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Structure Arrays and passing those arrays to a function

Posted on 2006-11-16
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Last Modified: 2010-04-15
I need some help with the proper technique to do the following

1. Build an array of structures of dynamic size (using malloc)
2. Alter that array through user defined functions (passing the array)

Thanks,
 
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Question by:bbcac
2 Comments
 
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momi_sabag earned 500 total points
ID: 17958680
hi

i guess when you say alter the array you mean alter it's values
lets say that your struct name is struct1, and that contains the data
all we need to remember that in C, an array is actually a pointer,
so this is what we do :

struct1** my_array;          // this will define the array
int num_of_items;             // num of items to allocate
int i;

my_array = (struct**) malloc (num_of_items * sizeof(struct1*));        // this will allocate an array of pointers that will  point to the data

for (i=0; i < num_of_items; ++i) {
   my_array[i] = (struct*) malloc (sizeof(struct1));
}

now you have the array of structs
whenever you want to pass the array to a function that will manipulate it, pass as argument the my_array variable and the num_of_items
that is because you can not pass an array of unknow size to a function (The compiler will fail)

momi
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LVL 46

Assisted Solution

by:Kent Olsen
Kent Olsen earned 500 total points
ID: 17958725

Hi bbac,

Ok.  One step at a time.

1....

//  Define a couple of structures of different sizes

typedef struct
{
  int   Type;
  int   MyInt;
}  struct1;

typedef struct
{
  int      Type;
  char   MyName[100];
}  struct 2;

//  Create a union of the structs

union
{
  struct1  IntStruct;
  struct2  NameStruct;
}  union_t;

//  List of union pointers.

uniont_t **UnionList = NULL;
union_t  *MyUnion;
int    UnionCount = 0;

// Now it's just a matter of putting the items in the list.

//  Add a struct 1

  MyUnion = (union_t *)malloc (sizeof (struct1));
  MyUnion->Type = 0;
  UnionList = (union_t **)realloc (UnionList, sizeof (UnionList*) * (UnionCount + 1);
  UnionList[UnionCount] = MyUnion;
  ++UnionCount;

//  Add a struct 2

  MyUnion = (union_t *)malloc (sizeof (struct2));
  MyUnion->Type = 1;
  UnionList = (union_t **)realloc (UnionList, sizeof (UnionList*) * (UnionCount + 1);
  UnionList[UnionCount] = MyUnion;
  ++UnionCount;


Good Luck,
Kent
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