Array of point to arrays of different size

[Edward Jensen]

Hi,

I have the following static arrays of different size in a class:

in header:
static double w2[2], x2[2];
static double w3[3], x3[3];
static double w4[4], x4[4];

in GaussLegendre.cpp:
double GaussLegendre::w2[2] = {1.0, 1.0};
double GaussLegendre::x2[2] = {-0.577350269189625, 0.577350269189625};
double GaussLegendre::w3[3] = {0.555555555555556, 0.8888888888888889,
0.555555555555556};
double GaussLegendre::x3[3] = {-0.774596669241483, 0.0, 0.774596669241483};
double GaussLegendre::w4[4] = {0.347854845137453, 0.652145154862546,
0.652145154862546, 0.347854845137453};
double GaussLegendre::x4[4] = {-0.861136311594053, -0.339981043584856,
0.339981043584856, 0.861136311594053};

Now I want to create a static array of pointers of x2, x3, x4 and w2,w2,w4.
I thought I could do that with just
declaring statis double* x[3] in my header and the following in .cpp:

double* GaussLegendre::x[3] = {&GaussLegendre::x2, &GaussLegendre::x3,
&GaussLegendre::x4};

but I get the following error: cannot convert from 'double (*)[2]' to
'double *

Why is that and how can it be done?

Thanks in advance,
Edward

 

[Salt_Peter]

Hi,

I have the following static arrays of different size in a class:

in header:
static double w2[2], x2[2];
static double w3[3], x3[3];
static double w4[4], x4[4];

in GaussLegendre.cpp:
double GaussLegendre::w2[2] = {1.0, 1.0};
double GaussLegendre::x2[2] = {-0.577350269189625, 0.577350269189625};
double GaussLegendre::w3[3] = {0.555555555555556, 0.8888888888888889,
0.555555555555556};
double GaussLegendre::x3[3] = {-0.774596669241483, 0.0, 0.774596669241483};
double GaussLegendre::w4[4] = {0.347854845137453, 0.652145154862546,
0.652145154862546, 0.347854845137453};
double GaussLegendre::x4[4] = {-0.861136311594053, -0.339981043584856,
0.339981043584856, 0.861136311594053};

Now I want to create a static array of pointers of x2, x3, x4 and w2,w2,w4.
I thought I could do that with just
declaring statis double* x[3] in my header and the following in .cpp:

double* GaussLegendre::x[3] = {&GaussLegendre::x2, &GaussLegendre::x3,
&GaussLegendre::x4};

but I get the following error: cannot convert from 'double (*)[2]' to
'double *

Why is that and how can it be done?

Thanks in advance,
Edward

You can point to a double using a double*, you certainly can't point
to an array of doubles of varying sizes. You can however point to the
first element of each array. Something as follows:

double* GaussLegendre::x[3] = { &GaussLegendre::x2[0],
&GaussLegendre::x3[0],
&GaussLegendre::x4[0] };

However, the elements of x know nothing about the contents of the
arrays that are part of.
To solve the issue, use dynamic containers. Lets take a std::deque of
std::vectors< double > for example (ignore the operator<< for now - it
just displays the populated containers). Notice how much simpler
coding without pointers is.

#include <iostream>
#include <ostream>
#include <deque>
#include <vector>
#include <algorithm>
#include <iterator>

namespace D
{
std::deque< std::vector< double > > dvd;
}

template < typename T >
std:stream& operator<<( std:stream& os,
std::deque< std::vector< T > >& r_d )
{
typedef typename std::deque< std::vector< T > >::iterator DIter;
for( DIter iter = r_d.begin(); iter != r_d.end(); ++iter )
{
std::copy( (*iter).begin(),
(*iter).end(),
std:stream_iterator< T >( os, " ") );
os << std::endl;
}
return os;
}

int main()
{
using namespace D;

std::vector< double > v1(10, 1.1); // 10 elements all initialized
std::vector< double > v2(11, 2.2);
std::vector< double > v3(12, 3.3);

dvd.push_back( v1 );
dvd.push_back( v2 );
dvd.push_back( v3 );

std::cout << dvd;
}

/*
1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1
2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2
3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3
*/

// Both the elements (std::vector< double >) and the deque itself
remain dynamic.

dvd[0].push_back( 9.9 ); // add an eleventh double to 1st collection

// How about loading a std::deque named d1 with
// four std::vector< double > containers
// all having 10 elements initialized to 0.1:

std::deque< std::vector< double > > d1(4, std::vector< double >(10,
0.1));

// Thats one line, and the containers remain dynamic.
// Still thinking of using primitive arrays and pointers?

