CSCI 1110.005 - Lecture Notes
Chapter 4
Program Input and the Software Design Process

Contents:

Input Streams

File Input and Output

Top-Down Design

Input Streams

• An input stream can be viewed as a continuous stream of characters that are consumed by the input mechanisms of the program.
  
  
• To use input and output streams in a C++ program, you must have the following include statement near the beginning of your program:

        #include <iostream.h> 



• iostream.h defines cin which is an istream and cout which is an ostream. cin is used to bring data into a program. cout is used to output data from a program.
  
  
• The input stream can be thought of as a list of input values that are separated by white-space characters such as space, tab, and end-of-line(EOL) characters.
  
  
• << is the output operator in C++. The << operator sends a value to the screen. Numeric values are translated to characters for output.
  
  
• >> is the extraction operator in C++. It is sometimes referred to as "get from". The >> operator gets the item from the input stream.
  
  
• Input examples:
  
  

                    ( i and j are int, ch is char, and x is float)

Statement	Data	Contents After Input
cin >> i;	32	i = 32
cin >> i >> j;	4 60	i = 4, j = 60
cin >> i >> ch       >> x;	25 A 16.9	i = 25, ch = 'A', x = 16.9
cin >> i >> ch       >> x;	25\n A\n 16.9	i = 25, ch = 'A', x = 16.9
cin >> i >> ch       >> x;	25A16.9	i = 25, ch = 'A', x = 16.9
cin >> i >> j       >> x;	12 8	i = 12, j = 8, waiting   for next input
cin >> i >> x;	46 32.4 15	i = 46, x = 32.4, 15 held   for next input statement

 
 


• Using the extraction operator, it is impossible to read a space character from the input stream since it acts as a delimiter between inputs. To read the next character, no matter what it is, use the cin.get function.
  
  

Statements	Data	Contents After Input
cin >> ch1;  cin >> ch2;  cin >> ch3	A B\n  CD\n	ch1 = 'A', ch2 = 'B', ch3 = 'C'
cin.get(ch1);      cin.get(ch2);  cin.get(ch3);	A B\n CD\n	ch1 = 'A', ch2 = ' ', ch3 = 'B'

 


• cin.ignore is used to skip characters in the input stream. The function takes two parameters:

    cin.ignore(200, '\n');

The first parameter is the maximum number of characters to skip. The second parameter is the skip-to character. The above statement will cause C++ to skip up to 200 characters in the input stream until a newline, /n, character is encountered. 



• Input and output of data in a program is like a two-way conversation with the program prompting the user for each input:
 
    cout << "Enter the part number: ";
  cin >> partNumber;
  cout << "Enter the quantity: ";
  cin >> quantity;
  cout << "Enter the unit price: ";
  cin >> unitPrice;
  cout << endl;

  totalPrice = unitPrice * quantity;

  cout << "The total price for " << quantity
       << " items of part number " << partNumber << endl
       << "at $" << setprecision(2) << unitPrice
       << " each is $" << totalPrice << '.' << endl;

  would have the following output to the screen. The blue characters were typed
   by the user.

    Enter the part number: 4671
    Enter the quantity: 10
    Enter the unit price: 27.25

    The total price for 10 items of part number 4671
    at $27.25 each is $272.50.
 

File Input and Output

• A file is a named area in secondary memory (such as a disk) that holds a collection of data. A C++ program can read from and write data to a file.
  
  
• To use a file, your program must do four things
1. Include the header file fstream.h.
   
   
2. Declare the files that are to be used.
   
   
3. Prepare each file for reading or writing by using a function named open.
   
   
4. Specify the name of the file in each input or output statement.
   
   
//**************************************************************
// Mileage program - page 149
// This program computes miles per gallon given four amounts
// for gallons used, and starting and ending mileage
//**************************************************************
#include <iostream.h>
#include <fstream.h>    // For file I/O

int main()
{
    float    amt1;         // Number of gallons for fillup 1
    float    amt2;         // Number of gallons for fillup 2
    float    amt3;         // Number of gallons for fillup 3
    float    amt4;         // Number of gallons for fillup 4
    float    startMiles;   // Starting mileage
    float    endMiles;     // Ending mileage
    float    mpg;          // Computed miles per gallon
    ifstream inMPG;        // Holds gallon amounts and mileages
    ofstream outMPG;       // Holds miles per gallon output

    // Open the files

    inMPG.open("inmpg.dat");
    outMPG.open("outmpg.dat");

    // Get data

    inMPG >> amt1 >> amt2 >> amt3 >> amt4
          >> startMiles >> endMiles;

    // Compute miles per gallon

    mpg = (endMiles - startMiles)
                        / (amt1 + amt2 + amt3 + amt4);

    // Output results

    outMPG << "For the gallon amounts" << endl;
    outMPG << amt1 << ' ' << amt2 << ' '
         << amt3 << ' ' << amt4 << endl;
    outMPG << "and a starting mileage of " << startMiles
           << endl;
    outMPG << "and an ending mileage of " << endMiles << endl;
    outMPG << "the mileage per gallon is " << mpg << endl;
    return 0;
} 



• Whenever something goes wrong with an input or output operation, the stream is said to be in a fail state. This can happen, for example, when you attempt to read and there is no more data in the file.
  
  
• The open function associates a stream variable with a physical file on disk.
  
  
• When the program attempts to open a file for reading, and the file does not exist, the file is in a fail state.
  
  
• When the program attempts to open a file for writing, and the file does exist, the program erases what was in the file and replaces it with any output the program directs to that file.
  
  
• When the program attempts to open a file for writing, and the file does not exist, it is created as an empty file that is used for output from the program.

Top-Down Design

• The number of details in even a simple program can be overwhelming. Without some type of systematic approach, the task becomes unmanagable.
  
  
• Top-Down Design is a technique for developing a program in which the problem is divided into more easily handled subproblems, the solutions of which create a solution to the overall problem.
  
  




• In Top-Down Design, the solution is first described in general or abstract steps, which are then defined in more concrete terms.
  
  
• An Abstract Step leaves some implementation details unspecified.
  
  
• A Concrete Step is fully specified.
  
  
• A Module is a collection of steps that solves a problem or subproblem. It can contain both concrete and abstract steps.
  
  
• Functional Cohesion is a property of a module in which all concrete steps are directed toward solving just one problem, and any significant subproblems are written as abstract steps.
  
  
• Modular Coupling is a is a property of a design that indicates how much one module has to know about other modules to work correctly.
  
  
• Low coupling and high cohesion is a sign of a good problem solution design.