Jay Savage wrote:
> On 3/3/07, John W. Krahn <[EMAIL PROTECTED]> wrote:
>> Jay Savage wrote:
>> > On 3/2/07, Robert Boone <[EMAIL PROTECTED]> wrote:
>> >> I think this is all you do:
>> >>
>> >> $piid = (split(/\t/, $row))[0];
>> >
>> > Split also takes an optional limit that keeps it from splitting the
>> > string into more than n parts. This keeps spilt from performing
>> > useless operations when you only want the first n-1 items, or when you
>> > want to lump all the items >= n into a single lump:
>> >
>> > $piid = (split(/\t/,$row,2)[0];
>> >
>> > Most of the time it probably doesn't matter, but adding a limit will
>> > be markedly more efficient if $row is particularly long or you are
>> > looping through an extremely long list of rows.
>> >
>> > As always, see perldoc -f split for the details.
>>
>> perldoc -f split
>>
>> [ snip ]
>>
>> The LIMIT parameter can be used to split a line partially
>>
>> ($login, $passwd, $remainder) = split(/:/, $_, 3);
>>
>> When assigning to a list, if LIMIT is omitted, or zero, Perl
>> supplies a LIMIT one larger than the number of variables in
>> the list, to avoid unnecessary work. For the list above
>> LIMIT would have been 4 by default. In time critical
>> applications it behooves you not to split into more fields
>> than you really need.
>>
>>
>> The limit is supplied automagically if the size of the list is know at
>> compile time like in your example above so using the limit argument is
>> superfluous.
>
> I read the doc to say that, given a list of size n, perl will perform
> n + 1 splits by default. limit has diminishing returns as n increases,
> but for a list of length one, not supplying a limit means double the
> work, since (n + 1) = 2n when n is 1.
>
> Furthermore, it's not clear to me what the default limit is in the
> case of a slice. Consider
>
> $piid = (split(/\t/,$row)[-1];
> $piid = (split(/\t/,$row)[4];
>
> It seems to me that in the case of a slice, split must split the
> entire string, and then return the appropriate element. Wanting a
> single element and wanting the first element are two differnt things.
>
> Maybe the compiler optimizes for the case of a slice with index [0]?
>
> It may, but it's not obvious to me from the docs that it does.
Yes, it appears that a list slice does not invoke the optimization:
$ perl -MO=Concise,-terse -e'
($x,$y) = split /:/, q[one:two:three:four:five:six:seven:eight]
'
LISTOP (0x818a978) leave [1]
OP (0x818b0c8) enter
COP (0x8170958) nextstate
BINOP (0x818aa10) aassign [4]
UNOP (0x818aa78) null [141]
OP (0x824e340) pushmark
LISTOP (0x818ab80) split [3]
PMOP (0x827fca8) pushre
SVOP (0x818a9c0) const [5] PV (0x81877b8)
"one:two:three:four:five:six:seven:eight"
SVOP (0x818a240) const [6] IV (0x816cd04) 3
UNOP (0x818ace0) null [141]
OP (0x8170900) pushmark
UNOP (0x818ac88) null [15]
PADOP (0x818abb8) gvsv GV (0x8187794) *x
UNOP (0x81708a8) null [15]
PADOP (0x818a9f0) gvsv GV (0x8187710) *y
-e syntax OK
$ perl -MO=Concise,-terse -e'
($x,$y) = ( split /:/, q[one:two:three:four:five:six:seven:eight] )[0,1]
'
LISTOP (0x818c4b8) leave [1]
OP (0x824e038) enter
COP (0x8170958) nextstate
BINOP (0x8170860) aassign [4]
UNOP (0x818a988) null [141]
OP (0x824e058) pushmark
BINOP (0x818aa20) lslice
UNOP (0x818ab90) null [141]
OP (0x824d688) pushmark
SVOP (0x824e358) const [5] IV (0x8187798) 0
SVOP (0x818b0d8) const [6] IV (0x816cce0) 1
UNOP (0x818aa88) null [141]
OP (0x824e078) pushmark
LISTOP (0x8205c78) split [3]
PMOP (0x8170818) pushre
SVOP (0x818a9d0) const [7] PV (0x81877c8)
"one:two:three:four:five:six:seven:eight"
SVOP (0x818a250) const [8] IV (0x8187738) 0
UNOP (0x818acf0) null [141]
OP (0x8170900) pushmark
UNOP (0x818ac98) null [15]
PADOP (0x818abc8) gvsv GV (0x81877a4) *x
UNOP (0x81708a8) null [15]
PADOP (0x818aa00) gvsv GV (0x8187720) *y
-e syntax OK
You will notice that the third argument to split using the list slice is 0.
John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall
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