[Tutor] Decision matrix

[Jeff Shannon]

Whoops, I'd intended to reply to the list as well, and only sent this to 
Poor Yorick.  

--Jeff S.

-------- Original Message --------
Subject: Re: [Tutor] Decision matrix
Date: Thu, 13 Feb 2003 09:46:08 -0800
From: Jeff Shannon <jeff@ccvcorp.com>
To: Poor Yorick <gp@pooryorick.com>
References: 
<7497DCA1C240C042B28F6657ADFD8E0901F850C3@i2km11-ukbr.domain1.systemhost.net> 
<3E4B57B2.8060607@pooryorick.com>

Poor Yorick wrote:

> alan.gauld@bt.com wrote:
>
> >Sounds like the definition of a dictionary to me...
> >
> Two problems with using a dictionary:
>
> 1.  I would like the flags to be accepted in any order.
> 2.  If I come up with other flags in the future, I would like to deal
> with them in the most simple possible manner.  Adding an exponentially
> larger number of permutations as dictionary keys doesn't seem like a
> good method. 


If you want each unique set of flags to call a single (probably unique) 
function, regardless of the order of those flags, you can always sort 
your flags before searching the dictionary.  This avoids the permutation 
explosion, and it's probably a lot simpler than your proposed 
Intersection() function.

flags = raw_input()
lflags = list(flags)
lflags.sort()
flags = ''.join(lflags)
result = Functions[flags]()

In order to find the intersection of a set of lists, you'll have to 
traverse each list once for each item in the first list, which (if you 
have a large number of flags) can grow to be a *lot* of list traversal. 
 Dictionary searching is much faster than list traversal, and is done in 
constant time regardless of the size of the dict (where list traversal 
is proportional to the size of the list).

Another possible issue with your Intersection() idea -- how are you 
intending to verify that a given set of flags always resolves to a 
*single* function?  With only a few flags, you can eyeball and match up, 
but when you've got a dozen flags, each of which has half a dozen 
potential functions, it'll become almost impossible to ensure that no 
group of flags will resolve to more than one function -- especially if 
someone later needs to modify the list of flags.  This will be a 
maintenance nightmare.  A dict, on the other hand, will make it very 
easy to see what function will be called by a given combination of 
flags, and there's no worries about accidentally selecting more than one 
function.

Almost any time you're looking to select one item out of a set of items 
based on some arbitrary parameter, a dictionary is the way to go.  The 
only trick is figuring out what the most efficient key scheme will be...  

Jeff Shannon
Technician/Programmer
Credit International