C++ Primer Plus

This special subscripting facility allows you to use a one-dimensional valarray object to represent two-dimensional data. For example, suppose you want to represent an array with 4 rows and 3 columns. You can store the information in a 12-element valarray object. Then a slice(0,3,1) object used as a subscript would represent elements 0, 1, and 2—that is, the first row. Similarly, a slice(0,4,3) subscript would represent elements 0, 3, 6, and 9—that is, the first column. Listing 16.21 illustrates some features of slice.

Listing 16.21. vslice.cpp

// vslice.cpp -- using valarray slices

#include

#include

#include

const int SIZE = 12;

typedef std::valarray vint;     // simplify declarations

void show(const vint & v, int cols);

int main()

{

    using std::slice;                // from

    using std::cout;

    vint valint(SIZE);               // think of as 4 rows of 3

    int i;

    for (i = 0; i < SIZE; ++i)

        valint[i] = std::rand() % 10;

    cout << "Original array:\n";

    show(valint, 3);                 // show in 3 columns

    vint vcol(valint[slice(1,4,3)]); // extract 2nd column

    cout << "Second column:\n";

    show(vcol, 1);                   // show in 1 column

    vint vrow(valint[slice(3,3,1)]); // extract 2nd row

    cout << "Second row:\n";

    show(vrow, 3);

    valint[slice(2,4,3)]  = 10;      // assign to 2nd column

    cout << "Set last column to 10:\n";

    show(valint, 3);

    cout << "Set first column to sum of next two:\n";

    // + not defined for slices, so convert to valarray

    valint[slice(0,4,3)]  = vint(valint[slice(1,4,3)])

                               + vint(valint[slice(2,4,3)]);

    show(valint, 3);

    return 0;

}

void show(const vint & v, int cols)

{

    using std::cout;

    using std::endl;

    int lim = v.size();

    for (int i = 0; i < lim; ++i)

    {

        cout.width(3);

        cout << v[i];

        if (i % cols == cols - 1)

            cout << endl;

        else

            cout << ' ';

    }

    if (lim % cols != 0)

        cout << endl;

}

The + operator is defined for valarray objects, such as valint, and it’s defined for a single int element, such as valint[1]. But as the code in Listing 16.21 notes, the + operator isn’t defined for slice-subscripted valarray units, such as valint[slice(1,4,3)]. Therefore, the program constructs full objects from the slices to enable addition:

vint(valint[slice(1,4,3)])    // calls a slice-based constructor

The valarray class provides constructors just for this purpose.

Here is a sample run of the program in Listing 16.21:

Original array:

  0   3   3

  2   9   0

  8   2   6

  6   9   1

Second column:

  3

  9

  2

  9

Second row:

  2   9   0

Set last column to 10:

  0   3  10

  2   9  10

  8   2  10

  6   9  10

Set first column to sum of next two:

 13   3  10

 19   9  10

 12   2  10

 19   9  10

Because values are set using rand(), different implementations of rand() will result in different values.

There’s more, including the gslice class to represent multidimensional slices, but this should be enough to give you a sense of what valarray is about.

The initializer_list Template (C++11)

The initializer_list template is another C++11 addition to the C++ library. You can use the initializer-list syntax to initialize an STL container to a list of values:

std::vector payments {45.99, 39.23, 19.95, 89.01};

This would create a container for four elements and initialize the elements to the four values in the list. What makes this possible is that the container classes now have constructors taking an initializer_list argument. A vector object, for example, has a constructor that accepts an initializer_list argument, and the previous declaration is the same as this:

std::vector payments({45.99, 39.23, 19.95, 89.01});

Here, the list is written explicitly as a constructor argument.

Normally, as part of the C++11 universal initialization syntax, you can invoke a class constructor using {} instead of () notation:

shared_ptr pd {new double}; // ok to use {} instead of ()

But this would create problems if there also is an initializer_list constructor:

std::vector vi{10};    // ??

Which constructor does this invoke?

std::vector vi(10);    // case A: 10 uninitialized elements

std::vector vi({10});  // case B: 1 element set to 10

The answer is that if the class does have a constructor accepting an initializer_list argument, then using the {} syntax invokes that particular constructor. So in this example, case B applies.

The initializer_list elements should all be of one type. However, the compiler will do conversions to match the type:

std::vector payments {45.99, 39.23, 19, 89};

// same as std::vector payments {45.99, 39.23, 19.0, 89.0};