C++ Primer Plus

void n_chars(char, int);     // prototype, style 2

However, providing variable names can make the prototype more understandable, particularly if two parameters are the same type. Then the names can remind you which argument is which:

double melon_density(double weight, double volume);

Listing 7.3 shows an example of a function with two arguments. It also illustrates how changing the value of a formal parameter in a function has no effect on the data in the calling program.

Listing 7.3. twoarg.cpp

// twoarg.cpp -- a function with 2 arguments

#include

using namespace std;

void n_chars(char, int);

int main()

{

    int times;

    char ch;

    cout << "Enter a character: ";

    cin >> ch;

    while (ch != 'q')        // q to quit

    {

        cout << "Enter an integer: ";

        cin >> times;

        n_chars(ch, times); // function with two arguments

        cout << "\nEnter another character or press the"

                " q-key to quit: ";

           cin >> ch;

    }

    cout << "The value of times is " << times << ".\n";

    cout << "Bye\n";

    return 0;

}

void n_chars(char c, int n) // displays c n times

{

    while (n-- > 0)         // continue until n reaches 0

        cout << c;

}

The program in Listing 7.3 illustrates placing a using directive above the function definitions rather than within the functions. Here is a sample run:

Enter a character: W

Enter an integer: 50

WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW

Enter another character or press the q-key to quit: a

Enter an integer: 20

aaaaaaaaaaaaaaaaaaaa

Enter another character or press the q-key to quit: q

The value of times is 20.

Bye

Program Notes

The main() function in Listing 7.3 uses a while loop to provide repeated input (and to keep your loop skills fresh). Note that it uses cin >> ch rather than cin.get(ch) or ch = cin.get() to read a character. There’s a good reason for this. Recall that the two cin.get()functions read all input characters, including spaces and newlines, whereas cin >> skips spaces and newlines. When you respond to the program prompt, you have to press Enter at the end of each line, thus generating a newline character. The cin >> ch approach conveniently skips over these newlines, but the cin.get() siblings read the newline following each number entered as the next character to display. You can program around this nuisance, but it’s simpler to use cin as the program in Listing 7.3 does.

The n_chars() function takes two arguments: a character c and an integer n. It then uses a loop to display the character the number of times the integer specifies:

while (n-- > 0)         // continue until n reaches 0

      cout << c;

Notice that the program keeps count by decrementing the n variable, where n is the formal parameter from the argument list. This variable is assigned the value of the times variable in main(). The while loop then decreases n to 0, but, as the sample run demonstrates, changing the value of n has no effect on times. Even if you use the name n instead of times in main(), the value of n in main() is unaffected by changes in the value of n in n_chars().

Another Two-Argument Function

Let’s create a more ambitious function—one that performs a nontrivial calculation. Also the function illustrates the use of local variables other than the function’s formal arguments.

Many states in the United States now sponsor a lottery with some form of Lotto game. Lotto lets you pick a certain number of choices from a card. For example, you might get to pick six numbers from a card having 51 numbers. Then the Lotto managers pick six numbers at random. If your choice exactly matches theirs, you win a few million dollars or so. Our function will calculate the probability that you have a winning pick. (Yes, a function that successfully predicts the winning picks themselves would be more useful, but C++, although powerful, has yet to implement psychic faculties.)

First, you need a formula. If you have to pick six values out of 51, mathematics says that you have one chance in R of winning, where the following formula gives R:

For six choices, the denominator is the product of the first six integers, or 6 factorial. The numerator is also the product of six consecutive numbers, this time starting with 51 and going down. More generally, if you pick picks values out of numbers numbers, the denominator is picks factorial and the numerator is the product of picks integers, starting with the value numbers and working down. You can use a for loop to make that calculation:

long double result = 1.0;

for (n = numbers, p = picks; p > 0; n--, p--)

    result = result * n / p ;

Rather than multiply all the numerator terms first, the loop begins by multiplying 1.0 by the first numerator term and then dividing by the first denominator term. Then in the next cycle, the loop multiplies and divides by the second numerator and denominator terms. This keeps the running product smaller than if you did all the multiplication first. For example, compare

(10 * 9) / (2 * 1)

with

(10 / 2) * (9 / 1)