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In std::vector, is reserve or resize required?

On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

The program below fails, but if the reserve(en)
is uncommented, it works. Is this as expected?

// vectst.cc 07/04/08

#include <iostream>
#include <vector>
using namespace std;

int main(int argc, const char* argv[])
{
int en = 10;
vector<int> vec;

// vec.reserve(en);
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
for (int jj = 0; jj < en; ++jj)
cout << vec[jj] << endl;

exit (0);
}

Thanks,
Mike.

Mike -- Email Ignored wrote:
> In std::vector, is reserve or resize required?
>
> On:
> Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
> Jul 27 18:10:34 EDT 2007 i686 athlon
> i386 GNU/Linux
> Using:
> g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)
>
> The program below fails, but if the reserve(en)
> is uncommented, it works. Is this as expected?
>
> // vectst.cc 07/04/08
>
> #include <iostream>
> #include <vector>
> using namespace std;
>
> int main(int argc, const char* argv[])
> {
> int en = 10;
> vector<int> vec;
>
> // vec.reserve(en);
> for (int jj = 0; jj < en; ++jj)
> vec[jj] = jj;
> for (int jj = 0; jj < en; ++jj)
> cout << vec[jj] << endl;
>
> exit (0);
> }
>
> Thanks,
> Mike.


No, you could do that (use reserve), or, use push_back()

int main(int argc, const char* argv[])
{
int en = 10;
vector<int> vec;

for (int jj = 0; jj < en; ++jj)
vec.push_back(jj);
for (int jj = 0; jj < en; ++jj)
cout << vec[jj] << endl;

return 0;
}

Mike -- Email Ignored wrote:

> In std::vector, is reserve or resize required?

No, but you need to fill the vector somehow. You can do that either upon
construction, or by using push_back, insert, or resize.

The reserve() method, on the other hand does not grow the vector.

>
> On:
> Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
> Jul 27 18:10:34 EDT 2007 i686 athlon
> i386 GNU/Linux
> Using:
> g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)
>
> The program below fails, but if the reserve(en)
> is uncommented, it works.

It does not. You are observing a manifestation of undefined behavior.

> Is this as expected?

There are no expectations as to how undefined behavior will show itself.

From a quality of implementation point of view, would want to see an abort
if you compile the program with assertions turned on.


> // vectst.cc 07/04/08
>
> #include <iostream>
> #include <vector>
> using namespace std;
>
> int main(int argc, const char* argv[])
> {
> int en = 10;
> vector<int> vec;
>
> // vec.reserve(en);
> for (int jj = 0; jj < en; ++jj)
> vec[jj] = jj;
> for (int jj = 0; jj < en; ++jj)
> cout << vec[jj] << endl;

Try something like

cout << vec.size() << endl;

and ponder the meaning of what you get.

>
> exit (0);
> }


Best

Kai-Uwe Bux

http://www.cplusplus.com/reference/stl/vector/operator[].html

On Jul 4, 7:44*am, Darío Griffo <dario.griffo.lis...@gmail.com> wrote:
> Mike -- Email Ignored wrote:
>
>
>
> > In std::vector, is reserve or resize required?
>
> > On:
> > * *Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
> > * * * Jul 27 18:10:34 EDT 2007 i686 athlon
> > * * * i386 GNU/Linux
> > Using:
> > * *g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)
>
> > The program below fails, but if the reserve(en)
> > is uncommented, it works. *Is this as expected?
>
> > // vectst.cc 07/04/08
>
> > #include <iostream>
> > #include <vector>
> > using namespace std;
>
> > int main(int argc, const char* argv[])
> > {
> > * *int * * * * * * * * *en = 10;
> > * *vector<int> * * * * *vec;
>
> > // * vec.reserve(en);
> > * *for (int jj = 0; jj < en; ++jj)
> > * * * vec[jj] = jj;
> > * *for (int jj = 0; jj < en; ++jj)
> > * * * cout << vec[jj] << endl;
>
> > * *exit (0);
> > }
>
> > Thanks,
> > Mike.
>
> No, you could do that (use reserve), or, use push_back()

reserve() does not make the vector larger. You must have meant "you
could use resize or push_back."

