Section 1.6. The C Program


1.6. The C++ Program

Now we are ready to solve our original bookstore problem: We need to read a file of sales transactions and produce a report that shows for each book the total revenue, average sales price, and the number of copies sold.

Exercises Section 1.5.2

Exercise 1.24:

Write a program that reads several transactions. For each new transaction that you read, determine if it is the same ISBN as the previous transaction, keeping a count of how many transactions there are for each ISBN. Test the program by giving multiple transactions. These transactions should represent multiple ISBNs but the records for each ISBN should be grouped together.


We'll assume that all of the transactions for a given ISBN appear together. Our program will combine the data for each ISBN in a Sales_item object named total. Each transaction we read from the standard input will be stored in a second Sales_item object named TRans. Each time we read a new transaction we'll compare it to the Sales_item object in total. If the objects refer to the same ISBN, we'll update total. Otherwise we'll print the value in total and reset it using the transaction we just read.

     #include <iostream>     #include "Sales_item.h"     int main()     {         //  declare variables to hold running sum and data for the next record         Sales_item total, trans;         //  is there data to process?         if (std::cin >> total) {             // if so, read the transaction records             while (std::cin >> trans)                 if  (total.same_isbn(trans))                    //  match: update the running total                    total = total + trans;                 else {                    //  no match: print & assign to total                    std::cout << total << std::endl;                    total = trans;                 }             //  remember to print last record             std::cout << total << std::endl;          } else {             //  no input!, warn the user             std::cout << "No data?!" << std::endl;             return -1;  //  indicate failure          }          return 0;     } 

This program is the most complicated one we've seen so far, but it uses only facilities that we have already encountered. As usual, we begin by including the headers that we use: iostream from the library and Sales_item.h, which is our own header.

Inside main we define the objects we need: total, which we'll use to sum the data for a given ISBN, and trans, which will hold our transactions as we read them. We start by reading a transaction into total and testing whether the read was successful. If the read fails, then there are no records and we fall through to the outermost else branch, which prints a message to warn the user that there was no input.

Assuming we have successfully read a record, we execute the code in the if branch. The first statement is a while that will loop through all the remaining records. Just as we did in the program on page 18, our while condition reads a value from the standard input and then tests that valid data was actually read. In this case, we read a Sales_item object into TRans. As long as the read succeeds, we execute the body of the while.

The body of the while is a single if statement. We test whether the ISBNs are equal, and if so we add the two objects and store the result in total. If the ISBNs are not equal, we print the value stored in total and reset total by assigning trans to it. After execution of the if, we return to the condition in the while, reading the next transaction and so on until we run out of records.

Once the while completes, we still must write the data associated with the last ISBN. When the while terminates, total contains the data for the last ISBN in the file, but we had no chance to print it. We do so in the last statement of the block that concludes the outermost if statement.

Exercises Section 1.6

Exercise 1.25:

Using the Sales_item.h header from the Web site, compile and execute the bookstore program presented in this section.

Exercise 1.26:

In the bookstore program we used the addition operator and not the compound assignment operator to add trans to total. Why didn't we use the compound assignment operator?




C++ Primer
C Primer Plus (5th Edition)
ISBN: 0672326965
EAN: 2147483647
Year: 2006
Pages: 223
Authors: Stephen Prata

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