C++ Primer Plus

The first form returns the current setting for field width. The second sets the field width to i spaces and returns the previous field width value. This allows you to save the previous value in case you want to restore the width to that value later.

The width() method affects only the next item displayed, and the field width reverts to the default value afterward. For example, consider the following statements:

cout << '#';

cout.width(12);

cout << 12 << "#" <<  24 << "#\n";

Because width() is a member function, you have to use an object (cout, in this case) to invoke it. The output statement produces the following display:

#          12#24#

The 12 is placed in a field 12 characters wide at the right end of the field. This is called right-justification. After that, the field width reverts to the default, and the two # characters and the 24 are printed in fields equal to their own size.

Caution

The width() method affects only the next item displayed, and the field width reverts to the default value afterward.

C++ never truncates data, so if you attempt to print a seven-digit value in a field with a width of two, C++ expands the field to fit the data. (Some languages just fill the field with asterisks if the data doesn’t fit. The C/C++ philosophy is that showing all the data is more important than keeping the columns neat; C++ puts substance before form.) Listing 17.4 shows how the width() member function works.

Listing 17.4. width.cpp

// width.cpp -- using the width method

#include

int main()

{

    using std::cout;

    int w = cout.width(30);

    cout << "default field width = " << w << ":\n";

    cout.width(5);

    cout << "N" <<':';

    cout.width(8);

    cout << "N * N" << ":\n";

    for (long i = 1; i <= 100; i *= 10)

    {

        cout.width(5);

        cout << i <<':';

        cout.width(8);

        cout << i * i << ":\n";

    }

    return 0;

}

Here is the output of the program in Listing 17.4:

      default field width = 0:

  N:   N * N:

  1:       1:

 10:     100:

100:   10000:

The output displays values right-justified in their fields. The output is padded with spaces. That is, cout achieves the full field width by adding spaces. With right-justification, the spaces are inserted to the left of the values. The character used for padding is termed the fill character. Right-justification is the default.

Note that the program in Listing 17.4 applies the field width of 30 to the string displayed by the first cout statement but not to the value of w. This is because the width() method affects only the next single item displayed. Also note that w has the value 0. This is because cout.width(30) returns the previous field width, not the width to which it was just set. The fact that w is 0 means that zero is the default field width. Because C++ always expands a field to fit the data, this one size fits all. Finally, the program uses width() to align column headings and data by using a width of five characters for the first column and a width of eight characters for the second column.

Fill Characters

By default, cout fills unused parts of a field with spaces. You can use the fill() member function to change that. For example, the following call changes the fill character to an asterisk:

cout.fill('*');

That can be handy for, say, printing checks so that recipients can’t easily add a digit or two. Listing 17.5 illustrates using this member function.

Listing 17.5. fill.cpp

// fill.cpp -- changing fill character for fields

#include

int main()

{

    using std::cout;

    cout.fill('*');

    const char * staff[2] = { "Waldo Whipsnade", "Wilmarie Wooper"};

    long bonus[2] = {900, 1350};

    for (int i = 0; i < 2; i++)

    {

        cout << staff[i] << ": $";

        cout.width(7);

        cout << bonus[i] << "\n";

    }

    return 0;

}

Here’s the output of the program in Listing 17.5:

Waldo Whipsnade: $****900

Wilmarie Wooper: $***1350

Note that, unlike the field width, the new fill character stays in effect until you change it.

Setting Floating-Point Display Precision

The meaning of floating-point precision depends on the output mode. In the default mode, it means the total number of digits displayed. In the fixed and scientific modes, to be discussed soon, precision means the number of digits displayed to the right of the decimal place. The precision default for C++, as you’ve seen, is 6. (Recall, however, that trailing zeros are dropped.) The precision() member function lets you select other values. For example, the following statement causes cout to set the precision to 2:

cout.precision(2);

Unlike the case with width(), but like the case for fill(), a new precision setting stays in effect until it is reset. Listing 17.6 demonstrates precisely this point.

Listing 17.6. precise.cpp

// precise.cpp -- setting the precision

#include

int main()

{

    using std::cout;

    float price1 = 20.40;

    float price2 = 1.9 + 8.0 / 9.0;

    cout << "\"Furry Friends\" is $" << price1 << "!\n";

    cout << "\"Fiery Fiends\" is $" << price2 << "!\n";

    cout.precision(2);