how to join array of integers?

[Robert Kern]

Grant Edwards wrote:
> On 2007-09-15, Robert Kern <robert.kern at gmail.com> wrote:
>> Grant Edwards wrote:
>>> On 2007-09-15, Erik Jones <erik at myemma.com> wrote:
>>>
>>>>>> print ''.join([str(i) for i in [1,2,3]])
>>>>> It's better to use generator comprehension instead of LC:
>>>>>
>>>>> ",".join(str(i) for i in [1, 2, 3])
>>>> Why is that?  That entire expression must be evaluated to
>>>> obtain the result, so what is the advantage of using a
>>>> generator comprehension v. a list comprehension?
>>> The generator avoids creating the intermediate list -- it
>>> generates the intermediate values on the fly. For short
>>> sequences it probably doesn't matter much.  For a very long
>>> list it's probably noticable.
>> Not true. str.join() creates a list from the iterator if it is
>> not already a list or a tuple.
> 
> So the iterator avoids creating an intermediate list, but the
> join method goes ahead and does it anyway?

Yup.

>> In Objects/stringobject.c, look at string_join(); it calls
>> PySequence_Fast() on the argument. Looking in
>> Objects/abstract.c, we see that PySequence_Fast()
>> short-circuits lists and tuples but builds a full list from
>> the iterable otherwise.
> 
> So what's the point of iterables if code is going to do stuff
> like that when it wants to iterate over a sequence?

Some code dealing with sequences can be recast in terms of iterators of unknown
length. Some can't. Some are better cast in terms of iterators than sequences;
some aren't.

>> map() seems to reliably be the fastest option,
> 
> Which is apparently going away in favor of the slower iterator
> approach?

It's only slower in this case. Of course, the main difference is where the loop
takes place, in C or in Python. imap() appears to give the same performance as
map().

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco