array of pointers to structures

Question

I'm trying to understand if my code is correct. I need to declare an array of pointers to structs, create a new struct and assign the values and print them. It seems to me that I'm not declaring array of pointers correctly. I need to know what I'm doing wrong. Thank you I'm getting this compile error: error: 'people' undeclared (first use in this function) And I've tried to insert struct data *list; into main but it wouldnt work

     char *book[] = { "x", "y", "z",};
     int number[] = { 1, 2, 3};

     struct data = { char *bookname; int booknumber;};

     function(char *x, int y)
     {
       static int count;

       struct data *list[3];

       //creating a new struct 
       list[count] = (struct data*) malloc( sizeof(struct data) );

       //assigning arguments
       list->bookname = x;
       list->booknumber = y;

       count++;
     }

     int main()
     {
       struct data *list[3];

       int i;
       for(i = 0; i < 3; i++)
       {
         function(book[i], number[i]);

         printf("name: %c number: %d", list[i]->bookname, list[i]->booknumber);
       }

@triclosan - right, `function` should be made to return a pointer to a `struct data`, which can then be printed (and, importantly, `free()`ed!) in main. — Kevin Vermeer, Jul 19 '12 at 15:57

score 3 · Answer 1 · answered Jul 19 '12 at 15:41

3

Since you want arrays, you need to declare arrays:

char *book[] = { "x", "y", "z",};
int number[] = { 1, 2, 3};

Another issue is

list = (struct data*) malloc( sizeof(struct data) );

//assigning arguments
list[count]->bookname = ...

Here, list is always going to have exactly one element. So if count is anything other than 0, you will be accessing an array out of bounds!

answered Jul 19 '12 at 15:41

chrisaycock

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off course, i just mistyped it. i ahve arrays in my actual program – Jack Morton Jul 19 '12 at 15:42
well, i have tried doing struct data *list[3]; but it wouldnt compile – Jack Morton Jul 19 '12 at 15:46
1

@JackMorton Make it a dynamic array: `malloc( sizeof(struct data) * 3 )` – chrisaycock Jul 19 '12 at 15:47

score 2 · Accepted Answer · answered Jul 19 '12 at 15:57

2

Please change the following piece of code

    // declaring array of pointers to structs //         
     struct data *list;         
    //not compiling        
    //struct data *list[3]; ---> There is no problem with this statement.        
   //creating a new struct         
   list = (struct data*) malloc( sizeof(struct data) );  ---> //This statement should compilation error due to declaration of struct data *list[3]

to

struct data *list[100]; //Declare a array of pointer to structures  
//allocate memory for each element in the array
list[count] = (struct data*) malloc( sizeof(struct data) );

answered Jul 19 '12 at 15:57

Jay

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The `[100]` limit is a regression. This should also be dynamically allocated, or handled by the caller. – Kevin Vermeer Jul 19 '12 at 15:59
@Kevin, The answer is just a sample. So, I have used 100. I have just given him a pointer as to what was causing him the so called compilation error. Hope this clears the air. – Jay Jul 20 '12 at 04:02

score 0 · Answer 3 · answered Jul 19 '12 at 15:42

I think you should write:

char *book[] = { "x", "y", "z"};

Because in your case you were declaring an array of chars and filling it with pointers, which actually makes no sense.

In the line of code above, it just means "declare an array of pointers".

Hope it helped...

score 0 · Answer 4 · answered Jul 19 '12 at 15:56

0

These are things are wrong in your program

struct data = { char *bookname; int booknumber;};

"=" should not be there

list = (struct data*) malloc( sizeof(struct data) );
list[count]->bookname = x;
list[count]->booknumber = y;

Here you are creating space for single list, so you cant do list[count]-> bookname, it should be list->bookname. Same with booknumber
And list is local to function, you cant access it in main.

answered Jul 19 '12 at 15:56

neel

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I'm sorry, 1st one is just a mistype again for me. it's in proper shape in the actual code – Jack Morton Jul 19 '12 at 15:58

array of pointers to structures

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