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Solution for Programming Exercise 2.4

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

Exercise 2.4:

Write a program that helps the user count his change. The program should ask how many quarters the user has, then how many dimes, then how many nickels, then how many pennies. Then the program should tell the user how much money he has, expressed in dollars.

Discussion

The program will need variables to represent the number of each type of coin. Since the number of coins has to be an integer, these variables are of type int. I'll call the variables quarters, dimes, nickels, and pennies.

The total value of the coins, when expressed in dollars, can be a non-integer number such as 1.57 or 3.02. Since the total value in dollars is a real number, I will use a variable of type double to represent it. The variable is named dollars

The outline of the program is clear enough:

Declare the variables.
Ask the user for the number of each type of coin, and read the responses.
Compute the total value of the coins, in dollars.
Display the result to the user.

The function TextIO.getlnInt() can be used to read each of the user's responses. The alternative function TextIO.getInt() could also be used, but it is less safe. Suppose, for example, that the user responds to the request to type in the number of quarters by entering "7 quarters". After TextIO.getlnInt() reads the number 7, it will discard the extra input "quarters". TextIO.getInt() will read the 7 correctly, but the extra input is not discarded. Later, when the program tries to read the number of dimes, it sees the left-over input and tries to read that, without giving the user a chance to type in another response. You might want to experiment and see what happens if you change getlnInt() to getInt(). (Of course, if the user's response is "I have 7 quarters" or "seven", then you are out of luck in any case.)

Since one quarter is worth 0.25 dollars, the number of dollars in N quarters is 0.25*N. Similarly, a dime is worth 0.10 dollars, a nickel is 0.05 dollars, and a penny is 0.01 dollars. So, to get the total value of all the user's coins, I just have to add up (0.25*quarters) + (0.10*dimes) + (0.05*nickels) + (0.01*pennies). This value is assigned to the variable, dollars, and that is the result that is displayed to the user. Formatted output, using TextIO.putf or System.out.printf can be used to specify that the output value should be printed with two digits after the decimal point.

Alternatively, I could first have computed the total number of cents in all the coins, and then divided by 100 to convert the amount into dollars:

int totalCents;   // Total number of cents in the coins.

totalCents = 25*quarters + 10*dimes + 5*nickels + pennies;
dollars = totalCents/100.0;

Since totalCents is of type int, it is essential here that I compute dollars as totalCents/100.0 and not as totalCents/100. The value computed by totalCents/100 is an integer. For example, if totalCents is 397, then totalCents/100 is 3. Using totalCents/100.0 forces the computer to compute the answer as a real number, giving 3.97.

A second version of the program, using a Scanner for input, is also given. (See Subsection 2.4.6.) A Scanner named stdio is created, and integers are read using the function stdio.nextInt(). This method corresponds to TextIO.getInt(), not TextIO.getlnInt(). To get something equivalent to TextIO.getlnInt(), we can follow the call to stdio.nextInt() with a call to stdio.nextLine(). The call to stdio.nextLine() will read on discard any extra characters on the same line after the integer that was read by stdio.nextInt().

In the second version of the program, in order to avoid any dependence on TextIO, I also use System.out instead of TextIO for output.

The Solution 
public class CountChange {
  
   /*  This program will add up the value of a number of quarters,
       dimes, nickels, and pennies.  The number of each type of 
       coin is input by the user.  The total value is reported
       in dollars.  This program depends on the non-standard class,
       TextIO.
   */

   public static void main(String[] args) {
   
      int quarters;   // Number of quarters, to be input by the user.
      int dimes;      // Number of dimes, to be input by the user.
      int nickels;    // Number of nickels, to be input by the user.
      int pennies;    // Number of pennies, to be input by the user.
      
      double dollars; // Total value of all the coins, in dollars.
      
      /* Ask the user for the number of each type of coin. */
      
      TextIO.put("Enter the number of quarters:  ");
      quarters = TextIO.getlnInt();
      
      TextIO.put("Enter the number of dimes:     ");
      dimes = TextIO.getlnInt();
      
      TextIO.put("Enter the number of nickels:   ");
      nickels = TextIO.getlnInt();
      
      TextIO.put("Enter the number of pennies:   ");
      pennies = TextIO.getlnInt();
      
      /* Add up the values of the coins, in dollars. */
      
      dollars = (0.25 * quarters) + (0.10 * dimes) 
                          + (0.05 * nickels) + (0.01 * pennies); 
      
      /* Report the result back to the user. */
      
      TextIO.putln();
      TextIO.put("The total in dollars is $");
      TextIO.putf("%1.2f", dollars);  // Formatted output!
      TextIO.putln();
   
   }  // end main()
 
}  // end class

A version using Scanner for input: 

import java.util.Scanner;

public class CountChange {
  
   public static void main(String[] args) {
   
      int quarters;   // Number of quarters, to be input by the user.
      int dimes;      // Number of dimes, to be input by the user.
      int nickels;    // Number of nickels, to be input by the user.
      int pennies;    // Number of pennies, to be input by the user.
      
      Scanner stdio = new Scanner( System.in );
      
      double dollars; // Total value of all the coins, in dollars.
      
      /* Ask the user for the number of each type of coin. */
      
      System.out.print("Enter the number of quarters:  ");
      quarters = stdio.nextInt();
      stdio.nextLine();
      
      System.out.print("Enter the number of dimes:     ");
      dimes = stdio.nextInt();
      stdio.nextLine();
      
      System.out.print("Enter the number of nickels:   ");
      nickels = stdio.nextInt();
      stdio.nextLine();
      
      System.out.print("Enter the number of pennies:   ");
      pennies = stdio.nextInt();
      stdio.nextLine();
      
      /* Add up the values of the coins, in dollars. */
      
      dollars = (0.25 * quarters) + (0.10 * dimes) 
                          + (0.05 * nickels) + (0.01 * pennies); 
      
      /* Report the result back to the user. */
      
      System.out.println();
      System.out.print("The total in dollars is $");
      System.out.printf("%1.2f", dollars);  // Formatted output!
      System.out.println();
   
   }  // end main()
 
}  // end class
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