[Tutor] A list of input arguments

[Mr Gerard Kelly]

Many thanks for your helpful answer Alan.

My only other question is, does there exist a convenient way to unpack a
collection of variable length?

If you know that there are 3 elements in the collection, you can specify:

play_for(waves(chord[0],chord[1],chord[2]),500)

But then if you want to change the number of elements in the list
"chord", say from 3 to 5, you have to change the code accordingly. Is
there some sort of way to instruct the code to unpack its elements for
any number of elements?

Like 
i=len(chord)
play_for(waves(chord[0],chord[1],...,chord[i-1]),500)

If there is a way to do this it would be great!

----- Original Message -----
From: Alan Gauld <alan.gauld at btinternet.com>
Date: Tuesday, January 13, 2009 7:16 pm
Subject: Re: [Tutor] A list of input arguments
> "Mr Gerard Kelly" <s4027340 at student.uq.edu.au> wrote
> 
> >I have a problem with understanding how lists, strings, tuples, 
> >number
> > types and input arguments all interact with each other.
> 
> OK, they are all different types. Some of them take a single value
> (sometimes known as a scalar type) and others take a collection
> of values (sometimes known as a sequence). Strings are unusual
> in that they can be considered as a scalar type or as a collection
> of characters!
> 
> Input parameters are simply local variables for the function to
> which they are attached. Other than the fact that you can assign
> them values from outside the function whe you call it they act just
> like normal variables.
> 
> def f():
>   print x
> 
> x = 42
> f()
> 
> Is the same (almost)) as
> 
> def f(x):
>   print x
> 
> > As an example, if I use three arguments, it looks like this:
> >
> > def main():
> >  play_for(waves(440,550,660), 5000)
> 
> 
> > def main():
> >  chord=[440,550,660]
> >  play_for(waves(chord), 5000)
> >
> > it doesn't work.
> 
> Because you are passing a single value (a list) into a function that
> expects 3 values.
> 
> > It doesn't work with a string or a tuple either.
> 
> Because strings and tuples are also single (container )entities
> 
> You must unpack your collection when calling the function:
> 
> play_for(waves(chord[0],chord[1],chord[2]),5000)
> 
> > The problem is that another part of the code needs to take
> > float(chord[0]), that is convert the first input value into the 
> > float
> > class, and the error message says "TypeError: float() argument 
> must 
> > be a
> > string or a number."
> 
> Thats right, and as you did above you must extract the first element
> since the code can't tell that yopu have passed it a collection 
> instead
> of a number!
> 
> > Is there any way to list the input arguments without listing them 
> > inside
> > the function's parentheses?
> 
> No, because the function expects 3 arguments so you must pass it 3.
> That is the contract (or interface) that exists betweeen the writer 
> of 
> the
> function and you the consumer of it.  Programming. particularly on
> larger projects with multiple teams, is all about defining interfaces
> and adhering to them! If you want thebenefit of the function you must
> respect its contract.
> 
> HTH,
> 
> 
> -- 
> Alan Gauld
> Author of the Learn to Program web site
> http://www.freenetpages.co.uk/hp/alan.gauld
> 
> 
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