Re: A89: Arrays in C

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Re: A89: Arrays in C


 > Really?  I've tried to do that w/ Borland C++ in my CS class at school and
 > it didn't work.  When I asked my CS teacher, he said it was impossible...

 > > and yes, you can instead do this:
 > >
 > > int a[]={9,2,7};

I think I know what your problem is with that.

What Robin's talking about is an initialiser, as in this:

 int x[ 3 ] = { 1, 2, 3 };

 void func() 
 {
  ...
 }

where x is a global variable and is initialised.
ANSI C does not allow initialisation of auto variables:

 void func()
 {
 int x[ 3 ] = { 1, 2, 3 };
 ...
 }

is not valid ANSI but [ of course :-) ] is valid gcc.
(In fact, gcc allows you any expression in initialisers.)

Now this is not valid in ANSI *or* gcc: [ :-( ]

 void func()
 {
 int x[ 3 ];
   ...
   x = { 1, 2, 3 };
 }

On the other hand, gcc allows you to create dynamic arrays, so this is 
actually valid (not in ANSI, though):

 int func( int select )
 {
   return ((int [3]) { 3, 6, 9 })[ select ];
 }

Using the dynamic array creation you can assign values to an array,
albeit often slower than doing it piece by piece:

 void func()
 {
 int x[ 3 ];

   memcpy( x, (int [3]) { 1, 2, 3 }, sizeof( x ) );
 }

The above is quicker than doing

  x[ 0 ] = 1; x[ 1 ] = 2; x[ 2 ] = 3;
  
because all initialisers are constants.

However,

 void func()
 {
 int x[ 3 ];

   memcpy( x, (int [3]) { foo(), bar(), baz() }, sizeof( x ) );
 }

will be slower than

  x[ 0 ] = foo();
  x[ 1 ] = bar();
  x[ 2 ] = baz();
  
because the compiler has to generate a temporary array then copy it to 
x, that is, what it will do is this:

  temp[ 0 ] = foo();
  temp[ 1 ] = bar();
  temp[ 2 ] = baz();
  memcpy( x, temp, sizeof( x ));
  
which is obviously slower.

Regards,

Zoltan
References: