arrays-ctors.texi

A C++ class library for scientific computing

@node Array ctors
@section Constructors

@subsection Default constructor
@cindex Array default ctor

@example
Array();
Array(GeneralArrayStorage<N_rank> storage)
@end example

The default constructor creates a C-style array of zero size.  Any attempt
to access data in the array may result in a run-time error, because there
isn't any data to access!

An optional argument specifies a storage order for the array.

Arrays created using the default constructor can subsequently be given data
by the @code{resize()}, @code{resizeAndPreserve()}, or @code{reference()}
member functions.

@subsection Creating an array from an expression

@example
Array(expression...)
@end example

You may create an array from an array expression.  For example,

@example
Array<float,2> A(4,3), B(4,3);   // ...
Array<float,2> C(A*2.0+B);
@end example

This is an explicit constructor (it will not be used to perform implicit
type conversions).  The newly constructed array will have the same storage
format as the arrays in the expression.  If arrays with different storage
formats appear in the expression, an error will result.  (In this case, you
must first construct the array, then assign the expression to it).

@subsection Constructors which take extent parameters
@cindex Array ctors with extent parameters

@example
Array(int extent1);
Array(int extent1, int extent2);
Array(int extent1, int extent2, int extent3);
...
Array(int extent1, int extent2, int extent3, ..., int extent11)
@end example

These constructors take arguments which specify the size of the array to be
constructed.  You should provide as many arguments as there are dimensions
in the array.@footnote{If you provide fewer than @code{N_rank} arguments,
the missing arguments will be filled in using the last provided argument.
However, for code clarity, it makes sense to provide all @code{N_rank}
parameters.}

An optional last parameter specifies a storage format:

@example
Array(int extent1, GeneralArrayStorage<N_rank> storage);
Array(int extent1, int extent2, GeneralArrayStorage<N_rank> storage);
...
@end example

For high-rank arrays, it may be convenient to use this constructor:
@cindex Array high-rank

@example
Array(const TinyVector<int, N_rank>& extent);
Array(const TinyVector<int, N_rank>& extent, 
      GeneralArrayStorage<N_rank> storage);
@end example

The argument @code{extent} is a vector containing the extent (length) of the
array in each dimension.  The optional second parameter indicates a storage
format.  Note that you can construct @code{TinyVector<int,N>} objects on the
fly with the @code{shape(i1,i2,...)} global function.  For example,
@code{Array<int,2> A(shape(3,5))} will create a 3x5 array.

A similar constructor lets you provide both a vector of base index values
(lbounds) and extents:

@example
Array(const TinyVector<int, N_rank>& lbound, 
      const TinyVector<int, N_rank>& extent);
Array(const TinyVector<int, N_rank>& lbound,
      const TinyVector<int, N_rank>& extent,
      GeneralArrayStorage<N_rank> storage);
@end example

The argument @code{lbound} is a vector containing the base index value (or
lbound) of the array in each dimension.  The argument @code{extent} is a
vector containing the extent (length) of the array in each dimension.  The
optional third parameter indicates a storage format.  As with the above
constructor, you can use the @code{shape(i1,i2,...)} global function to
create the @code{lbound} and @code{extent} parameters.

@subsection Constructors with Range arguments
@cindex Array ctor with Range args

These constructors allow arbitrary bases (starting indices) to be set:

@example
Array(Range r1);
Array(Range r1, Range r2);
Array(Range r1, Range r2, Range r3);
...
Array(Range r1, Range r2, Range r3, ..., Range r11);
@end example

For example, this code:

@example
Array<int,2> A(Range(10,20), Range(20,30));
@end example

will create an 11x11 array whose indices are 10..20 and 20..30.  An optional
last parameter provides a storage order:

@example
Array(Range r1, GeneralArrayStorage<N_rank> storage);
Array(Range r1, Range r2, GeneralArrayStorage<N_rank> storage);
...
@end example

@subsection Referencing another array
@cindex Array referencing another array

This constructor makes a shared view of another array's data:
@cindex Array creating a reference of another array

@example
Array(Array<T_numtype, N_rank>& array);
@end example

After this constructor is used, both @code{Array} objects refer to the
@emph{same data}.  Any changes made to one array will appear in the other
array.  If you want to make a duplicate copy of an array, use the
@code{copy()} member function.

