Re: C++ Primer exercise 3.13
On Jul 19, 4:14 pm, Andre Kostur <nntps...@kostur.net> wrote:
=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?= <Erik-wikst...@telia.com> wrote in=
news:ToIni.4399$ZA.1859@newsb.telia.net:
On 2007-07-19 13:41, arnuld wrote:
On Thu, 19 Jul 2007 09:23:21 +0000, Erik Wikstr=F6m wrote:
Replace the check to a less than (iter < ivec.end()), since
you are increasing the iterator with 2 each step you will
step over ivec.end() if the number of elements is odd..
Caveat: this is not good general advice. Testing for != end()
is better than testing for < end() as it is how one does the
same sort of test for the other containers. May work for
vectors, won't work for the other containers.
Agreed, sort of. It's what one expects, and code should provide
the least surprises to the reader. But he needs random access
iterators for the + 1 as well, so the code won't work for other
containers anyway.
Finally, assuming that the += 2 steps past the end() iterator,
isn't it technically undefined behaviour to make the
comparison as you would be comparing a pointer to an element
within an array (one-past-the-end pointer is still considered
to be "within the array") to a pointer that isn't in the same
array (and isn't 0)?
It's not only undefined behavior; it crashes with both g++ and
VC++ when you turn on debugging options. (You should be using
the debugging options whenever possible, especially when
learning. Regretfully, code compiled with the debugging options
is not compatible with code compiled without. The debugging
options have significant runtime overhead, and a bottleneck
anywhere in the application means that you can't use them
anywhere.)
{
if((iter + 1) == ivec.end())
{
std::cout << *iter << std::endl;
Or add break; here.
I think this may be a better choice.
Using a break to leave a loop is NEVER a good choice.
Then again, I'd probably using indexing for this instead of
iteration (which does pretty much tie me to vector...). Or
perhaps use a while loop instead of a for.
There's a very nice solution with a while loop:
while ( iter != ivec.end() ) {
{
int first = *iter ++ ;
if ( iter != ivec.end() ) {
int second = *iter ++ ;
std::cout << first + second << std::endl ;
} else {
std::cout << "odd number of elements, last was: "
<< first << std::endl ;
}
}
I'm not overly enthusiastic about spreading the increments out
all over the loop, but it seems preferable here to the
alternatives. It also avoids the requirement for a random
access iterator, so you can replace vector with just about any
container. (I still rather prefer the idea of using a container
of std::pair, however, and handling the odd case at the end of
input.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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