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Chapter 2, A more elaborate example

To illustrate several of the fundamental points of Pike we will now introduce an example program, that will be extended as we go. We will build a database program that keeps track of a record collection and the songs on the records. In the first version we hard-code our "database" into the program. The database is a mapping where the index is the record name and the data is an array of strings. The strings are of course the song names. The default register consists of one record.

mapping (string:array(string)) records =
    "Star Wars Trilogy" : ({
        "Fox Fanfare",
        "Main Title",
        "Princess Leia's Theme",
        "Here They Come",
        "The Asteroid Field",
        "Yoda's Theme",
        "The Imperial March",
        "Parade of the Ewoks",
        "Luke and Leia",
        "Fight with Tie Fighters",
        "Jabba the Hut",
        "Darth Vader's Death",
        "The Forest Battle",
We want to be able to get a simple list of the records in our database. The function list_records just goes through the mapping records and puts the indices, i.e. the record names, in an array of strings, record_names. By using the builtin function sort we put the record names into the array in alphabetical order which might be a nice touch. For the printout we just print a header, "Records:", followed by a newline. Then we use the loop control structure for to traverse the array and print every item in it, including the number of the record, by counting up from zero to the last item of the array. The builtin function sizeof gives the number of items in an array. The printout is formatted through the use of sprintf which works more or less like the C function of the same name.
void list_records()
    int i;
    array (string) record_names=sort(indices(records));

        write(sprintf("%3d: %s\n", i+1, record_names[i]));
If the command line contained a number our program will find the record of that number and print its name along with the songs of this record. First we create the same array of record names as in the previous function, then we find the name of the record whose number (num) we gave as an argument to this function. Next we put the songs of this record in the array songs and print the record name followed by the songs, each song on a separate line.
void show_record(int num)
    int i;
    array (string) record_names = sort(indices (records));
    string name=record_names[num-1];
    array (string) songs=records[name];
    write(sprintf("Record %d, %s\n",num,name));
        write(sprintf("%3d: %s\n", i+1, songs[i]));
The main function doesn't do much; it checks whether there was anything on the command line after the invocation. If this is not the case it calls the list_records function, otherwise it sends the given argument to the show_record function. When the called function is done the program just quits.
int main(int argc, array (string) argv)
    if(argc <= 1)
    } else {
        show_record((int) argv[1]);

2.1 Taking care of input

2.2 Communicating with files

2.3 Completing the program

2.4 Then what?

2.5 Simple exercises

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