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

Torsten Rennett Torsten at Rennett.de
Thu Sep 6 21:03:39 UTC 2001

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;
   for (int i=0; i<10; 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++)

> 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();
    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

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
In german:
        Objektorientiertes Programmieren in C++ 
        Ein Tutorial für Ein- und Umsteiger 
        Addison-Wesley Germany, 2001
        ISBN 3-8273-1771-1 

In english (coming soon):
        Object-Oriented Programming in C++

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


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