Re: help understanding iterators with vector::insert

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 31 Jul 2007 00:06:42 -0700

Message-ID:

<ZcBri.61$1T2.60@newsfe04.lga>

"pwalker" <pwalker@nospam.com> wrote in message
news:46aeaaf1$1@dnews.tpgi.com.au...

Hi everyone,

I am trying to get my head around iterators used on vectors. Let's take
the code below:

-------------
std::vector<int> v1;
std::vector<int> v2;

v1.push_back( 1 );
v1.push_back( 2 );
v1.push_back( 3 );

v2.push_back( 10 );
v2.push_back( 20 );
v2.push_back( 30 );

v1.insert(v1.end(), v2.begin(), v2.begin());

for(int j = 0; j < v1.size(); j++)
std::cout << v1[j] << " ";
-------------

I'm confused why we dont get the output 1 2 3 10

Doesn't the insert method insert to the back of v1 everything between
v2.begin() and v2.begin(), which is namely 10?

I realise that to get the desired result I need to instead use
v1.insert(v1.end(), v2.begin(), v2.begin()+1) but i fail to understand the
logic in this.

I know that v2.begin() points to 10, and v2.end() (which is the same as
v2.begin()+3) points to one-beyond-30.

I guess all it comes down to is if I specify v1.insert(v1.end(),
v2.begin(), v2.begin()+N) it will append to the back of v1 (namely as
position one-beyond-3) the first N values of v2.

Any help on understanding this whole concept would be great!

Because the third paramater is one past the one to be inserted, the reason
being so you can use .end() which is one past the end of a container. Thsi
way we can code:

v1.insert( v1.end(), v2.begin(), v2.end() );
which is more common and not have to do
v1.insert( v1.end(), v2.begin(), v2.end() - 1 );