[Tutor] recursivity and lists

[sina sareth]

Hi ThereI have those projects and I would like to get some directions the way to tackle them.
Thanks
1)  XML parsing into database. 
2) Do the same thing but this time, get ALL the quotes and save them as separate database entries (you will have to do a for loop for this). 
3) Use Django to build a view where you can download all Quotes in CSV form. 
4. Use Django to build a view where you can update quote values based on a call to the API (they change every day) 

    On Friday, March 4, 2016 11:14 AM, Alan Gauld <alan.gauld at btinternet.com> wrote:
 

 On 04/03/16 15:00, Gaston wrote:

> Both work but I am not satisfied. Is there not a method that could do 
> the job of '+' operator (namely concatenate in a copy)?

Why would you want a method? An operator is a perfectly legitimate
feature and is at least as readable as a method. It is certainly no less
"functional". And of course, in Python + usually translates
to the __add__() method anyway.


As to how your initial function works can I suggest you sit down
with a pen and paper and work through the code iteration by
iteration. Start with 2 element lists and then try 3 elements.
By the time you've done that it should become clear.
Keep track of the contents of list1 and list2 and the
output for each call of the function.

Recursive can be a seductive technique but it can be a real
brain twister to work out and debug when things don;t go
as expected! The pen/paper manual approach is the most
reliable technique I've found.

Here is what I mean for a 2 element list pair:

list1        list2        f(list1,list2)
-----------------------------------------------
[1]        [2]        f([1],[]).append(2) # first call of f()
[1]        []        [1].append(2) -> list1 becomes [1,2]
[1,2]          []              f([1,2],[])  # 2nd call of f() -> [1.2]

Now try that for 2 elements per list, then 3.
In effect you move elements from list 2 to list 1, or vice versa.
And you iterate over the list multiple times.
It's very inefficient as well as non intuitive.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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