Stuctures of variable length

[steflhermitte]

Dear cpp-ians,

I am working with a structure:

struct meta_segment
{
long double id;
long double num;
long double mean;
bool done;
};

but I want to store multiple elements for some of the elements of my
structure. I was thinking of using arrays in my structure. E.g., when I
have 5 elements for 'num' and 'mean':

struct meta_segment
{
long double id;
long double num[5];
long double mean[5];
bool done;
};

The problem is that only at run-time the program knows how long my
'num' and 'mean' will be. So what I want to do is make a structure,
where I can incorporate the length into the structure and use that
length as an argument.

struct meta_segment
{
long double id;
long double num[NbElements];
long double mean[NbElements];
bool done;
};

I something like this possible? Or should I look for other solutions to
solve this problem?

Thank you very much in advance,
Stef

[Allan Bruce]

"steflhermi tte" <st************ ***@agr.kuleuve n.ac.be> wrote in message
news:11******** *************@g 14g2000cwa.goog legroups.com...

Dear cpp-ians,

I am working with a structure:

struct meta_segment
{
long double id;
long double num;
long double mean;
bool done;
};

but I want to store multiple elements for some of the elements of my
structure. I was thinking of using arrays in my structure. E.g., when I
have 5 elements for 'num' and 'mean':

struct meta_segment
{
long double id;
long double num[5];
long double mean[5];
bool done;
};

The problem is that only at run-time the program knows how long my
'num' and 'mean' will be. So what I want to do is make a structure,
where I can incorporate the length into the structure and use that
length as an argument.

struct meta_segment
{
long double id;
long double num[NbElements];
long double mean[NbElements];
bool done;
};

I something like this possible? Or should I look for other solutions to
solve this problem?

Thank you very much in advance,
Stef

Why dont you use a class and allocate the memory in the constructor? You
dont have to add any methods, and making the variables public means you can
use the class in exactly the same way you would use your structure.
Allan

[Karl Heinz Buchegger]

steflhermitte wrote:

Dear cpp-ians,
[snip]
I something like this possible? Or should I look for other solutions to
solve this problem?

You want a dynamically sizeable array.
Use std::vector for this.

#include <vector>

struct meta_segment
{
long double id;
std::vector< long double > num;
std::vector< long double > mean;
bool done;
};
Now the num and mean members can grow dynamically as needed.

int main()
{
meta_segment TheData;

TheData.num.pus h_back( 5.0 ); // add 1 entry to num
TheData.num.pus h_back( 7.0 ); // and another one

for( int i = 0; i < 200; ++i ) // what the heck ...
TheData.num.pus h_back( i ); // ... add a lot of them
}

--
Karl Heinz Buchegger
kb******@gascad .at

[Rolf Magnus]

steflhermitte wrote:

Dear cpp-ians,

I am working with a structure:

struct meta_segment
{
long double id;
long double num;
long double mean;
bool done;
};

but I want to store multiple elements for some of the elements of my
structure. I was thinking of using arrays in my structure. E.g., when I
have 5 elements for 'num' and 'mean':

struct meta_segment
{
long double id;
long double num[5];
long double mean[5];
bool done;
};

The problem is that only at run-time the program knows how long my
'num' and 'mean' will be. So what I want to do is make a structure,
where I can incorporate the length into the structure and use that
length as an argument.

struct meta_segment
{
long double id;
long double num[NbElements];
long double mean[NbElements];
bool done;
};

I something like this possible? Or should I look for other solutions to
solve this problem?

You can use std::vector:

#include <vector>
#include <iostream>

struct meta_segment
{
long double id;
std::vector<lon g double> num;
std::vector<lon g double> mean;
bool done;
};

int main()
{
meta_segment seg;
seg.num.push_ba ck(12345);
seg.num.push_ba ck(54321);
std::cout << seg.num[1] << '\n';
}

[Shezan Baig]

steflhermitte wrote:

The problem is that only at run-time the program knows how long my
'num' and 'mean' will be. So what I want to do is make a structure,
where I can incorporate the length into the structure and use that
length as an argument.

use std::vector

[Robbie Hatley]

"steflhermi tte" <st************ ***@agr.kuleuve n.ac.be> wrote in message
news:11******** *************@g 14g2000cwa.goog legroups.com...

