The hello program, which you saw earlier, doesn’t include many features you’d expect in a "real" program. To illustrate more of C++, let's consider the program named f2c which converts Fahrenheit temperatures to Celsius.

This is an IPO or Input Processing Output program, typical of those that we'll be building in these first few weeks. A computer is an information processor; a Cuisinart for words, numbers and ideas. Instead of vegetables, you feed it input; raw numbers, facts, figures and symbols (data). Your computer program processes that data, and turns it into information: organized, meaningful and useful output.

Below you'll find the code for the f2c program. Use the arrow on the left to show and hide the code as we discuss its various features.

/**
 * @file f2c.cpp
 * @author Stephen Gilbert
 * @version CS 150 Reader
 * Converts Fahrenheit to Celsius.
 */
#include <iostream>
#include <iomanip>
using namespace std;

double convert(double temp);

int main()
{
    cout << "Enter a temperature in Fahrenheit: ";
    double fahr;
    cin >> fahr;
    double celsius = convert(fahr);
    cout << "Converted: " << fahr << "F -> "
        << celsius << "C" << endl;
    return 0;
﹜

/**
 * Converts a temperature from Fahrenheit to Celsius.
 * @param temp the Fahrenheit temperature to convert.
 * @return the Celsius temperature.
 */
double convert(double temp)
{
    return (temp - 32) * 5.0 / 9.0;
﹜

Comments

A comment is text provided for readers of your code; comments are ignored by the compiler. C++ has two kinds of comments:

• Single-line (//) are comments that end when the line ends.
• Paired (/* … */) are comments which can enclose several lines.

Documentation comments(/** … */) are paired comments, which start with /** instead of /*. The are processed using a tool called Doxygen. The f2c program begins with a documentation file comment (lines 1-6), which describes the program as a whole, and uses three Doxygen tags:

• @file contains the name of the program file and allows Doxygen to locate and identify the functions which it needs to document.
• @author should your CS 150 login id (such as sgilbert).
• @version should contain your CS 150 section (such as Spring '24 MWAM)

Functions also have documentation comments appearing before each one. Such comments start with a line describing the purpose of the functions, and then contain tags which describe the function inputs and outputs:

• @param is the name and description of each input, or parameter.
• @return describes the meaning of the output produced by the function.

Lines 24-28 in the sample code contain the documentation comment for the convert function. You'll learn more about documentation comments in the homework.

Below you'll find the code for the f2c program. Use the arrow on the left to show and hide the code as we discuss its various features.

/**
 * @file f2c.cpp
 * @author Stephen Gilbert
 * @version CS 150 Reader
 * Converts Fahrenheit to Celsius.
 */
#include <iostream>
#include <iomanip>
using namespace std;

double convert(double temp);

int main()
{
    cout << "Enter a temperature in Fahrenheit: ";
    double fahr;
    cin >> fahr;
    double celsius = convert(fahr);
    cout << "Converted: " << fahr << "F -> "
        << celsius << "C" << endl;
    return 0;
}

/**
 * Converts a temperature from Fahrenheit to Celsius.
 * @param temp the Fahrenheit temperature to convert.
 * @return the Celsius temperature.
 */
double convert(double temp)
{
    return (temp - 32) * 5.0 / 9.0;
}

Libraries are collections of useful, pre-written components (functions and classes). The standard library comes with your C++ compiler. It is divided into a number of "packages", known as headers in C++. Here are the headers you'll use most often. You'll meet more as the semester goes on. Find the whole list at cppreference.com.

// input-output 
#include <iostream> //reading and printing
#include <iomanip>  // formatting output
#include <cmath>    // all math functions
#include <string>   // The C++ string class

// input-output
//reading and printing 
#include <iostream> 
// formatting output
#include <iomanip>  
// all math functions
#include <cmath>    
// The C++ string class
#include <string>   

 using namespace std; 

Below you'll find the code for the f2c program. Use the arrow on the left to show and hide the code as we discuss its various features.

