Dynamic Dictionary Creation

[Carl Banks]

Bob van der Poel wrote:
> 
> I'm writing a program which needs to convert note lengths specified in
> somewhat standard musical notation to midi ticks. I have a function
> which does this with
> a dictionary lookup. An abreviated version is shown here:
> 
> def getNoteLen(x):
>   global TicksQ
>   ntb = { '1': TicksQ *
> 4,                                                               
>      '2': TicksQ * 2,                                   
>      '4': TicksQ,
>      '8': TicksQ
> }                                                                  
>
>   return ntb[str(x)]  
> 
> What I'm concerned about is the initialization of the table. Am I
> correct in assuming that each time I access the function the values in
> the table will be recalculated?

Yep.  Not only that, but the dictionary itself will be recreated every
time the function is called.  For this example, that could be more
time consuming than the arithmetic.

 
> I suppose I could make the table global and avoid this?

That's what I suggest you do.  Globals are not evil when used to store
a table of unchanging data.


> I could avoid runtime math by replacing TicksQ with a value, but I've
> always been taught not to use literal magic numbers, and python doesn't
> have a DEFINE statement.

Even if you did that, Python would still regenerate the dictionary
every function call.  That is far from efficient.  I suggest you keep
TicksQ and just make the table global.

There is one other optimization you can make.  Dictionaries do not
require strings as keys; you can use integers.  So you can do this:

ntb = { 1: TicksQ * 4,
        2: TicksQ * 2,
        3: TicksQ,
        4: TicksQ }

def getNoteLen(x):
    return ntb[x]

And, because getNoteLen is now a trivial function, I would get rid of
it and just use the dictionary directly.


-- 
CARL BANKS