[Tutor] double ended queue
Martin A. Brown
martin at linux-ip.net
Sat Nov 4 21:31:04 EDT 2017
Hello and welcome duplicative Atux,
>i am new to the area and i am struggling with a small project that i have.
>i need some help please.
New to Python, perhaps. New to programming, as well?
>i need to create a program that runs constantly unless the user
>presses q to end it.
This is a good way to learn how to handle loops which can handle
various conditions. I mention below a module called cmd, which
ships as a part of the Python library, but it may be good to
understand first how to handle the looping constructs before you
move on to the cmd module.
Consider a loop like you indicated:
> if input("\n\n\nType a number to add it to the queue or q to exit: ") == 'q':
There are quite a few valuable lessons in this simple task.
* consider the what kind of data you are asking for; is it an
integer, a string or a floating point value? Alan mentioned
that in his message
* be careful with the kind of data your user is entering, but it
also teaches you the value of knowing what the data type is (an
important lesson in most programming languages)
* how do you loop successfully and how do you break out of the
loop; you'll want to know about 'break' and you might find
'continue' useful, as well
# -- Python3
queue = list()
status = ''
instructions = "\n\nAdd a number to the queue or q to exit: "
# -- convert the string inputs to int(), float() or whatever type you want
# the program to operate on
response = input(status + instructions)
response = response.lower() # -- lower case the string
if response == 'q':
number = int(response) # -- or float(response)
status = "Ignoring non-integer input: " + response
status = "queue = " + str(queue)
>the program asks the user for a number and puts the number in a
>queue, then it prints the queue with the new element at the end.if
>the number is with 01,02 then it will be added at the left hand
>side without the 0 at the beginning, otherwise at the right hand
Steven has demonstrated how you could use the list() data structure
to do what you describe. Note that Python comes with additional
data structures and what you describe is known in Python as a deque.
It's a bit less common of a data structure, so you could benefit
from learning how to perform the prepend and append operations like
Steven has suggested.
If you haven't noticed that you can run the Python interpreter
directly, you may find this helpful simply to play with the data
You can use a list() as you can see in the core documentation (and
as Stephen mentioned), however, even in the main documentation is a
pointer to the collections.deque, which is a more specific sort of
data structure geared for exactly the use case you present.
Important note; If humans are interacting with this, though, it's
unlikely that any of the efficiency gains of a deque over a list are
necessary, but here are links to the documentation:
>the user can remove an item from the end of the queue by typing r.
>if the user types '0r' it will be removed from the beginning of the
Once you have the addition of items to the queue and the looping
worked out to your satisfaction, maybe you could share your progress
and there might be somebody to provide a bit more help on the next
step of your learning
>but i a stuck on how to continue asking and adding to the list
Martin A. Brown
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