Re: iterator on for loop with multiple ending conditions

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++.moderated

Date:

12 Dec 2006 17:47:30 -0500

Message-ID:

<eln0ja$6q3$1@news.datemas.de>

janeywit wrote:

I'm no expert in C++ and I've quite a while tracking down a problem
which I believe I am summarising here. If anybody can help me out I'd
be very grateful!
The following piece of code doesn't work as I expect:

       int testIndex=0;
       vector<int> testyVec;
       testyVec.push_back(1);
       testyVec.push_back(2);
       testyVec.push_back(3);
       testyVec.push_back(4);
       testyVec.push_back(5);

      typedef vector<int>::iterator vecIterator;
       vecIterator vi;

       for(vi=testyVec.begin(); testIndex<3, vi!=testyVec.end();
++vi)
       {
          cout<<"itr: testIndex is "<<testIndex<< " and testyVec item
is "<<(*vi)<<"\n";
          testIndex++;
      }

I expect the for loop to end after 3 iterations due to one of the
ending conditions "testIndex<3".

Why? The expression 'a,b' has the value of 'b', the 'a' is evaluated
and thrown out. It does not contribute to the value of the overall
expression unless 'b' somehow depends on it. In your case the '!='
part does not depend on evaluation of the '<' part, so the left-hand
side of the comma is _ignored_. You could have just omitted it.

It doesn't. It goes on until the
second ending condition is reached. In fact if I switch the order of
the ending conditions then it always ignores the first one. Am I
using incorrect syntax... is there some bug ... or what am I missing?!
(I know I can do this by putting && between the two conditions but I
thought the other syntax was also correct?)

The syntax is correct. The semantics differ.

V
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