C++ Primer Plus

To add two times, you first add the minute members. Integer division by 60 yields the number of hours to carry over, and the modulus operation (%) yields the number of minutes left. Listing 7.11 incorporates this approach into the sum() function and adds a show_time() function to display the contents of a travel_time structure.

Listing 7.11. travel.cpp

// travel.cpp -- using structures with functions

#include

struct travel_time

{

    int hours;

    int mins;

};

const int Mins_per_hr = 60;

travel_time sum(travel_time t1, travel_time t2);

void show_time(travel_time t);

int main()

{

    using namespace std;

    travel_time day1 = {5, 45};    // 5 hrs, 45 min

    travel_time day2 = {4, 55};    // 4 hrs, 55 min

    travel_time trip = sum(day1, day2);

    cout << "Two-day total: ";

    show_time(trip);

    travel_time day3= {4, 32};

    cout << "Three-day total: ";

    show_time(sum(trip, day3));

    return 0;

}

travel_time sum(travel_time t1, travel_time t2)

{

    travel_time total;

    total.mins = (t1.mins + t2.mins) % Mins_per_hr;

    total.hours = t1.hours + t2.hours +

                 (t1.mins + t2.mins) / Mins_per_hr;

    return total;

}

void show_time(travel_time t)

{

    using namespace std;

    cout << t.hours << " hours, "

         << t.mins << " minutes\n";

}

Here travel_time acts just like a standard type name; you can use it to declare variables, function return types, and function argument types. Because variables such as total and t1 are travel_time structures, you can apply the dot membership operator to them. Note that because the sum() function returns a travel_time structure, you can use it as an argument for the show_time() function. Because C++ functions, by default, pass arguments by value, the show_time(sum(trip, day3)) function call first evaluates the sum(trip, day3) function call in order to find its return value. The show_time() call then passes sum()’s return value, not the function itself, to show_time(). Here’s the output of the program in Listing 7.11:

Two-day total: 10 hours, 40 minutes

Three-day total: 15 hours, 12 minutes

Another Example of Using Functions with Structures

Much of what you learn about functions and C++ structures carries over to C++ classes, so it’s worth looking at a second example. This time let’s deal with space instead of time. In particular, this example defines two structures representing two different ways of describing positions and then develops functions to convert one form to the other and show the result. This example is a bit more mathematical than the last, but you don’t have to follow the mathematics to follow the C++.

Suppose you want to describe the position of a point on the screen or a location on a map relative to some origin. One way is to state the horizontal offset and the vertical offset of the point from the origin. Traditionally, mathematicians use the symbol x to represent the horizontal offset and y to represent the vertical offset (see Figure 7.6). Together, x and y constitute rectangular coordinates. You can define a structure consisting of two coordinates to represent a position:

Figure 7.6. Rectangular coordinates.

struct rect

{

      double x;           // horizontal distance from origin

      double y;           // vertical distance from origin

};

A second way to describe the position of a point is to state how far it is from the origin and in what direction it is (for example, 40 degrees north of east). Traditionally, mathematicians have measured the angle counterclockwise from the positive horizontal axis (see Figure 7.7). The distance and angle together constitute polar coordinates. You can define a second structure to represent this view of a position:

struct polar

{

       double distance;   // distance from origin

       double angle;      // direction from origin

};

Figure 7.7. Polar coordinates.

Let’s construct a function that displays the contents of a type polar structure. The math functions in the C++ library (borrowed from C) assume that angles are in radians, so you need to measure angles in that unit. But for display purposes, you can convert radian measure to degrees. This means multiplying by 180/π, which is approximately 57.29577951. Here’s the function:

// show polar coordinates, converting angle to degrees

void show_polar (polar dapos)

{

    using namespace std;

    const double Rad_to_deg = 57.29577951;

    cout << "distance = " << dapos.distance;

    cout << ", angle = " << dapos.angle * Rad_to_deg;

    cout << " degrees\n";

}

Notice that the formal variable is type polar. When you pass a polar structure to this function, the structure contents are copied into the dapos structure, and the function then uses that copy in its work. Because dapos is a structure, the function uses the membership (dot) operator (see Chapter 4) to identify structure members.