slice_array

Library:  Numerics

Does not inherit

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Members

operator%=() operator&=() operator>>=()

operator<<=() operator*=() operator+=()

operator-=() operator/=() operator=()

operator^=() operator|=() slice_array()

A numeric array class for representing a BLAS-like slice from a valarray

#include <valarray>

namespace std {
  template <class T>
  class slice_array;
}

The valarray helper class slice_array gives a slice view into a valarray. A slice_array is only produced by applying the slice subscript operator to a valarray. The elements in a slice_array are references to selected elements in the valarray (so changing an element in the slice_array really changes the corresponding element in the valarray). A slice_array does not itself hold any distinct elements. The template cannot be instantiated directly since all its constructors are private. However, you can easily copy a slice_array to a valarray using either the valarray copy constructor or the assignment operator. Reference semantics are lost at that point.

namespace std {

  template <class T> class slice_array {
    public:

    // types
    typedef T value_type;

    // destructor
    ~slice_array();

    // public assignment
    void operator=(const valarray<T>& array) const;
  
    // computed assignment
    void operator*=(const valarray<T>& array) const;
    void operator/=(const valarray<T>& array) const;
    void operator%=(const valarray<T>& array) const;
    void operator+=(const valarray<T>& array) const;
    void operator-=(const valarray<T>& array) const;
    void operator^=(const valarray<T>& array) const;
    void operator&=(const valarray<T>& array) const;
    void operator|=(const valarray<T>& array) const;
    void operator<<=(const valarray<T>& array) const;
    void operator>>=(const valarray<T>& array) const;
 
    // other
    void operator=(const T&);  

    private:
    // constructors
    slice_array();
    slice_array(const slice_array<T>&);
    // operator=
    slice_array<T>& operator=(const slice_array<T>& array);
  };
}

slice_array();
slice_array(const slice_array&);

All slice_array constructors are private and cannot be called directly. This prevents copy construction of slice_arrays.

void operator=(const valarray<T>& x) const;

Assigns values from x to the selected elements of the valarray that self refers to. Remember that a slice_array never holds any elements itself, it simply refers to selected elements in the valarray used to generate it.

slice_array<T>& 
operator=(const slice-_array<T>& x);

Private assignment operator. Cannot be called directly, thus preventing assignment between slice_arrays.

void operator=(const T& x);

Assigns x to the selected elements of the valarray that self refers to.

void operator*=(const valarray<T>& val) const;
void operator/=(const valarray<T>& val) const;
void operator%=(const valarray<T>& val) const;
void operator+=(const valarray<T>& val) const;
void operator-=(const valarray<T>& val) const;
void operator^=(const valarray<T>& val) const;
void operator&=(const valarray<T>& val) const;
void operator|=(const valarray<T>& val) const;
void operator<<=(const valarray<T>& val) const;
void operator>>=(const valarray<T>& val) const;

Applies the indicated operation using elements from val to the selected elements of the valarray that self refers to. Remember that a slice_array never holds any elements itself; it simply refers to selected elements in the valarray used to generate it.

//
//  slice_array.cpp
//

#include <valarray.h>  // Includes valarray and provides stream inserter.

typedef std::valarray<int> valarray_t;

int main(void) {

    valarray_t::value_type ibuf[9] = {0,1,2,3,4,5,6,7,8};

    // Create a valarray initialized to the sequence of
    // integers above.
    valarray_t vi (ibuf,9);
    
    std::cout << "original valarray vi\n\n" << vi << "\n\n";

    // Display a slice of the valarray.
    std::cout << "vi[slice(0,3,3)]\n\n"
              << vi[std::slice(0,3,3)] << "\n\n";
    
    // Unfortunately, slice_arrays were not designed to be 
    // easily combined.  An explicit conversion to valarray 
    // is required, leaving us with this awkward casting
    // syntax.
    vi[std::slice (0,3,3)] =
            static_cast<valarray_t>(vi[std::slice (1,3,3)]) +
            static_cast<valarray_t>(vi[std::slice (2,3,3)]);  

    // The operation above uses slices ([0-2],3,3) to treat
    // our valarray as a 3-dimensional array:
    //
    //  0 1 2                 3 = 1 + 2 
    //  3 4 5                 9 = 4 + 5
    //  6 7 8                15 = 7 + 8          
    //  | | |                 |   |   | 
    //  | | +-slice(2,3,3)    |   |   +- slice_array for 
    //  | |                   |   |      arithmetic operand.
    //  | +---slice(1,3,3)    |   +----- slice_array for 
    //  |                     |          arithmetic operand.
    //  +-----slice(0,3,3)    +---slice_array assignment
    //                            refers to valarray.
    
    std::cout << "vi[slice(0,3,3)] = vi[slice(1,3,3)] + "
              << "vi[slice(2,3,3)]\n\n"
              << vi << std::endl;
    
    return 0;
}


Program Output:

original valarray vi

[0,1,2,3,4,5,6,7,8]

vi[slice(0,3,3)]

[0,3,6]

vi[slice(0,3,3)] = vi[slice(1,3,3)] + vi[slice(2,3,3)]

[3,1,2,9,4,5,15,7,8]

slice, valarray, gslice, gslice_array, mask_array, indirect_array

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 26.3.5