@subsection Constructing an array from an expression

Arrays may be constructed from expressions, which are described in
@ref{Array expressions}.  The syntax is:

@example
Array(...array expression...);
@end example

For example, this code creates an array B which contains the square roots of
the elements in A:

@example
Array<float,2> A(N,N);   // ...
Array<float,2> B(sqrt(A));
@end example

@subsection Creating an array from pre-existing data
@cindex Array creating from pre-existing data

When creating an array using a pointer to already existing data, you have
three choices for how Blitz++ will handle the data.  These choices are
enumerated by the enum type @code{preexistingMemoryPolicy}:
@cindex Array creating a reference of another array

@example
enum preexistingMemoryPolicy @{ 
  duplicateData, 
  deleteDataWhenDone, 
  neverDeleteData 
@};
@end example
@findex preexistingMemoryPolicy
@findex duplicateData
@findex deleteDataWhenDone
@findex neverDeleteData

If you choose @code{duplicateData}, Blitz++ will create an array object
using a copy of the data you provide.  If you choose
@code{deleteDataWhenDone}, Blitz++ will not create a copy of the data; and
when no array objects refer to the data anymore, it will deallocate the data
using @code{delete []}.  Note that to use @code{deleteDataWhenDone}, your
array data must have been allocated using the C++ @code{new} operator -- for
example, you cannot allocate array data using Fortran or @code{malloc}, then
create a Blitz++ array from it using the @code{deleteDataWhenDone} flag.
The third option is @code{neverDeleteData}, which means that Blitz++ will
not never deallocate the array data.  This means it is your responsibility
to determine when the array data is no longer needed, and deallocate it.
You should use this option for memory which has not been allocated using the
C++ @code{new} operator.

These constructors create array objects from pre-existing data:

@example
Array(T_numtype* dataFirst, TinyVector<int, N_rank> shape,
      preexistingMemoryPolicy deletePolicy);
Array(T_numtype* dataFirst, TinyVector<int, N_rank> shape,
      preexistingMemoryPolicy deletePolicy, 
      GeneralArrayStorage<N_rank> storage);
@end example

The first argument is a pointer to the array data.  It should point to the
element of the array which is stored first in memory.  The second argument
indicates the shape of the array.  You can create this argument using the
@code{shape()} function.  For example:

@example
double data[] = @{ 1, 2, 3, 4 @};
Array<double,2> A(data, shape(2,2), neverDeleteData);   // Make a 2x2 array
@end example

@findex shape()

The @code{shape()} function takes N integer arguments and returns a
@code{TinyVector<int,N>}.

By default, Blitz++ arrays are row-major.  If you want to work with data
which is stored in column-major order (e.g. a Fortran array), use the second
version of the constructor: 

@cindex Array creating from Fortran arrays

@example
Array<double,2> B(data, shape(2,2), neverDeleteData,
                  FortranArray<2>());
@end example

This is a tad awkward, so Blitz++ provides the global object
@code{fortranArray} which will convert to an instance of
@code{GeneralArrayStorage<N_rank>}:

@example
Array<double,2> B(data, shape(2,2), neverDeleteData, fortranArray);
@end example

Another version of this constructor allows you to pass an arbitrary
vector of strides:

@example
Array(T_numtype* _bz_restrict dataFirst, TinyVector<int, N_rank> shape,
      TinyVector<int, N_rank> stride, 
      preexistingMemoryPolicy deletePolicy,
      GeneralArrayStorage<N_rank> storage = GeneralArrayStorage<N_rank>())
@end example

@subsection Interlacing arrays
@cindex Array interlacing
@findex interlaceArrays()
@findex allocateArrays()

For some platforms, it can be advantageous to store a set of arrays
interlaced together in memory.  Blitz++ provides support for this through
the routines @code{interlaceArrays()} and @code{allocateArrays()}.  An
example:

@example
Array<int,2> A, B;
interlaceArrays(shape(10,10), A, B);
@end example

The first parameter of @code{interlaceArrays()} is the shape for the arrays
(10x10).  The subsequent arguments are the set of arrays to be interlaced
together.  Up to 11 arrays may be interlaced.  All arrays must store the
same data type and be of the same rank.  In the above example, storage is
allocated so that @code{A(0,0)} is followed immediately by @code{B(0,0)} in
memory, which is folloed by @code{A(0,1)} and @code{B(0,1)}, and so on.

A related routine is @code{allocateArrays()}, which has identical syntax:

@example
Array<int,2> A, B;
allocateArrays(shape(10,10), A, B);
@end example

Unlike @code{interlaceArrays()}, which always interlaces the arrays, the
routine @code{allocateArrays()} may or may not interlace them, depending on
whether interlacing is considered advantageous for your platform.  If the
tuning flag @code{BZ_INTERLACE_ARRAYS} is defined in
@code{<blitz/tuning.h>}, then the arrays are interlaced.

Note that the performance effects of interlacing are unpredictable: in some
situations it can be a benefit, and in most others it can slow your code
down substantially.  You should only use @code{interlaceArrays()} after
running some benchmarks to determine whether interlacing is beneficial for
your particular algorithm and architecture.

@subsection A note about reference counting
@cindex Array reference counting
@cindex reference counting

Blitz++ arrays use reference counting.  When you create a new array, a
memory block is allocated.  The @code{Array} object acts like a handle for
this memory block.  A memory block can be shared among multiple @code{Array}
objects -- for example, when you take subarrays and slices.  The memory
block keeps track of how many @code{Array} objects are referring to it.
When a memory block is orphaned -- when no @code{Array} objects are
referring to it -- it automatically deletes itself and frees the allocated
memory.