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From: ajrobb@ecr.mu.oz.au (Andrew_J ROBBIE)
Subject: Re: STL and set_union algorithm question? (newbie to STL!)
Date: 1998/05/01
Message-ID: <6i9dgp$a2n$1@mulga.cs.mu.OZ.AU>#1/1
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Approved: Fergus Henderson <fjh@cs.mu.oz.au>
References: <6g2te9$a7n$1@ukwsv3.ggr.co.uk>
X-Original-Date: 30 Apr 1998 08:42:01 GMT
Organization: Computer Science, The University of Melbourne
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Newsgroups: comp.std.c++


NJ Biggs (njb1441@ggr.co.uk) wrote:
: I am new to STL and am after a little help.

Join the club...

: Can anyone please post a short example of using the set_union algorithm on
: two set<int> objects that places the result in a third set<int>.

: Sort of...
: set<int> s1;
: set<int> s2;
: set<int> result;

: set_union(s1.begin(), s1.end(), s2.begin(), s2.end(), result.end());

Firstly, you should have the destination iterator (the 5th arg) being at
the start of where you wish the output to go. Secondly, the container must
have space to put the items in. Thirdly, I can't see how it can work for
ordered containers.

Re-forming your code:
    set<int> s1;
    set<int> s2;
    vector<int> result(s1.size() + s2.size());
    vector<int>::const_iterator it_last;
    ostream_iterator it_out(cout, "\n");

    it_last = set_union(s1.begin(), s1.end(),
        s2.begin(), s2.end(), result.begin());

    // Display the result:
    copy(result.begin(), it_last, it_out);

    // or make a new set:
    set<int> s3(result.begin(), it_last);

In this code, I made the destination big enough to hold the largest
possible number of elements. It seems terribly kludgy though.

I wish there was a better way to do this though. What I would like is to
have some pseudo-iterator which knows about the whole container, so can
therefore call container->insert() when the set_union() code assigns to
it. In fact, I would like to be able to do what ostream_iterator does:

    set_union(s1.begin(), s1.end(), s2.begin(), s2.end(), it_out);

Can any gurus tell me how I can get this to occur for iterators for sets,
etc ?

TIA,
Andrew
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