargo:########40
notmaxanymore = self.max
notmaxanymore.next = node
node = self.max
self.nextmax = notmaxanymore
self.length += 1
return
if node.cargo < self.min.cargo:
notminanymore = self.min
node.next = notminanymore
node = self.min
self.length += 1
return
else:
comparisonnode = self.min.next
comparisonprev = self.min
for i in range (2, self.length):
if node.cargo < comparisonnode.cargo:
node.next = comparisonnode
comparisonprev.next = node
self.length += 1
break
comparisonnode = comparisonnode.next
comparisonprev = comparisonprev.next
def remove (self):
maxtoremove = self.max
self.max = self.nextmax
self.max.next = None
length -=1
return maxtoremove.cargo
def print_list(self):
tempnode = self.min
print "[",
while not tempnode == None:
print tempnode,
tempnode = tempnode.next
print "]"
testlist = Priority_Queue_For_Linked()
testlist.insert(12)
testlist.insert(22)#######86
When you insert your first item you have a problem in your assignments:
if self.length == 0:
node = self.max = self.min
self.length = 1
return
This will not change the value of self.max or self.min and they will stay as None, which of course won't have cargo. You also lose the node you are trying to add. I assume you wanted to do this:
self.max = self.min = node