Homework 4: Music Database

Objectives

Dynamic memory
Linked lists

Description

For this assignment, you will write a program that maintains a database of music, using a linked list.

The program has four commands:

Command	Purpose
`a` title,artist	Add the given song to the database.
`d` title,artist	Delete the given song from the database.
`l`	List everything in the database.
`q`	Quit the program.

Sample Run

User input looks like this:

    % gcc -std=c99 music.c -o music
    % ./music
    Welcome to the song database!
    Command: l
    There are no songs.

    Command: a Yellow Submarine,Beatles
    "Yellow Submarine" by "Beatles" added.

    Command: a Hey Jude,Beatles
    "Hey Jude" by "Beatles" added.

    Command: a Mamma Mia,Abba
    "Mamma Mia" by "Abba" added.

    Command: a Reform School Musical,Various Artists
    "Reform School Musical" by "Various Artists" added.

    Command: l
    Artist                   Title
    ------                   -----
    Abba                     Mamma Mia
    Beatles                  Hey Jude
    Beatles                  Yellow Submarine
    Various Artists          Reform School Musical

    Command: d A Good Song,Eminem
    "A Good Song" by "Eminem" not found.

    Command: d Hey Jude,Beatles
    "Hey Jude" by "Beatles" deleted.

    Command: l
    Artist                   Title
    ------                   -----
    Abba                     Mamma Mia
    Beatles                  Yellow Submarine
    Various Artists          Reform School Musical

    Command: q
    Happy listening!

Required Stuff

You must use this structure for your linked list:

struct Song {

    char title[24];

    char artist[24];

    struct Song *next;

};

[Get Code]

You must write the following functions:

void show_all(struct Song *head): Print the entire linked list.
void add_song(struct Song **head, char *title, char *artist): Add this title to the list.
void delete_song(struct Song **head, char *title, char *artist): Delete this title from the list.

You may write other functions, as needed. You won’t get far without writing main().

Requirements

show_all() takes a struct Song *, but add_song() and delete_song() take a struct Song **. Really.
On input, a comma separates the title from the artist. Both the title and the artist may contain spaces.
As you can see from the output, the linked list is sorted, first by artist, then by song title.
Make sure to free() all the memory that you got from malloc(). This is true even if an error occurred (e.g., inserting a duplicate song).
Emit an error message if the command isn’t recognized, or is otherwise defective.
Emit an error message if the user tries to insert the same song twice.
Emit an error message if the user tries to delete something that hasn’t been inserted.
Emit an error message if malloc() fails.
Note what the “l” command should emit if there are no songs.
You do not have to check for a title or artist being too long.

Hints

Don’t get fooled into thinking “I must sort the list.” You never really sort it—you just keep it sorted at all times. You can think of it as an ongoing insertion sort.
I’d read the whole command line into a string using fgets() and pick it apart using strchr().
Make sure to test the following tricky cases, because you know that we will:
- Inserting at the beginning of the list.
- Inserting at the end of the list.
- Deleting the first item of the list.
- Deleting the last item of the list.

I’d do the program in this order:

Implement show_all().
Implement the parsing (picking apart) for the “a” command, and just use printf() to display what the title & artist are.
Implement add_song(), but don’t insert the song in alphabetical order, at first. Just insert it at the beginning of the list.
Change add_song() to insert the song in the proper place.
Implement delete_song().

Grading

This assignment is worth 10 points. You’ll lose points if you submit a file that:

doesn’t compile
doesn’t work like the sample run
isn’t named music.c
uses language features that we have not yet covered
changes any of the prompts or messages
contains poor code:
- cryptic variable names
- missing/useless comments
- extra { } for no reason
and many, many, more!

How to submit your homework:

Go to the same directory as the music.c file, and do this:

    ~cs157/bin/checkin HW4 music.c

How to check your answer after you turn it in:

    ~cs157/bin/peek HW4 music.c

How to receive negative points:

Turn in someone else’s work.

CS157