Using functions in C++ - DVD Info

A good way to organize and structure computer programs is dividing them into smaller parts using functions. Each function carries out a specific task of the problem that we are solving.

You've seen that all programs written in C++ must contain the main function where the program begins. You've probably already used functions such as pow, sin, cos, or sqrt from the cmath library. Since in almost all of the upcoming lab experiences you will continue using pre-defined functions, you need to understand how to work with them. In future exercises, you will learn how to design and validate functions. In this laboratory experience, you will search and display information contained in a DVD data base to practice declaring simple functions and calling pre-defined functions.

Objectives:

1. Identify the parts of a function: return type, name, list of parameters, and body.
2. Call pre-defined functions by passing arguments by value ("pass by value"), and by reference ("pass by reference").
3. Implement a simple function that utilizes parameters by reference.

Pre-Lab:

Before you get to the laboratory you should have:

1. Reviewed the following concepts:

   a. the basic elements of a function definition in C++

   b. how to call functions in C++

   c. the difference between parameters that are passed by value and by reference

   d. how to return the result of a function.

2. Studied the concepts and instructions for the laboratory session.

3. Taken the Pre-Lab quiz, available in Moodle.

Functions

In mathematics, a function $f$ is a rule that is used to assign to each element $x$ from a set called domain, one (and only one) element $y$ from a set called range. This rule is commonly represented with an equation, $y=f(x)$. The variable $x$ is the parameter of the function and the variable $y$ will contain the result of the function. A function can have more than one parameter, but only one result. For example, a function can have the form $y=f(x_1,x_2)$ where there are two parameters, and for each pair $(a,b)$ that is used as an argument in the function, the function will only have one value of $y=f(a,b)$. The domain of the function tells us the type of value that the parameter should have and the range tells us the value that the returned result will have.

Functions in programming languages are similar. A function has a series of instructions that take the assigned values as parameters and performs a certain task. In C++ and other programming languages, functions return only one result, as it happens in mathematics. The only difference is that a programming function could possibly not return any value (in this case the function is declared as void). If the function returns a value, we use the instruction return. As in math, you need to specify the types of values that the function's parameters and result will have; this is done when declaring the function.

Function header

The first sentence of a function is called the header and its structure is as follows:

type name(type parameter01, ..., type parameter0n)

For example,

int example(int var1, float var2, char &var3)

would be the header of the function called example, which returns an integer value. The function receives as arguments an integer value (and will store a copy in var1), a value of type float (and will store a copy in var2) and the reference to a variable of type char that will be stored in the reference variable var3. Note that var3 has a & symbol before the name of the variable. This indicates that var3 will contain the reference to a character.

Calling a function

The following line assigns the value returned by the function example to an integer variable x:

x = example(2, 3.5, unCar);

Note that when calling the function we don't include the type of the variables in the arguments as in the function's definition. The third parameter &var3 is a reference variable. Thus, what is being sent to the third argument when calling the function is a reference to the variable unCar. Any assignments that are made to the variable var3 will change the contents of the variable unCar.

You can also use the function's result without having to store it in a variable.

The following line sends the return value of function example to the cout object, so that it is displayed in the terminal:

cout << "The result of the function example is:" << example(2, 3.5, unCar);`

or use it in an arithmetic expression:

y = 3 + example(2, 3.5, unCar);

Overloaded Functions

Overloaded functions are functions that have the same name, but different signatures. The signature of a function is composed of the name of the function and the types of parameters it receives. The signature does not include the return type.

The following function prototypes have the same signature:

int example(int, int) ;
void example(int, int) ; 
string example(int, int) ;

Note that each has the same name, example, and receives the same amount of parameters of the same type (int, int).

The following function prototypes have different signatures:

int example(int) ;
int elpmaxe(int) ;

Note that even though the functions have the same amount of parameters with the same type int, the name of the functions is different.

The following function prototypes are overloaded versions of the function example:

int example(int) ;
void example(char) ;
int example(int, int) ;
int example(char, int) ;
int example(int, char) ;

All of the above functions have the same name, example, but different parameters. The first and second functions have the same amount of parameters, but their arguments are of different types. The fourth and fifth functions have arguments of type char and int, but they have a different order in each case.

In that last example, the function example is overloaded since there are five functions with different signatures but with the same name.

Default Values

Values by default can be assigned to the parameters of the functions starting from the rightmost parameter. It is not necessary to initialize all of the parameters, but the ones that are initialized should be consecutive: parameters in between two parameters cannot be left uninitialized. This allows calling the function without having to send values in the positions that correspond to the initialized parameters.

