Re: Comma operator in for loop
On 9/24/2010 9:11 AM, Ralf Goertz wrote:
Vladimir Jovic wrote:
Victor Bazarov wrote:
On 9/24/2010 5:13 AM, Ralf Goertz wrote:
I guess I don't really understand the concept of commas as sequence
points but now I am in a position where I might have use for the comma
in a statement other than a declaration. Is the following legal:
#include<iostream>
#include<set>
using namespace std;
int main() {
set<unsigned> s;
s.insert(42);
s.insert(84);
set<unsigned>::iterator i;
for (i=s.begin(),++i;i!=s.end();++i)
cout<<*i<<endl;
return 0;
}
It compiles and does what I want, but it could still be undefined
behaviour. So is it okay to use ",++i" before the first ";" of the
for-loop, or is it saver to check for i!=s.begin() in the *body* of the
loop?
It seems fine, and if it does what you need, keep it. If you just
needed to skip the first element, I'd probably do
i = ++(s.begin());
I would do this:
for (i=s.begin() + 1;i!=s.end();++i)
cout<<*i<<endl;
but it's not much different from your solution.
That was my first idea, but it doesn't compile:
error: no match for 'operator+' in 's.std::set<_Key, _Compare, _Alloc>:
:begin [with _Key = unsigned int, _Compare = std::less<unsigned int>, _Alloc = s
td::allocator<unsigned int>]() + 1'
The set iterator is not of the random-access kind, you cannot add a
number to it.
V
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