for( std::vector<std::string>::iterator v_iter = some_vec.begin(); v_iter < some_vec.end(); v_iter ++ )
{
// Stuff
}
Ripped off from various people [edited by - wild_pointer on September 29, 2002 4:31:47 AM]
question on iterators
Author
If I have something like
How would I figure out which iteration I am on? I suppose I could declare a counter and increment it each time, but that seems redundant.
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Ripped off from various people [edited by - wild_pointer on September 29, 2002 4:31:47 AM]
Ripped off from various people [edited by - wild_pointer on September 29, 2002 4:31:47 AM]
a) std::vector<std::string>::difference_type d = v_iter - some_vec.begin(); but that's only possible for Random Iterators (such as those for arrays, vectors and deque). Or use distance
b) calling some_vec.end() at each iteration is inefficient.
c) Prefer standard algorithms : for_each( some_vec.begin(), some_vec.end(), DoStuff ); where DoStuff it a function that is the body of your loop.
d) Or use a function object for better performance
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[edited by - Fruny on September 29, 2002 5:05:32 AM]
b) calling some_vec.end() at each iteration is inefficient.
c) Prefer standard algorithms : for_each( some_vec.begin(), some_vec.end(), DoStuff ); where DoStuff it a function that is the body of your loop.
d) Or use a function object for better performance
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[edited by - Fruny on September 29, 2002 5:05:32 AM]
quote:Original post by wild_pointer
How would I figure out which iteration I am on?
Ancillary to Fruny''s comments, you usually don''t need to know which iteration you''re on. If you absolutely must, you may use the expression ( v_iter - vec.begin() ), but only for random iterators or forward iterators in a strictly forward progressing loopm and only if the sequence held by the container is not modified.
All the conditions make it obvious that an alternative solution is a better idea.
Author
I see... I guess I''ll rethink what I''m doing. Thank you.
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Ripped off from various people
[My site|SGI STL|Bjarne FAQ|C++ FAQ Lite|MSDN|Jargon]
Ripped off from various people
what was your reason for wanting to know?
if you need to know - of course put a counter in
but it might not be necessary to know
also prefer ++v_iter as it doesn''t make a copy of the iterator and is more efficient.
peace
if you need to know - of course put a counter in
but it might not be necessary to know
also prefer ++v_iter as it doesn''t make a copy of the iterator and is more efficient.
peace
quote:Original post by wild_pointer
v_iter < some_vec.end();
Bad, bad! *slaps wrists*
Use v_iter != some_vec.end(); instead. Otherwise when you start using list iterators you''ll get into trouble, as asking whether one is ''less than'' another is not guaranteed to give you the correct order.
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quote:Original post by FrunyYou sure about that? I thought x.end() for any container is just some inlined function that returns a member variable and calculates nothing. So it''s as fast as it gets (unless the STL implementation is crappy, or I''m missing something).
b) calling some_vec.end() at each iteration is inefficient.
It has to construct the iterator and return it by value each and every time.
That''s probably less than good.
That''s probably less than good.
For readability, you should probably stick to increment/decrement counters. Plus theres the performance issue that a cache''d loop counter would be faster than some Accessor function.
If you absolutely must:
using namespace std;vector<string>::iterator iter = some_vec.begin();vector<string>::iterator end = some_vec.end();for(int i=0;iter!=end;++iter,++i){ // i will now represent the current iteration} This topic is closed to new replies.
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