Examples of Function Headers and Valid Function Calls:

1. Header: int example(int var1, float var2, int var3 = 10) Here var3 is initialized to 10.

   Calls:

   a. example(5, 3.3f, 12) This function call assigns the value 5 to var1, the value 3.3 to var2, and the value of 12 to var3.

   b. example(5, 3.3f) This function call sends the values for the first two parameters and the value for the last parameter will be the value assigned by default in the header. That is, the values in the variables in the function will be as follows: var1 will be 5, var2 will be 3.3, and var3 will be 10.

2. Header: int example(int var1, float var2 = 5.0f, int var3 = 10) Here var2 is initialized to 5 and var3 to 10.

   Calls:

   a. example(5, 3.3f, 12) This function call assigns the value 5 to var1, the value 3.3 to var2, and the value 12 to var3.

   b. example(5, 3.3f) In this function call only the first two parameters are given values, and the value for the last parameter is the value by default. That is, the value for var1 within the function will be 5, that of var2 will be 3.3, and var3 will be 10.

   c. example(5) In this function call only the first parameter is given a value, and the last two parameters will be assigned values by default. That is, var1 will be 5, var2 will be 5.0, and var3 will be 10.

Example of a Valid Function Header with Invalid Function Calls:

1. Header: int example(int var1, float var2 = 5.0f, int var3 = 10)

   Invocation:

   a. example(5, , 10) This function call is invalid because it leaves an empty space in the middle argument.

   b. example() This function call is invalid because var1 was not assigned as a default value. A valid call to the function example needs at least one argument (the first).

Examples of Invalid Function Headers:

1. int example(int var1 = 1, float var2, int var3) This header is invalid because the default values can only be assigned starting from the rightmost parameter.

2. int example(int var1 = 1, float var2, int var3 = 10) This header is invalid because you can't place parameters without values between other parameters with default values. In this case, var2 doesn't have a default value but var1 and var3 do.

DVD Movies and the DVD Data Base

DVD stands for "digital versatile disk" or "digital video disk", which is an optical disc format for storing digital information invented by Philips, Sony, Toshiba, and Panasonic in 1995. The DVD offers larger storage capacity than compact disks (CD) but has the same dimensions. DVDs can be used to store any kind of digital data but they are mainly famous for their use in the distribution of movies.

Which of the example functions would be called by the following instruction: example(42, 'a')? int example(int) ; int example(int, int) ; int example(char, int) ; int example(int, char) ; None of the above The provided arguments are of type int and char (in that exact order). Thus, the following version is called: int example(int, char);.

Consider a function with header int foo01(int a, int b, int c = 5). Would the following be a valid invocation? cout << foo01(1, 2) << endl; Yes No Cannot determine with the provided information. Yes, the invocation is valid because the function foo01 can be invoked with either the first two argumensts or all three arguments since the last argument has a default value.

What will be the value of arg01 after line 19 is executed?

4 3 none; the void command erases the value Since arg01 is passed by value, its value cannot be modified by the instructions of function ejercicio. Its final value is 4.

What is the value of arg02 after line 19 is executed?

5 2 non of the above; the function ejercicio is of type "void", thus the variable will have no value Since arg02 is passed by reference, any assignment that is made to its corresponding parameter (var02) is also an assignment to arg02. At the end of function ejercicio var02 is assigned a value of 2. Therefore, the value of arg02 after the function call in line 19 is 2.

Laboratory Session:

In this laboratory session we will be using a database of DVD movies maintained by http://www.hometheaterinfo.com/dvdlist.htm. This database contains 44MB of information for movies that have been distributed in DVD. Some of the stored information in the database for each DVD is: DVD title, publishing studio, date of publication, type of sound, versions, price, rating, year and genre. The information for each movie is stored in the following format:

DVD_Title|Studio|Released|Status|Sound|Versions|Price|Rating|Year|Genre|Aspect|UPC|DVD_ReleaseDate|ID|Timestamp

For example,

Airplane! (Paramount/ Blu-ray/ Checkpoint)|Paramount||Discontinued|5.1 DTS-HD|
LBX, 16:9, BLU-RAY|21.99|PG|1980|Comedy|1.85:1|097361423524|2012-09-11 00:00:00|
230375|2013-01-01 00:00:00

Exercise 1 - Familiarize Yourself with the Defined Functions

The first step in this laboratory experience is to familiarize yourself with the functions that are already defined in the code. Your tasks require that you imitate what the functions do, for this reason it is important that you understand how to call, declare and define the functions.

Instructions

1. Load the project DVDInfo into QtCreateor. There are two ways to do this:

   • Using the virtual machine: Double click the file DVDInfo.pro located in the folder home/eip/labs/functions-dvdinfo of your virtual machine.
   • Downloading the project's folder from Bitbucket: Use a terminal and write the command git clone http://bitbucket.org/eip-uprrp/functions-dvdinfo to download the folder functions-dvdinfo from Bitbucket. Double click the file DVDInfo.pro located in the folder that you downloaded to your computer.

