Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!udel!princeton!phoenix!asjoshi
From: asjoshi@phoenix.Princeton.EDU (Amit S. Joshi)
Newsgroups: comp.lang.c
Subject: Unions and structures in C
Message-ID: <1520@phoenix.Princeton.EDU>
Date: 22 Jan 88 22:44:42 GMT
Organization: Princeton University, NJ
Lines: 40
Keywords: C Turbo C

Hello,

I have a question about unions and structures in C:
Suppose I have the following fragment of a program :

struct one { float x,y; };

union two { float x[2]; 
	    struct one one; };

main () {
	union two two;

	two.one.x = 1.0; two.one.y = 2.0;
	printf(" x = %f,  y = %f\n",two.one.x,two.one.y);
	printf(" x[%d] = %f, x[%d] = %f\n",1,two.x[0],2,two.x[1]);
}
My question is what would the second printf statement print ?
Would I get "x[1] = 1.0, x[2] = 2.0" or "x[1] = 2.0, x[2] = 1.0" or
would I get that two.x[?] is undefined ? Or what? Does the order in
which the the elements of the structure are present correspond to the 
other elements declared in the union ? Is any such correspondence
guaranteed ? I know from some test cases that in TurboC I seem to get a
correspondence between the parts of the union.   

The reason I need this is that if the order or correspondence were known
then I could use very informative names in the structure and use arrays
where array processing is more efficient, e.g. suppose x and y were the
coordinates of an object then using x and y for code where there is a
distinction between the axes is nice whereas when I want to say
calculate the motion I might wish to integrate where the array
representation is more convinient.

Thanks,


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Amit Joshi	BITNET	|	Q3696@PUCC.BITNET
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