Solution for Programmming Exercise 3.5
This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.
Exercise 3.5:
Suppose that a file contains information about sales figures for a company in various cities. Each line of the file contains a city name, followed by a colon (:) followed by the data for that city. The data is a number of type double. However, for some cities, no data was available. In these lines, the data is replaced by a comment explaining why the data is missing. For example, several lines from the file might look like:
San Francisco: 19887.32 Chicago: no report received New York: 298734.12
Write a program that will compute and print the total sales from all the cities together. The program should also report the number of cities for which data was not available. The name of the file is "sales.dat".
To complete this program, you'll need one fact about file input with TextIO that was not covered in Subsection 2.4.5. Since you don't know in advance how many lines there are in the file, you need a way to tell when you have gotten to the end of the file. When TextIO is reading from a file, the function TextIO.eof() can be used to test for end of file. This boolean-valued function returns true if the file has been entirely read and returns false if there is more data to read in the file. This means that you can read the lines of the file in a loop while (TextIO.eof() == false).... The loop will end when all the lines of the file have been read.
Suggestion: For each line, read and ignore characters up to the colon. Then read the rest of the line into a variable of type String. Try to convert the string into a number, and use try..catch to test whether the conversion succeeds.
The statement TextIO.readFile("sales.dat") can be used at the beginning of the program to make TextIO read from the file instead of from the user's input. This statement throws an IllegalArgumentException if the file can't be found. Although it's not required by the statement of the problem, it would be nice to detect the exception and print an error message about the missing file. This can be done as follows, using System.exit to end the program if the error occurs:
try {
TextIO.readFile("sales.dat");
}
catch (IllegalArgumentException e) {
System.out.println("Can't open file \"sales.dat\" for reading!");
System.out.println("Please make sure the file is present before");
System.out.println("running the program.");
System.exit(1); // Terminates the program.
}
We have two main quantities to keep track of in the program: The total amount of sales and the number of cities for which data is missing. These quantities can be represented by two variables, salesTotal of type double and missingCount of type int. These variables are initialized to zero before processing any data from the file, and their values are output at the end of the program, after reading the entire file.
The main processing loop of the program is the while loop mentioned in the statement of the exercise, which begins while (TextIO.eof() == false). (In my program, I've chosen to use the equivalent but more elegant test while ( ! TextIO.eof() ); you should try to understand why this is equivalent -- and why it is more elegant!) In pseudocode form, we want a loop that does the following:
while there are more lines in the file:
read the name of the city and the colon that follows it
read the rest of the line into a string
try to convert the string into a number
if the conversion succeeds, add the number to the total sales
otherwise add 1 to the count of missing data
To read past the name of the city, we can simply read characters using TextIO.getChar() until we get to the colon:
do { // Read past characters up to the ':'.
ch = TextIO.getChar();
} while (ch != ':');
where ch is a variable of type char. We can read the rest of the line using TextIO.getln(). (As a side note, it's important here that we read the entire line, including the carriage return at the end. TextIO.getln() does this. If we neglected to read the carriage return at the end of the line, TextIO.eof() would not detect the end of the file correctly. After the data from the last line is read, the carriage return character would still be there in the file waiting to be read; since there is still something left to read in the file, TextIO.eof() would be false false and the while loop would continue, even though all the cities have already been processed. In fact, the program would crash while looking for the next ':', which is not there. Text processing can be surprisingly subtle!)
Once we have the information in the form of a String, we can try to convert the string into a number with Double.parseDouble. If the string is not a number, this will throw an exception of type NumberFormatException. We can catch the error in a try..catch statement. The "missing data" processing is done in the catch part of this statement. In the program, this takes the form:
try {
sales = Double.parseDouble(dataString);
salesTotal += sales; // This is skipped if the conversion fails.
}
catch (NumberFormatException e) {
// The dataString is not a number, so it counts as
// missing data. Add 1 to the missing data count.
missingCount++;
}
(Note that my solution assumes that the data file (if it exists at all) has exactly the format that is specified in the problem. The problem would be more difficult if we had to check the format of the file, to see whether it conforms to the specification.)
/**
* This program reads from a file named "sales.dat". Each line of the
* file contains the name of a city, followed by a colon, followed by
* either a number giving the amount of sales in that city or by a
* message saying why the sales figure is not available. The program
* prints the total sales for all cities and the number of cites for
* which the figure was not available.
*/
public class SalesFigures {
public static void main( String[] args ) {
/* Open file for reading; if it can't be opened, end the program */
try {
TextIO.readFile("sales.dat");
}
catch (IllegalArgumentException e) {
System.out.println("Can't open file \"sales.dat\" for reading!");
System.out.println("Please make sure the file is present before");
System.out.println("running the program.");
System.exit(1); // Terminates the program.
}
/* Read the file, keeping track of total sales and missing data. */
double salesTotal; // Total of all sales figures seen so far.
int missingCount; // Number of cities for which data is missing.
salesTotal = 0;
missingCount = 0;
while ( ! TextIO.eof() ) { // process one line of data.
char ch; // For reading past the name of the city.
String dataString; // Contents of line, after the city name.
double sales; // The sales figure for the city.
do { // Read past characters up to the ':'.
ch = TextIO.getChar();
} while (ch != ':');
dataString = TextIO.getln(); // Get the rest of the line.
try {
sales = Double.parseDouble(dataString);
salesTotal += sales; // This is skipped if the conversion fails.
}
catch (NumberFormatException e) {
// The dataString is not a number, so it counts as
// missing data. Add 1 to the missing data count.
missingCount++;
}
} // end while
/* Report the results. */
System.out.printf("Total sales recorded from all cities: $%1.2f\n\n", salesTotal);
if (missingCount == 0)
System.out.println("Data was received from all cities.");
else if (missingCount == 1)
System.out.println("Data was missing from 1 city.");
else
System.out.printf("Data was missing from %d cities.\n", missingCount);
} // end main()
} // end class SalesFigures