initialize an array of elements that contain another array


W

wenmang

hi,
I have following:
struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char * name;
array1 *myArray;
};

Now, I try to create an array of array2 by initialization:
array2 collection[] =
{
10, "first",
{1, "first.first"},
20, "second",
{2, "second.second"}
};

compiler generates following similar error:
error: a value of type "1" cannot be used to initialize an entity of
type "array1 *"

How am I going to initialize something like above. Thx
 
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L

Lew Pitcher

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Hash: SHA1


hi,
I have following:
struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char * name;
array1 *myArray;
};

Now, I try to create an array of array2 by initialization:
array2 collection[] =
{
10, "first",
{1, "first.first"},
20, "second",
{2, "second.second"}
};

compiler generates following similar error:
error: a value of type "1" cannot be used to initialize an entity of
type "array1 *"

How am I going to initialize something like above. Thx

You are going to explicitly malloc() space for your array1 entries, and
populate the myArray pointers in array2 with the malloc()ed values.

You are going to do this in program logic.

You are *not* going to do this with an initializer.

HTH
- --
Lew Pitcher


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W

wenmang

Lew said:
You are going to explicitly malloc() space for your array1 entries, and
populate the myArray pointers in array2 with the malloc()ed values.

You are going to do this in program logic.

You are *not* going to do this with an initializer.

HTH
- --
Lew Pitcher


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actually, I want an array1 inside array2 without specifying the size of
array1. The size of array1 is determined through initialization list,
can I do that?
struct array2
{
int id;
char *name;
array1 myArray[]; - will not compile here
};
 
W

wenmang

struct type1
{
int id;
char *name;
};


struct type2
{
int id;
char * name;
type1 *myType1Collection; //should be arrary of type1 here: type1
myType1Collcetion[]?

};

The actual array size for type1 is a variable and can only determined
through initialization list after reading from external input through a
code generation routine. How can I achieve that?
 
W

wenmang

struct type1
{
int id;
char *name;
};


struct type2
{
int id;
char * name;
type1 *myType1Collection; //should be arrary of type1 here: type1
myType1Collcetion[]?

};

The actual array size for type1 is a variable and can only determined
through initialization list after reading from external input through a
code generation routine. How can I achieve that?
 
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S

Szabolcs Borsanyi

hi,
I have following:
struct array1
{
int id;
char *name;
};

This is ok.
struct array2
{
int id;
char * name;
array1 *myArray;
};

The last member makes me worry. It is not an array but a pointer. Moreover,
you did not define the array1 type. If you are using a C++ compiler, this
is not a problem, but, well, in C, you'd better write
struct array1 *myArray;
But as already told, this is not an array. An array looks like
struct array1 myArray[5];

but then you fix the number of elements. You could also write
struct array1 myArray[];
but then you would like to define an array out ouf struct array2 which.
Should not all the members of this array be of the same type?
(Different sizes of myArrays make them incompatible.)

Szabolcs Borsanyi
 
K

Keith Thompson

I have following:
struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char * name;
array1 *myArray;
};

Now, I try to create an array of array2 by initialization:
array2 collection[] =
{
10, "first",
{1, "first.first"},
20, "second",
{2, "second.second"}
};

The first thing you should do is stop compiling your code with a C++
compiler; either that, or ask in comp.lang.c++.

You haven't declared a type "array1", so the declaration of the third
element of your "struct array2" is illegal. You've declared a type
called "struct array1".

The following would be legal:

struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char *name;
struct array1 *myArray;
};

struct array2 collection[] = ...

[...]
How am I going to initialize something like above. Thx

C allows you to omit some braces in an initializer, but it's not
generally a good idea to take advantage of this. Enclosing the
initializers for each object and subobject in braces can make the code
easier to understand; it can also make it easier for the compiler to
catch some errors.

Assuming you want two elements in your array, you can declare it like
this:

struct array2 collection[] =
{ { 10, "first", ? },
{ 20, "second", ? } };

The question marks, of course, aren't legal; they need to be replaced
by something that initializes the myArray member of the struct array2
object.

What you want to do is initialize the myArray member (a pointer to
struct array1) so it points to an object of type struct array1. To do
this, you would need to allocate an object of type struct array1. You
can't really do this in an initializer.

Here's one possible approach:

struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char *name;
struct array1 *myArray;
};

struct array1 a1_0 = { 1, "first.first" };
struct array1 a1_1 = { 2, "second.second" };

struct array2 collection[] =
{ { 10, "first", &a1_0 },
{ 20, "second", &a1_1 } };

Incidentally, "array1" and "array2" aren't very good names for
structures. I hope you're using something clearer in your real code.
 
B

Barry Schwarz

hi,
I have following:
struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char * name;
array1 *myArray;
};

array2 has three members.
Now, I try to create an array of array2 by initialization:
array2 collection[] =
{
10, "first",

By omitting the braces on this line, you are telling the compiler that
this is not the complete list of initialization values for the first
element of the array. Since there are two values in this list, they
are assigned to id and name. The next value will be assigned to
myArray (which has type pointer to struct).

{1, "first.first"},

This is not a suitable value for myArray. If myArray were a struct
instead of a pointer to struct, it would probably work.
20, "second",
{2, "second.second"}
};

compiler generates following similar error:
error: a value of type "1" cannot be used to initialize an entity of
type "array1 *"

How am I going to initialize something like above. Thx

If you really want myArray to be a pointer, then you will need to
define a number of objects of type struct array1 (probably initialized
with the braced values above) and replace the braced initialization
values above with the address of the appropriate object (e.g.,
&array1_1).


Remove del for email
 
S

Simon Biber

hi,
I have following:
struct array1
{
int id;
char *name;
};

struct array2
{
int id;
char * name;
array1 *myArray;
}; ....
How am I going to initialize something like above. Thx

Hi Wenmang. (IIRC that means 'illiterate' in Chinese?)

Nobody here mentioned that you can use a feature from C99 to create an
automatic struct and take its address as part of the initialiser.

#include <stdio.h>

struct foo
{
int id;
char *name;
};

struct bar
{
int id;
char *name;
struct foo *pfoo;
};

int main(void)
{
struct bar myBar = {
10,
"hello",
& (struct foo) {20, "world"},
};
printf("{%d, %s, {%d, %s}}\n",
myBar.id, myBar.name,
myBar.pfoo->id, myBar.pfoo->name);
return 0;
}

Note that the unnamed struct object that was created has function scope.
If you do the initialisation inside a function and then return from the
function, the pointer will become invalid.
 
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