 

[Andrey Tarasevich]

Now I want to create a static array of pointers of x2, x3, x4 and w2,w2,w4.
I thought I could do that with just
declaring statis double* x[3] in my header and the following in .cpp:

double* GaussLegendre::x[3] = {&GaussLegendre::x2, &GaussLegendre::x3,
&GaussLegendre::x4};

but I get the following error: cannot convert from 'double (*)[2]' to
'double *

Why is that

In C and C++, when you apply the unary '&' to an array, you get a value
of pointer-to-array type. For example, if array 'a' is declared as

int a[10];

the expression '&a' has type 'int (*)[10]' - a pointer to an array of 10
'int's

int (*pa)[10] = &a; // OK

This type is not convertible to 'int*'

int *pi = &a; // ERROR

This is exactly the error your code produces.

and how can it be done?

Since you are trying to point to arrays of _different_ sizes, using
pointers of pointer-to-array type is not an option. Instead, just use an
array of "regular" pointers (which is what you have already) and make
the point to the very first element of each array

double* GaussLegendre::x[3] = {
&GaussLegendre::x2[0],
&GaussLegendre::x3[0],
&GaussLegendre::x4[0]
};

or alternatively (and equivalently)

double* GaussLegendre::x[3] = {
GaussLegendre::x2,
GaussLegendre::x3,
GaussLegendre::x4
};

(note that there's no '&' operator in the latter case).

 

[James Kanze]

I have the following static arrays of different size in a class:
in header:
static double w2[2], x2[2];
static double w3[3], x3[3];
static double w4[4], x4[4];
in GaussLegendre.cpp:
double GaussLegendre::w2[2] = {1.0, 1.0};
double GaussLegendre::x2[2] = {-0.577350269189625, 0.577350269189625};
double GaussLegendre::w3[3] = {0.555555555555556, 0.8888888888888889,
0.555555555555556};
double GaussLegendre::x3[3] = {-0.774596669241483, 0.0, 0.774596669241483};
double GaussLegendre::w4[4] = {0.347854845137453, 0.652145154862546,
0.652145154862546, 0.347854845137453};
double GaussLegendre::x4[4] = {-0.861136311594053, -0.339981043584856,
0.339981043584856, 0.861136311594053};

So presumably that bit in header was within the defintion of the
class GaussLegendre.

Now I want to create a static array of pointers of x2, x3,
x4 and w2,w2,w4. I thought I could do that with just
declaring statis double* x[3] in my header and the following
in .cpp:
double* GaussLegendre::x[3] = {&GaussLegendre::x2, &GaussLegendre::x3,
&GaussLegendre::x4};
but I get the following error: cannot convert from 'double (*)[2]' to
'double *

You should get an error about defining a variable twice, and/or
defining it with a different type than it was declared with.

You can point to a double using a double*, you certainly can't
point to an array of doubles of varying sizes. You can however
point to the first element of each array. Something as
follows:

double* GaussLegendre::x[3] = { &GaussLegendre::x2[0],
                                &GaussLegendre::x3[0],
                                &GaussLegendre::x4[0] };

That's certainly legal, but it's far more idiomatic to use the
implicit conversion of array to pointer here, as Sam suggested.
While it may be a misfeature, it's use is ubiquious, and
unavoidable if you are using C style arrays.

However, the elements of x know nothing about the contents of the
arrays that are part of.

Which is as it should be. (But I think you're trying to say
something else. You surely don't mean that the value in x[1]
should know something about x as a whole.)

To solve the issue, use dynamic containers. Lets take a
std::deque of std::vectors< double > for example (ignore the
operator<< for now - it just displays the populated
containers). Notice how much simpler coding without pointers
is.

As a general rule, std::vector should be preferred, so his
double[] would become std::vector<double>, and he really can
have an array of pointers to arrays, std::vector< std::vector<
double >* >. I'd want to know more about his particular use,
however, before doing that here. I have a sneaky suspicion that
he's forgotten the const, and there is one place where C style
arrays are better: they allow static initialization. If all of
these tables are in fact const, it might make more sense (and it
would certainly require less code) to leave them as C style
arrays, i.e. something like:

double const GaussLegendre::w2[2] = {1.0, 1.0};
// ...
double const *const GaussLegendre::x2[2] = { ... } ;

Typically, I'd also leave off the size of the dimension, and
let the compiler calculate it. But again, I'm not sure here; I
think the actual size may be important, in which case, it's
better explicitly stating it.