Re: std::for_each + break
On Nov 22, 10:14 am, Kai-Uwe Bux <jkherci...@gmx.net> wrote:
terminator wrote:
[...]
I guess, the way to do this idiomatically in STLeese is something like:=
std::for_each( begin, std::find_if( begin, end, condition ), action )=
;
Now, I see that the OP did not specify whether there is a
need for performing an action on the elements of the
sequence or whether the task at hand is simply to find the
first where a condition holds. In the later case,
std::find_if is of course perfectly fine.
that is double looping :too much run time.
Before you come to that conclusion, you might want to measure.
Note that during the first loop, only the condition is
evaluated and during the second loop only the action is
evaluated. That means, as long as incrementing iterators is
cheap compared to evaluating the condition and performing the
action, the double looping is should not be expected to be
significantly more expensive.
Moreover, optimization in compilers has come a long way and I
wouldn't be surprised if the compiler would actually fold the
two loops into one (especially since the first is only used to
determine the upper bound for the second).
More likely it won't. You have two loops, each of which fits
into a cache line. Replacing it with a single, larger loop
which no longer fits into the cache line is *not* an
optimization.
Of course, the compiler knows about things like cache line
width, and will do the optimization if it improves things, and
not do it if it would slow them down.
[...]
Upthread, I suggested:
for ( ... ) {
...
}
and I still find it readable and fast.
And a really good compiler might even break it into two loops,
if that would speed things up:-).
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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