/**
 * @file f2c.cpp
 * @author Stephen Gilbert
 * @version CS 150 Reader
 * Converts Fahrenheit to Celsius.
 */
#include <iostream>
#include <iomanip>
using namespace std;

double convert(double temp);

int main()
{
    cout << "Enter a temperature in Fahrenheit: ";
    double fahr;
    cin >> fahr;
    double celsius = convert(fahr);
    cout << "Converted: " << fahr << "F -> "
        << celsius << "C" << endl;
    return 0;
}

/**
 * Converts a temperature from Fahrenheit to Celsius.
 * @param temp the Fahrenheit temperature to convert.
 * @return the Celsius temperature.
 */
double convert(double temp)
{
    return (temp - 32) * 5.0 / 9.0;
}
  

A function is a named section of code that performs an operation. Every C++ program must contain exactly one function with the name main, which is automatically called when your program starts up.

• Each statement in the body of the main function is then run (or executed), one after another, in order. This concept is called sequence.
• When main has finished its work, execution of the program ends.

The main function contains six different statements.

1. Line 15 is an output statement. It prints a prompt, telling the user what to enter. cout is the standard output stream (similar to System.out in Java or stdout in Python). The characters in quotes (generically called a string literal) are sent to the screen using the insertion operator (<<).
2. Line 16 is a variable definition for fahr, a floating-point number, called double in C++, which is uninitialized.
3. Line 17 is an input statement. cin is similar to a Scanner object in Java, or a file object in Python. It reads a sequence of characters from the keyboard and stores the converted value in fahr using the >> (or extraction) operator.
4. Line 18 has both a variable definition and a function call. The line calls the function named convert, passing a copy of fahr as an argument. Then, it defines a variable, celsius, and initializes it with the returned value.
5. Lines 19 and 20 are a single output statement, spread over two lines. The statement combines text and variables to produce the desired result.
6. Line 21 is a return statement which ends the program and returns a value to the operating system. 0 indicates success, while anything else signals failure. You may omit this return statement in main, but not in any other function.

Below you'll find the code for the f2c program. Use the arrow on the left to show and hide the code as we discuss its various features.

/**
 * @file f2c.cpp
 * @author Stephen Gilbert
 * @version CS 150 Reader
 * Converts Fahrenheit to Celsius.
 */
#include <iostream>
#include <iomanip>
using namespace std;

double convert(double temp);

int main()
{
    cout << "Enter a temperature in Fahrenheit: ";
    double fahr;
    cin >> fahr;
    double celsius = convert(fahr);
    cout << "Converted: " << fahr << "F -> "
        << celsius << "C" << endl;
    return 0;
}

/**
 * Converts a temperature from Fahrenheit to Celsius.
 * @param temp the Fahrenheit temperature to convert.
 * @return the Celsius temperature.
 */
double convert(double temp)
{
    return (temp - 32) * 5.0 / 9.0;
}

In addition to main, the f2c program uses the function convert which the main function calls (on line 18) to carry out its work. Before main can call convert, though, the compiler must know what kind of arguments the function requires, and what kind of value it will return. This information is called the function's interface, and it is supplied by adding a function declaration or prototype, appearing before the main function, (on line 11).

double convert(double temperature);

The prototype provides the information needed to call the function: its name along with the types of its inputs and outputs. C++ requires prototype declarations so the compiler can check whether calls to functions are compatible with the definitions of those functions.

The convert function definition, starting on line 29, repeats the interface information from the prototype, butdoes not end in a semicolon. Instead, the definition header is followed by a body (just like main) consisting of a list of statements surrounded (or delimited) by braces {}.

double convert(double temperature)
{
    return (temp - 32) * 5.0 / 9.0;
}

The body of convert is a single return statement which uses a formula or expression to convert the input Fahrenheit temperature into the output Celsius temperature, and return it to the caller.