array and vector

[Michael]

Hi,

I could use vector to get an array to random_shuffle it. Is it possible
to define an array to random_shuffle it? I tried but it did not work.
Can I use array[] instead of vector to use algorithm?

Could you please help me out? Thanks.

Michael

1. vector (works)

#include <iostream>
#include <vector>
#include <algorithm>

using std::cout;

void show_val(int x)
{
cout<<x<<endl;
}

int main()
{

vector<inta;

for (int i=1; i<11;i++)
a.push_back(i);

random_shuffle( a.begin(), a.end());

for_each(a.begi n(), a.end(), show_val);

return 0;

}

2. array (not work)

#include <iostream>
#include <vector>
#include <algorithm>

using std::cout;

void show_val(int x)
{
cout<<x<<endl;
}


int main()
{

int a [100];

for (int i=0; i<100;i++)
a[i]=i+1;

random_shuffle( a.begin(), a.end());

for_each(a.begi n(), a.end(), show_val);

return 0;

}

[Alf P. Steinbach]

Re: array and vector

* Michael:

>
int main()
{
>
int a [100];
>
for (int i=0; i<100;i++)
a[i]=i+1;
>
random_shuffle( a.begin(), a.end());

A raw array does not have member functions.

You could write

random_shuffle( a, a+100);

But raw arrays and pointers have numerous pitfalls for the novice (and
for the experienced): in general it's not a good idea to use raw arrays,
no matter how experienced one is, and given that you assumed there were
member functions, in particular it's not a good idea for you to use raw
arrays, except to learn about them.

std::vector is much more safe.

Additional suggestion, not related to the code above: instead of writing
v[i], write v.at(i), which performs a range check and so is more safe.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Michael]

Re: array and vector


Alf P. Steinbach wrote:

* Michael:

int main()
{

int a [100];

for (int i=0; i<100;i++)
a[i]=i+1;

random_shuffle( a.begin(), a.end());

>
A raw array does not have member functions.
>
You could write
>
random_shuffle( a, a+100);
>
But raw arrays and pointers have numerous pitfalls for the novice (and
for the experienced): in general it's not a good idea to use raw arrays,
no matter how experienced one is, and given that you assumed there were
member functions, in particular it's not a good idea for you to use raw
arrays, except to learn about them.
>
std::vector is much more safe.
>
Additional suggestion, not related to the code above: instead of writing
v[i], write v.at(i), which performs a range check and so is more safe.
>
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

Thanks for your advice. I tried the following code by using at(). Why
not working?

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>

using std::cout;


int main(void)
{

vector<intv( 5, 1 );
for( int i = 0; i < 10; i++ ) {
cout << "Element " << i << " is " << v.at(i) << endl;
}

}

Thanks in advance,
Michael

[Alf P. Steinbach]

Re: array and vector

* Michael:

[Quoting signature, overquoting]

Please don't quote signatures -- corrected.

Also, please quote only the relevant parts you're replying to.


* Michael:

* Alf P. Steinbach:

>Additional suggestion, not related to the code above: instead of writing
>v[i], write v.at(i), which performs a range check and so is more safe.

>
Thanks for your advice. I tried the following code by using at(). Why
not working?
>
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
>
using std::cout;
>
>
int main(void)
{
>
vector<intv( 5, 1 );
for( int i = 0; i < 10; i++ ) {
cout << "Element " << i << " is " << v.at(i) << endl;
}
}

When corrected so that it compiles (you need to write std::vector and
std::endl) this program has Undefined Behavior. But, due to the use of
std::vector::at instead of operator[] or a raw array, that UB can be
easily converted to predictable behavior -- see below. Btw., the
'void' argument list is C'ism that's better a-voided in C++. ;-)

The program indexes the vector beyond its current size, using 'at'. The
result of that is an exception. Here that exception results in
Undefined Behavior (most probably a crash) because you don't catch the
exception -- look up the 'try' and 'catch' keywords.

With a raw array you would have Undefined Behavior with no means of
turning it into something predictable & well defined.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Michael]

Re: array and vector


Alf P. Steinbach wrote:

When corrected so that it compiles (you need to write std::vector and
std::endl) this program has Undefined Behavior. But, due to the use of
std::vector::at instead of operator[] or a raw array, that UB can be
easily converted to predictable behavior -- see below. Btw., the
'void' argument list is C'ism that's better a-voided in C++. ;-)
>
The program indexes the vector beyond its current size, using 'at'. The
result of that is an exception. Here that exception results in
Undefined Behavior (most probably a crash) because you don't catch the
exception -- look up the 'try' and 'catch' keywords.
>
With a raw array you would have Undefined Behavior with no means of
turning it into something predictable & well defined.
>

Thanks for your help! However, even if I change the upbound of i, it
still did not work. Confused...

