[Tutor] Lists in lists

[Brian van den Broek]

Kent Johnson said unto the world upon 16/09/06 04:35 PM:
> Brian van den Broek wrote:
>> Morten Juhl Johansen said unto the world upon 16/09/06 08:29 AM:
>>> # Newbie warning
>>> I am making a timeline program. It is fairly simple.
>>> I base it on appending lists to a list.
>>> Ex.
>>> [[year1, "headline1", "event text1"], [year2, "headline2", "event text2"]]
>>>
>>> This seemed like a brilliant idea when I did it. It is easy to sort.
>>> Now, if I want to OUTPUT it, how do I indicate that I want to extract
>>> first entry in a list in a list? How do I print the separate entries?
>>>
>>> Yours,
>>> Morten
>>>
>> Hi Morten,
>>
>> Andrei answered the question you asked; I'd like to make a suggestion 
>> involving a bit of reworking.
>>
>> You might think about structuring your timeline data as a dictionary, 
>> rather than a list. So:
>>
>>  >>> timeline_data = {
>> ...      800: ["Charlemagne Crowned Holy Roman Emperor", 'event_text'],
>> ...     1066: ["Battle at Hastings", 'event_text']}
>>
>>
>> This makes it very easy to access a given year's data:
>>
>>  >>> timeline_data[800]
>> ['Charlemagne Crowned Holy Roman Emperor', 'event_text']
>>
>> and
>>
>>  >>> timeline_data[800][0]
>> 'Charlemagne Crowned Holy Roman Emperor'
>>
>> will get you the headline alone.
>>
>> You expressed a liking for the lists as they are easy to sort. On 
>> recent versions of python one can easily obtain a sorted list of 
>> dictionary keys, too:
>>
>>  >>> d = {1:2, 3:4, 43545:32, -3434:42}
>>  >>> d
>> {1: 2, 3: 4, -3434: 42, 43545: 32}
>>  >>> sorted(d)
>> [-3434, 1, 3, 43545]
>>  >>>
>>
>> (Older versions of Python can do the same, but with a bit more 
>> keyboard action.)
>>
>> So, if you wanted to print the headlines in increasing year order:
>>
>>  >>> for year in sorted(timeline_data):
>> ...     print timeline_data[year][0]
>> ...
>> Charlemagne Crowned Holy Roman Emperor
>> Battle at Hastings
>>  >>>
>>
>>
>> You say you are new to Python. Well, it might not now be obvious why 
>> dictionaries are especially useful, but they are *central* to the 
>> pythonic approach. The sooner you become comfortable with them, the 
>> better (IMHO).
> 
> I agree that dicts are extremely useful, but I don't think they add 
> anything in this case unless there is actually a need for keyed access. 
> A list of lists (or tuples) seems very appropriate to me. A good 
> alternative might be a list of Bunches.
> http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52308
> 
> Kent


Hi Kent and all,

I should have included the reason why I thought a dict might be better 
here. (I did send it in a private email after the post.)

A lot of ways I could imagine the time-line data being used might 
involve wanting to access some one year, rather than the entire time-line.

So, if you wanted to get the headline for the year 800,

 >>> print timeline_data[800][0]

seems *way* better than something like:

 >>> for year_data in timeline_data_as_list_of_lists:
...    if year_data[0] == 800:
...       print year_data[1]
...       break

which would be what the original list structure seems to require.

It may be a case of over-design for needs that won't arise, though.

Best to all,

Brian vdB