maximum() efficency

[Mitja Trampus]

Steve R. Hastings wrote:
> I was looking at a Python function to find the maximum from a list. 
> The original was more complicated; I simplified it.  The built-in max()
> function can replace the simplified example, but not the original.

But you forgot to shuw us the original...

[snip several implementations]

> Of course, it's a little ugly to use a[0] instead of "maxval".  And I'm
> trying not to index a list, and here I am indexing another list.  So:
> 
> def maximum(lst):
>   for v in lst:
>     try:
>       if maxval > v:
>         maxval = v
>     except NameError:
>       maxval = v
>   return maxval
> 
> In my testing on my computer, the above function works perfectly.  Even
> if you do this:
> 
>>>> maxval = 100
>>>> print maximum(cmp, [2, 0, 3, 1])
> 3
> 
> In other words, you get a NameError in the function even if there is a
> global variable with that name.  That's with Python 2.4, however... is
> this function guaranteed to work in all Python versions?  

Not 100% sure, but I think yes. People with longer experience in python can answer 
definitely. Or you can try it out yourself.
The reason it works is that this:
	def f():
		print a
	a=7; f()
works and prints 7 (global a is used), while this
	def f():
		print a
		a = 12
	a=7; f()
does NOT work. Python notices that a is going to get assigned to inside f and treats it as 
a local variable. This is also the case with your code. Try out the above examples without 
a=7 and notice the different error messages...

> The clear winner was the iterator version. It was much faster than the
> others, and in my opinion it is simpler and easier to understand than any
> of the others.

I would have done it in the same way, but probably without the iterators. I.e., like this:

def maximum(lst):
	try: m = lst[0]
	except (TypeError, IndexError): raise Exception "Non-sequence or empty sequence given to 
maximum")

	# (you forgot the above sanity checks in your code.
	# not strictly necessary, but nice to have.)

	for x in lst:
		if x>m: m=x
	return m