[LC++]STL vector push_back() and resize()

[Torsten Rennett]

Mark Phillips wrote:
> 
> Suppose I want a vector with the i-th element being i squared.
> 
> One way to do it would be as follows:
> 
>   vector<int> vec;
>   for (int i=0; i<10; i++)
>     vec.push_back(i*i);
> 
> Another way of doing it would be:
> 
>   vector<int> vec;
>   vec.resize(10);
>   for (int i=0; i<10; i++)
>     vec[i]=i*i;
> 
> Do both these methods end up with the same final result, or does
> the first method end up allocating more memory than it needs?

Yes, that's right, it depends on the actual implementation.

> Also, is one method more efficient than the other?

The first one may reallocate and the second calls the default ctor
which is not necessary, but not too bad for ints ;-) ...

I think the fastes would be to use reserve() instead of resize():
   vector<int> vec;
   vec.reserve(10);
   for (int i=0; i<10; i++)
     vec.push_back(i*i);
This prevents reallocating and no default ctor is called.

Or to give the needed size in the ctor:
   vector<int> vec(10);
   for (int i=0; i<10; i++)
     vec[i]=i*i;

> Final question.  If I do:
> 
>   vector<int> vec;
>   vec.resize(10);
>   for (int i=0; i<10; i++)
>     vec.push_back(i*i);
> 
> does this give me the wrong result.  Ie does it create a vector
> with 20 elements in it?

Just test it!
  for (vector<int>::const_iterator it = vec.begin(); 
       it != vec.end();
       ++it)
    clog << *it << endl;
(I guess you'll get 20 elements, with the first 10 all 0.)

> I think there are a few technicalities with the STL vector class
> that I don't understand properly.

I can highly recommend the following book as tutorial and reference:

        Nicolai M. Josuttis:
        The C++ Standard Library: A Tutorial and Reference
        ISBN: 0-201-37926-0
        http://www.josuttis.com/libbook/index.html
        http://cseng.aw.com/book/0,3828,0201379260,00.html

Josuttis is an active member of the C++ Standard Committee library
working group.

And from the same author there is also a rather new book about learning
C++:
In german:
        Objektorientiertes Programmieren in C++ 
        Ein Tutorial für Ein- und Umsteiger 
        Addison-Wesley Germany, 2001
        ISBN 3-8273-1771-1 
        http://www.josuttis.com/cppbuch/index.html

In english (coming soon):
        Object-Oriented Programming in C++
        http://www.josuttis.com/cppbook/index.html


I found this the best books on the topic I've ever seen.
Have a look on the listed web pages for other comments.

Torsten

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