std::vector: reserve required?

[Mike -- Email Ignored]

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.

[=?ISO-8859-1?Q?Dar=EDo_Griffo?=]

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

return 0;
}

[Kai-Uwe Bux]

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

[callumurquhart]

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

[acehreli]

On Jul 4, 7:44*am, Darío Griffo <dario.griffo.l [e-mail] 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

[acehreli]

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

[Mike -- Email Ignored]

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.

[=?ISO-8859-1?Q?Dar=EDo_Griffo?=]

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

[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,

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(-8);
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.beg in(), v.end(), std::ostream_it erator<int>(std ::cout,
" "));
}
LR

[red floyd]

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<>::reser ve 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<intv;

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;
}