Ali

On Jul 4, 7:54*am, callumurquh...@googlemail.com wrote:
> http://www.cplusplus.com/reference/stl/vector/operator[].html

If the OP did not concentrate on the vec.reserve(en), the document you
show could be used to help with the problem. But because the OP does
expect some behavior from vec.reserve(en), the problem is not with
operator[].

Ali

On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:

> On Jul 4, 7:54Â*am, callumurquh...@googlemail.com wrote:
>> http://www.cplusplus.com/reference/stl/vector/operator[].html
>
> If the OP did not concentrate on the vec.reserve(en), the document you
> show could be used to help with the problem. But because the OP does
> expect some behavior from vec.reserve(en), the problem is not with
> operator[].
>
> Ali

Another test using:
vec.reserve(en);
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
cout << "vec.size() = " << vec.size() << endl;

prevents the crash, but vec.size() returns zero, showing that
in this case, reserve() really does not work.

Mike.

acehr...@gmail.com wrote:

> reserve() does not make the vector larger. You must have meant "you
> could use resize or push_back."
>
> Ali

Yes, you're right

Mike -- Email Ignored wrote:
> On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:
>
>> On Jul 4, 7:54 am, callumurquh...@googlemail.com wrote:
>>> http://www.cplusplus.com/reference/stl/vector/operator[].html
>> If the OP did not concentrate on the vec.reserve(en), the document you
>> show could be used to help with the problem. But because the OP does
>> expect some behavior from vec.reserve(en), the problem is not with
>> operator[].
>>
>> Ali
>
> Another test using:
> vec.reserve(en);
> for (int jj = 0; jj < en; ++jj)
> vec[jj] = jj;
> cout << "vec.size() = " << vec.size() << endl;
>
> prevents the crash, but vec.size() returns zero,

What does vec.capacity() return?

> showing that
> in this case, reserve() really does not work.

I think it doesn't do what you expect. But why do you think it doesn't work?

Please consider this:
#include <iostream>
#include <vector>

int main() {
const unsigned int en = 10;
std::vector<int> v;
//
std::cout << "a " << v.size() << " " << v.capacity() << std::endl;
v.reserve(en);
std::cout << "b " << v.size() << " " << v.capacity() << std::endl;
//
for(unsigned int i=0; i<en; i++) {
v[i] = i;
}
//
v.push_back(-;
std::cout << "c " << v.size() << " " << v.capacity() << std::endl;
//
for(unsigned int i=0; i<en; i++) {
std::cout << "[" << i << "] " << v[i] << std::endl;
}
}


Also, please consider this:
#include <iostream>
#include <vector>

int main() {
const unsigned int en = 10;
std::vector<int> v;
std::cout << "a " << v.size() << " " << v.capacity() << std::endl;
v.resize(en); // there's more than one way to do it
std::cout << "b " << v.size() << " " << v.capacity() << std::endl;
//
v[0] = -8;
v[9] = -7;
v.push_back(-9);
std::cout << "c " << v.size() << " " << v.capacity() << std::endl;
std::copy(v.begin(), v.end(), std:stream_iterator<int>(std::cout,
" "));
}


LR

Mike -- Email Ignored wrote:
> On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:
>
>> On Jul 4, 7:54 am, callumurquh...@googlemail.com wrote:
>>> http://www.cplusplus.com/reference/stl/vector/operator[].html
>> If the OP did not concentrate on the vec.reserve(en), the document you
>> show could be used to help with the problem. But because the OP does
>> expect some behavior from vec.reserve(en), the problem is not with
>> operator[].
>>
>> Ali
>
> Another test using:
> vec.reserve(en);
> for (int jj = 0; jj < en; ++jj)
> vec[jj] = jj;
> cout << "vec.size() = " << vec.size() << endl;
>
> prevents the crash, but vec.size() returns zero, showing that
> in this case, reserve() really does not work.
>

No, reserve() does work. You misunderstand how it does.
vector<>::reserve changes the CAPACITY -- that is, you can use
push_back() or resize() up to the amount you've reserved without
the vector reallocating. It does not change the SIZE of the vector.

Try this:

#include <iostream>
#include <ostream>
#include <vector>

using namespace std;

int main()
{
vector<int> v;

cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << "\n";

v.reserve(200);

cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << "\n";

v.resize(200);

cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << endl;

return 0;
}