Multidimensional Static Array Initialization Question ...

[Tim Graham]

Hi everyone,

I have a question regarding initialization of multi-dimensional arrays in C.
In trying some experiments a few days ago, I found that on all the C compilers
that I have access to (Sun-3 UNIX BSD 3.5, VAX Mt. Xinu UNIX 4.3), the 
following definition is illegal:

static int	test_array[][] = { { 1, 2, 3, 4, 5 }, { 1, 2 } };

With further experimenting, I determined that in an n-dimensional static
array initialization, the last n-1 dimensions (the n-1 "most quickly
varying" array indexes) must be specified by constant expressions.  That is,
the only array dimension which can be determined at compile time by
the form of the initializer is the first dimension.  For example, the
following definition is legal:

static int test_array[][5] = { { 1, 2, 3, 4, 5 }, { 1, 2 } };

Being thoroughly curious about what was going on, I went and consulted
my K&R (1st edition) and H&S.  In K&R, I found no mention of initializations
like the first one above, but in H&S I found a reference (p. 67) which
says the following:


	"The length of the array, a constant expression, may be 
	 omitted as long as it is not needed to allocate storage.
	 It is not needed when:
		
		1. The object being declared is a formal parameter 
		   of a function.
		2. The declarator is accompanied by an initializer 
		   from which the length of the array can be deduced.
		3. The declaration is not a defining occurrence; that 
		   is, it is an external declaration that refers to 
		   an object defined elsewhere.

	 An exception to these cases is that the declaration of any
	 n-dimensional array must include the sizes of the last n-1
	 dimensions so that the accessing algorithm can be determined."


I also found a reference (p. 96) which says the following:


	"When multidimensional arrays are used, it is necessary to
	 specify the bounds of all but the first dimension, so
	 that the proper address arithmetic can be calculated.

	    extern int matrix[][10]; /* ?-by-10 array of int */

	 If such bounds are not specified, the declaration is
	 in error."


Why is it that non-specification of array size declarators is illegal in
the presence of complete initializers?  It seems clear that there is 
enough information present at compile time to allocate the appropriate
memory and to calculate the proper address arithmetic.  The calculation
does not even appear to be that difficult - a fairly simple recursion
would do the trick.

It would be very nice if it was possible to make declarations like the first
one above.  In a particular application which I have been working on, I
would like to represent the types of expected parameters to various commands 
as integers, and store a list of integers for each command as a row in a 2-D 
array.  That is, I would like to do something like the following:

			      P  P  P  P  P .............
			      a  a  a  a  a .............
			      r  r  r  r  r .............
			      m  m  m  m  m .............
			      1  2  3  4  5 .............

			      |  |  |  |  | .............
			      |  |  |  |  | .............
			      V  V  V  V  V .............

	Command 1 ------> { { 1, 5, 7, 4, 5 },
	Command 2 ------>   { 2, 5, 6 },
	Command 3 ------>   { 1, 5, 4, 3, 5, 2, 5, 1 },
	Command 4 ------>   { 5, 5, 4, 4, 3, 2 }
	Command 5 ------>   { 1 } }

It would be very nice to declare this as a static 2-D array, with the 
initializers determining the array bounds.  This way, if someone wished
to add a command in the future, it would merely be necessary to add the
expected parameter types as a row to this array, and array bounds and other
such bothersome details would not have to be recalculated.  It seems to
me that since such initialization is possible for 1-D arrays it would
be more consistent if it were possible for 2-D arrays.

Are there any good reasons why this is not done?  If not, has this issue 
already been brought up to the ANSI-C committee?  If not, how does one go 
about bringing it up, and what are the chances of getting something like this 
incorporated for the current version of the standard?

Any help that anyone can give me will be appreciated.

Thank you,

Tim Graham

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