Re: help understanding iterators with vector::insert

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++

Date:

Tue, 31 Jul 2007 11:59:55 +0200

Message-ID:

<5h8fgjF3jn03jU1@mid.individual.net>

Jim Langston wrote:
:: "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 );

And also, the first version works even if v2 is empty. The last one
does definitely not!

Bo Persson

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