Data Structure in C with pointers: I want to store an array of information in each position

Question

How can I define a data structure in C, where for each position I want to store: an int array of size M, an int array of size N and a double value? I tried the following with no success yet.

At the beginning of my code I defined my data structure (LOCOPT) as follows:

typedef struct LOCOPT
{
  int    *org;
  int    *des;
  double value;
} LOCOPT;

LOCOPT *locopt;

Then, I initialized the locopt using calloc as follows:

for( locopt_ctr=0 ; locopt_ctr<locopt_max ; locopt_ctr++ )
{
    locopt[locopt_ctr].org = (LOCOPT *) calloc(M, sizeof(LOCOPT));
    locopt[locopt_ctr].des = (LOCOPT *) calloc(N, sizeof(LOCOPT));
}

When I tried to use it in the code it did not work: I want to do this:

for( m=0 ; m<M ; m++ )
locopt[locopt_ctr].org[m] = origin[m]; // regardless of what origin[m] is.

I need to use calloc several times instead of once only and I don't know how to do that. I need to use the loop properly. Any ideas please?

`locopy` is a pointer to ... nothing. Either declare it as a non-pointer or malloc some at least one `LOCOPT` struct for it. — ooga, Feb 05 '14 at 21:18
@self What do you mean by init? The three coding parts are all what I have regarding this issue. — Wael, Feb 05 '14 at 21:20
@user3276935 I suggest looking at some questions showing how to allocate memory for simple types. — this, Feb 05 '14 at 21:22
@self, I know how to allocate memory for simple types. I did that before. I don't know how to handle this one though. — Wael, Feb 05 '14 at 21:25
Find the total size of this data structure array which is I think locopt_max * (Size of one LOCOPT) — Wajahat, Feb 05 '14 at 21:30

s.bandara · Answer 1 · 2014-02-06T17:51:27.380

2

There are two problems with your allocations, one is about the memory for the locopt array itself, and the other is about how you allocate memory for des and org in each structure.

First, we don't see if you ever allocate memory for your locopt structure array. If that has L elements, you could do

locopt = calloc(L, sizeof(LOCOPT));

Then, you still need to allocate memory for the fields org and des in it like this (for each structure, as you do), but

locopt[locopt_ctr].org = calloc(M, sizeof(int));

and likewise for des. LOCOPT is the name of your structure, but the type of your arrays is int.

edited Feb 06 '14 at 17:51

answered Feb 05 '14 at 21:32

s.bandara

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I tried, but I still have an error. "a value of type "void *" cannot be assigned to a value of type "int *". Any idea please? – Wael Feb 05 '14 at 21:40
Is this C or C++? If this is C++, you need to cast as done in the answer by @DavidW. See also http://stackoverflow.com/a/3559772/1916766. – s.bandara Feb 05 '14 at 21:55

score -1 · Accepted Answer · answered Feb 05 '14 at 21:40

You're trying to do too much at once. Let's break this down to some simpler pieces first:

You've declared LOCOPT *locopt. Before you can use locopt itself as a pointer (or in array syntax) to anything, you have to allocate memory for it, eg

locopt[locopt_ctr] = (LOCOPT *) calloc(1,sizeof(LOCOPT));

But that's only part of the effort.

If you want to store a list of items in each instance of locopt, then you need to call calloc again on the members of the newly allocated locopt instance :

locopt[locopt_ctr].org = (int *) calloc(M, sizeof(int)); locopt[locopt_ctr].des = (int *) calloc(N, sizeof(int));

Bottom line - if you've got a pointer, you've got to allocate it eventually.

Thank you. Both answers made it work. At least for now the code is running and it seems to be working. Thank you everyone. — Wael, Feb 05 '14 at 21:44
Careful, given the way `locopt` was defined, you are dereferencing a dangling pointer. — s.bandara, Feb 05 '14 at 21:51

Data Structure in C with pointers: I want to store an array of information in each position

2 Answers2