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>

using std::cout;

int main(void)
{

vector<intv( 5, 1 );
for( int i = 0; i < 10; i++ ) {
cout << "Element " << i << " is " << v.at(i) << endl;
}

}

Thanks,
Michael

[Michael]

Re: array and vector


Michael wrote:

Thanks for your help! However, even if I change the upbound of i, it
still did not work. Confused...
>
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
>
using std::cout;
>
int main(void)
{
>
vector<intv( 5, 1 );
for( int i = 0; i < 3; i++ ) {
cout << "Element " << i << " is " << v.at(i) << endl;
}
>
}
>

Sorry about the typo, the upbound changed to 3, it did not work either.

Michael

[Alf P. Steinbach]

Re: array and vector

* Michael:

* Michael:
>

>Thanks for your help! However, even if I change the upbound of i, it
>still did not work. Confused...
>>
>#include <iostream>
>#include <vector>
>#include <algorithm>
>#include <cstring>
>>
>using std::cout;
>>
>int main(void)
>{
>>
> vector<intv( 5, 1 );
> for( int i = 0; i < 3; i++ ) {
> cout << "Element " << i << " is " << v.at(i) << endl;
> }
>>
>}
>>

Sorry about the typo, the upbound changed to 3, it did not work either.

1. See the FAQ item "How do I post a question about code that doesn't
work correctly?", currently at <url:
http://www.parashift.c om/c++-faq-lite/how-to-post.html#faq-5.8>. In
particular, "did not work" says nothing about what you expected, and
what happened or did not happen contrary to that expectation.

2. Remove unnecessary headers, <algorithmand <cstring>.

3. Add necessary header <ostream(althou gh your compiler probably does
not require it).

4. Remove the 'using' directive.

5. Remove the 'void' C'ism.

6. Add 'std::' before 'vector', 'cout' and 'endl'.

7. See the FAQ item "Should I use using namespace std in my code?",
currently at <url:
http://www.parashift.c om/c++-faq-lite/coding-standards.html# faq-27.5>.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Michael]

Re: array and vector


Alf P. Steinbach wrote:

>
2. Remove unnecessary headers, <algorithmand <cstring>.
>
3. Add necessary header <ostream(althou gh your compiler probably does
not require it).
>
4. Remove the 'using' directive.
>
5. Remove the 'void' C'ism.
>
6. Add 'std::' before 'vector', 'cout' and 'endl'.
>
7. See the FAQ item "Should I use using namespace std in my code?",
currently at <url:
http://www.parashift.c om/c++-faq-lite/coding-standards.html# faq-27.5>.

Thanks for your advice!
The error I got was:
no matching function for call to `vector<int,all ocator<int::at (int
&)'

I did not see anything wrong with my code though. Could you please help
me?

Many thanks,
Michael

[Salt_Peter]

Re: array and vector


Michael wrote:

Alf P. Steinbach wrote:

2. Remove unnecessary headers, <algorithmand <cstring>.

3. Add necessary header <ostream(althou gh your compiler probably does
not require it).

4. Remove the 'using' directive.

5. Remove the 'void' C'ism.

6. Add 'std::' before 'vector', 'cout' and 'endl'.

7. See the FAQ item "Should I use using namespace std in my code?",
currently at <url:
http://www.parashift.c om/c++-faq-lite/coding-standards.html# faq-27.5>.

>
Thanks for your advice!
The error I got was:
no matching function for call to `vector<int,all ocator<int::at (int
&)'
>
I did not see anything wrong with my code though. Could you please help
me?
>
Many thanks,
Michael

did you not even read the post above?
there is no such thing as a vector.

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

int main()
{
const int sz(10);
std::vector<int v( sz, 1 );

for( int i = 0; i < sz; i++ )
{
std::cout << "v[ " << i << " ] = " << v.at(i);
std::cout << std::endl;
}
return 0;
}

___
Now lets generate an exception and then catch it...
there is no 6th element in the vector.

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

int main()
{
std::vector<int v( 5, 1 );

try
{
for( int i = 0; i < 6; i++ )
{
std::cout << "v[ " << i << " ] = " << v.at(i);
std::cout << std::endl;
}
}
catch ( const std::exception& e )
{
std::cout << "\n*** error: " << e.what();
std::cout << std::endl;
}
return 0;
}

/*

v[ 0 ] = 1
v[ 1 ] = 1
v[ 2 ] = 1
v[ 3 ] = 1
v[ 4 ] = 1

*** error: invalid vector<Tsubscri pt

*/