Thread: Structure array question

Hi,
Im doing an assignment that needs us to create a program to make a movie database.

In main() there is the structure array declarations, a main menu and 2 movie entries (so its not empty)

My main menu consists of a switch statement based on the user input.
From there it is passed to 1 of 3 functions that will either search database, add entry or print entire list of database.

Im having a hard time setting up my structure array and Im just looking for pointers of what I'm doing wrong.

the array is supposed to be able to fit 100 movies.

The code Im posting is only my global declarations, main and first function.

Whats happening right now:
-main menu works, hands to functions fine.
-passing to first function doesn't ask to enter anything
-initial 2 entries don't work and have error message "unused variable entry"

Code:

struct movie_list entry[size] = {{"Children of Men",109,8,"Science Fiction"},                                     {"Donnie Darko",133,6,"Science Fiction"}

any hints and pointers appreciated. Thanks!*edit* I'm using Eclipse and mingw

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define size 100 //Size of array defined


void enter_movie(void); //Prototypes for 3 functions
void print_movie(void);
void search_movie(void);
int counter;


struct movie_list { //Movie structure prototype
    char title[30];
    float length;
    float rating;
    char genre[20];
}entry[size];


int main(void)
{
    int exit; //This variable is for the while controlling while loop. It breaks out the while loop to end the program
    int menu;     // variable for menu selection
    struct movie_list entry[size] = {{"Children of Men",109,8,"Science Fiction"},
                                     {"Donnie Darko",133,6,"Science Fiction"}
    };


    exit = 1; //sets parameter for while loop expectation to be met.


    while(exit >=1){ //while loop to bring end of function operation below back to main menu


        printf("The Movie List Builder V1.0\n\n");
        printf("What would you like to do? \n Enter selection number between 1-4 below:\n");
        printf("1: Add movie entry\n");
        printf("2: Show entire movie list\n");
        printf("3: Search for movie\n");
        printf("4: Exit program\n");
        scanf("%d", &menu);


        switch(menu)
        {      case 1 : //If user chooses 1 enter entry function is called
            enter_movie();
            break;


        case 2 :
            print_movie();//If user chooses 2 search movie function is called
            break;


        case 3 :
            search_movie();  //If user chooses 3 print all entries function is called
            break;


        case 4 :
            exit = 0;  //If user chooses 4 the variable exit changes to 0 and the while loop ends.
            //leads to program ending.
            break;


        default:       //default if user doesn't input a number between 1-4
            printf("\n Error:Selection is not between 1-4.");
            break;
        }} //end of while loop and switch statement


        printf("Have a nice day!");
        system("pause");
        return 0;
    } //main() end




    void enter_movie (void) { //start of add entry function
        int num;


        printf("How many movies entries would you like to make?");
        scanf ("%d" ,&num);
        for (counter=2;counter>=num+2;counter++){


        printf("Enter Movie Title:");
        scanf(" %s",&entry[counter].title[30]);
        printf("Enter Movie Length (in minutes):");
        scanf(" %g",&entry[counter].length);
        printf("Enter Movie Rating (Between 1-10):");
        scanf(" %g",&entry[counter].rating);
        printf("Enter Movie Genre:");
        scanf(" %s",&entry[counter].genre[20]);
            }
        return;
    } //End of add entry function

This looks problematic:

Code:

void enter_movie(void);

The function accepts no arguments and has no return value. Therefore, for it to do anything useful, it must affect global state. This is okay if the global state is standard output, e.g., if you will call the function to print a menu or some other message. But here the function is named "enter_movie", and indeed it turns out that you are using global variable named entry. Unfortunately, global variables tend to make it harder for you to reason about your program, and consequently can make it harder to develop and maintain your program. Instead, make entry a local variable, e.g., local to the main function.

Your struct type is named struct movie_list, but it is not a movie list; it models a single movie. Therefore, declare it as such:

Code:

#define MOVIE_ENTRIES_MAX_SIZE 100

struct movie {
    char title[30];
    float length;
    float rating;
    char genre[20];
};

struct movie_list {
    struct movie entries[MOVIE_ENTRIES_MAX_SIZE];
    size_t n; /* number of entries in use */
};

You can then declare two functions:

Code:

void enter_movie(struct movie *movie);
void enter_movie_list(struct movie_list *movies);

In the main function, you can then declare:

Code:

struct movie_list movies;

Originally Posted by Glassjaw

So the main chunk of code you posted, and the functions are global?

Yes. That is fine because the macro names a constant, i.e., not global state. Note that I used a descriptive name as well as the convention for macro names to be fully uppercase: you definitely do not want a name collision with macros. The functions have to be global (or rather, at file scope: you can have static functions that are technically not global): standard C does not allow for nested functions.

Originally Posted by Glassjaw

and then you just declare it in main?

As in the variable of type struct movie_list named movies? Yes.

Okay,
Im still a little confused between which iteration of the structure you use for different things...

for example if I was to declare the first movie.. Would I put something like this in main?

Code:

strcpy(movie_list entry[0].title, "Children of men");
        movie_list entry[0].length = 109;
        movie_list entry[0].rating = 8;
        movie_list entry[0].genre, "Science Fiction");

and then relevent printf statements or whatever I wanted to do?



and what about passing in a function or to a function (say the beginning of my functions above or going to the function like in my switch statements?

would I use:

Code:

void enter_movie(struct movie *movie)

and what does the * in *movie do?



I tried changing the code to the variables you recommended and I have errors everywhere
so I'm still clearly not fully understanding
Thank you and sorry for so many questions! this is confusing!

Originally Posted by Glassjaw

Im still a little confused between which iteration of the structure you use for different things...

Basically, struct movie models a single movie; struct movie_list models the list of movies.

Originally Posted by Glassjaw

for example if I was to declare the first movie.. Would I put something like this in main?

No, struct movie_list is the struct type. The object is named movies. Therefore:

Code:

strcpy(movies.entries[0].title, "Children of men");
movies.entries[0].length = 109;
movies.entries[0].rating = 8;
strcpy(movies.entries[0].genre, "Science Fiction");

Originally Posted by Glassjaw

and what about passing in a function or to a function (say the beginning of my functions above or going to the function like in my switch statements?

For example:

Code:

enter_movie_list(&movies);

Originally Posted by Glassjaw

and what does the * in *movie do?

It declares that movie is a pointer to a struct movie object.