Dear cpp-ians,

I am working with a structure:

struct meta_segment
{
long double id;
long double num;
long double mean;
bool done;
};

but I want to store multiple elements for some of the elements of my
structure. I was thinking of using arrays in my structure. E.g., when I
have 5 elements for 'num' and 'mean':

struct meta_segment
{
long double id;
long double num[5];
long double mean[5];
bool done;
};

The problem is that only at run-time the program knows how long my
'num' and 'mean' will be. So what I want to do is make a structure,
where I can incorporate the length into the structure and use that
length as an argument.

struct meta_segment
{
long double id;
long double num[NbElements];
long double mean[NbElements];
bool done;
};

I something like this possible? Or should I look for other solutions to
solve this problem?

Thank you very much in advance,
Stef

Arrays in structs or classes are a poor idea.
(No copy constructor, fixed size, etc.)

If you need variable-length members in structs,
use std::vector instead of arrays:

#include <iostream>
#include <iomanip>
#include <vector>
using std::vector;
using std::cout;
using std::endl;
using std::setprecisi on;
struct meta_segment
{
long double id;
std::vector<lon g double> num;
std::vector<lon g double> mean;
bool done;
};
int main()
{
meta_segment S;
S.num.push_back (3278.2054);
S.num.push_back (7284.0355);
S.mean.push_bac k(6342.3967);
S.mean.push_bac k(3968.2853);
cout << "S.num[0] = " << setprecision(10 ) << S.num[0] << endl;
cout << "S.num[1] = " << setprecision(10 ) << S.num[1] << endl;
cout << "S.mean[0] = " << setprecision(10 ) << S.mean[0] << endl;
cout << "S.mean[1] = " << setprecision(10 ) << S.mean[1] << endl;
return 0;
}
--
Cheers,
Robbie Hatley
Tustin, CA, USA
email: lonewolfintj at pacbell dot net
web: home dot pacbell dot net slant earnur slant

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[John Carson]

"Robbie Hatley" <lonewolfintj at pacbell dot net> wrote in message
news:42******** **@spool9-west.superfeed. net

Arrays in structs or classes are a poor idea.
(No copy constructor, fixed size, etc.)

What do you mean by no copy constructor? The array itself doesn't have one
of course, but array members are successfully copied when one struct
containing an array is used to initialise another, e.g.,

#include <iostream>

struct Test
{
int x[5];
};

int main()
{
Test t1;
for(int i=0; i<5; ++i)
t1.x[i]=i;

Test t2(t1);
for(int i=0; i<5; ++i)
std::cout << t2.x[i] << std::endl;

return 0;
}
--
John Carson

[BigBrian]

> I something like this possible?

Yes, its definately possible.

Or should I look for other solutions to
solve this problem?

IMHO, yes. Use std::vector.

[Me]

<snip about NbElements is not an integral constant expression>

struct meta_segment
{
long double id;
long double num[NbElements];
long double mean[NbElements];
bool done;
};

I something like this possible? Or should I look for other solutions to
solve this problem?

Some C++ compilers allow C99's flexible array members as an extension.
If your compiler does, you can try:

struct meta_stuff {
long double num;
long double mean;
};

struct meta_segment {
bool done;
long double id;
meta_stuff stuff[];
};

meta_segment *m = (meta_segment*) malloc(sizeof(m eta_segment) +
sizeof(meta_stu ff)*NbElements) ;

[steflhermitte]

Thanks folks, you helped me a lot!

I'm am not an experienced c++-user, so things that are evident are not
so evident for me.

I opted for making a class. I made:

TEST.H
------------------------------------------------
#ifndef TEST_H_
#define TEST_H_

using namespace std;
namespace test
{
class metasegment
{
public:
// constructor
metasegment(uns igned int NbLayers);
// porperties
int id;
int *num;
int *mean;
bool done;
};
};
#endif
------------------------------------------------

TEST.CPP
------------------------------------------------
#include "test.h"
#include <iostream>
#include <stdlib.h>

namespace test
{
// constructor and destructor
metasegment::me tasegment(unsig ned int NbLayers)
{
id=0;
num = new int[NbLayers];
mean = new int[NbLayers];
done=0;
}
};
------------------------------------------------

Now I want to make a vector based on this class:

vector <metasegment(10 )> testvector;

but is does not work. I assume I have to work with typedef, but I don't
know how to solve this problem. Any advice?

Thanks again for your help!

Kind regards,
Stef