Iterator question

[Alex__655321]

Hello All

I hope I'm correct posting an STL question here - if not feel free to
direct me to somewhere more appropriate.

I'm writing some code using an std::set which I believe is the best
container to use for this particular problem.

However I have a case where I need to iterate through the set at an
arbitrary starting point and traverse all other elements in it.

For example - if I have a list of 5 elements and wished to start at
element 3, I would need my iterator to go

3,
4.
5,
1,
2

I am wondering if there is a standard mechanism for doing this or
maybe another container type that may be appropriate or am I better
off using a bidirectional iterator and handling the traversal myself?
I'm asking from an efficiency perspective.

Thank you for taking the time to read this.

Alex...

 

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

Hello All

I hope I'm correct posting an STL question here - if not feel free to
direct me to somewhere more appropriate.

I'm writing some code using an std::set which I believe is the best
container to use for this particular problem.

However I have a case where I need to iterate through the set at an
arbitrary starting point and traverse all other elements in it.

For example - if I have a list of 5 elements and wished to start at
element 3, I would need my iterator to go

3,
4.
5,
1,
2

I am wondering if there is a standard mechanism for doing this or
maybe another container type that may be appropriate or am I better
off using a bidirectional iterator and handling the traversal myself?
I'm asking from an efficiency perspective.

Thank you for taking the time to read this.

If you have a way to get an iterator to the element in question (3 in
your example) then you can always do something like this (though it
might not be the most elegant way it'll work):

std::set<int>::iterator it = getIterator(set, 3); // Get the iterator
for (; it != set.end(); ++it)
/*... */

std::set<int>::iterator it2;
for (it2 = set.begin(); it2 != it; ++it2)
/* same thing as above */

 

[Victor Bazarov]

If you have a way to get an iterator to the element in question (3 in
your example) then you can always do something like this (though it
might not be the most elegant way it'll work):

std::set<int>::iterator it = getIterator(set, 3); // Get the iterator
for (; it != set.end(); ++it)
/*... */

std::set<int>::iterator it2;
for (it2 = set.begin(); it2 != it; ++it2)

OOPS... 'it' == 'end()' here... You likely wanted to preserve its value...

/* same thing as above */

Better

std::set<...>::iterator it = myset.find ..., e = myset.end();
for (size_t i = 0, s = myset.size(); i < s; ++i)
{
/* do something with 'it' */
if (++it == e)
it = myset.begin(); // loop around
}

It's a bit less efficient that two loops, but at least it's all in one
place.

V

 

[Alex__655321]

Thanks guys - this is a great help.