2. Configure the project. The file main.cpp has the function calls that you will use in the next exercises. The declarations and definitions of the functions that will be called can be found in the files movie.h and movie.cpp.

3. Double click on the file movie.h that contains this project's function prototypes. Go to movie.h and identify which functions are overloaded and describe why.

   Study the function prototypes and documentation in movie.h so that you understand the task they carry out and the data types they receive and return. For each of the following functions, identify the data types they receive and return:

    showMovie
    showMovies 
    getMovieName
    getMovieByName

   You can find the function definitions in the file movie.cpp. Note that some versions of the function showMovie use the object called fields of the QStringList class. The purpose of this object is to provide easy access to information fields of each movie, using an index between 0 and 14. For example, you may use fields[0] to access a movie’s title, fields[1] to access a movie’s studio, fields[8] to access its year, and so forth.

Which of the following functions are overloaded in movie.h? getMovieByName showMovies getMovieInfo The function showMovies is overloaded. There are two versions of showMovies.

Exercise 2 - Call and Modify Functions

In this exercise you will modify the main function and some of the pre-defined functions so that they display only certain movies from the database, display only part of the information, or display the information in a specific format.

Instructions

1. Notice the following line in the main funcion.

   filemanip file("dvd_csv.txt") ;

   Its purpose is to associate the object called file with the data file dvd_csv.txt. When openning a data file in a program it is often necessary to specify the file's absolute path to guarantee that the program finds the file regardless of where it is executed.

   Find out the absolute path of dvd_csv.txt and substitute it for the current "dvd_csv.txt" string. For example, after the change the line might look as follows:

   filemanip file("/home/eip/functions-dvdinfo/dvd_csv.txt") ;

   Compile and run the program. If you see errors such as Cannot open dvd_csv.txt, then the path is incorrect. Correct the path until you get no errors when executing.

2. Perform the following implementations in the main function (in the main.cpp file):

   • Add code so that the program displays the movies that have positions from 80 to 100.
   • Add code so that the program displays only the movies that have “forrest gump” in the title.
   • Add code so that the program displays only the movie in position 75125 using function composition and the function showMovie.
   • Add code so that the program displays the name and the rating of the movie in position 75125.

3. Modify the getMovieInfo function in the file movie.cpp so that it receives an additional parameter by reference where the name of the movie will be assigned. Remember that you must also modify the function's prototype in movie.h.

4. Add the necessary code to the main function so that, using getMovieInfo, it displays the name, rating, year and the genre of the movie in position 75125 in one line. Hint: note that the function getMovieInfo has parameters that are passed by reference.

Exercise 3 - Define and Implement Functions

The functions whose prototypes are in movie.h are implemented in the file movie.cpp. In this exercise you will use the files movie.h, movie.cpp, and main.cpp to define and implement additional functions. As you implement these functions, remember to use good programming techniques and document your program.

Instructions

1. Study the functions that are already implemented in movie.cpp so that they may be used as examples for the functions you will create.

2. Implement a function getMovieStudio that receives a string with the information of a movie and returns the name of the film's studio. Remember to add the function's prototype in the file movie.h. Invoke the function getMovieStudio in main() to display the name and studio of the movie in the position 75125 and demonstrate its functionality.

3. Implement an overloaded function getMovieInfo that returns the name of the studio as well as the name, rating, year and genre. Call the function getMovieInfo in main() to display the name, studio, rating, year and genre of the movie in the position 75125 and demonstrate its functionality.

4. Implement a function showMovieInLine that displays the information displayed by showMovie, but in a single line. The function should have a parameter to receive a string of information of the movie. Call the function showMovieInLine in main() to display the information for the movie in position 75125 to demonstrate its functionality.

5. Implement a function showMoviesInLine that displays the same information displayed by showMovies (all of the movies within a range of positions) but in a single line per movie. For example, a function call would be: showMoviesInLine(file, 148995, 149000);. Call the function showMoviesInLine in main() to display the information and demonstrate its functionality.

Deliverables

Use "Deliverables" in Moodle to hand in the files main.cpp, movie.cpp, and movie.h with the function calls, changes, implementations and declarations that you made in Exercises 2 and 3. Remember to use good programming techniques, include the names of the programmers involved, and document your program.

References

[1] http://mathbits.com/MathBits/CompSci/functions/UserDef.htm

[2] http://www.digimad.es/autoria-bluray-dvd-duplicado-cd-video.html

[3] http://www.soft32.com/blog/platforms/windows/keep-your-dvd-collection-up-to-date-with-emdb-erics-movie-database/

[4] http://www.hometheaterinfo.com/dvdlist.htm