Re: Can SWITCH be avoided by using BIT OPERATORS or anything else
?
* mast4as:
On Apr 20, 4:06 pm, "Alf P. Steinbach" <al...@start.no> wrote:
* mast4as:
Hi everyone
It seems that one day I saw some code where the developper managed to
use bit operations to avoid using a switch which I would like to avoid
for performance reasons. Here is what I am trying to do.
typedef float color[3];
typedef struct Test
{
float a;
float b;
color c;
float d;
};
typedef enum Type
{
TYPE_A = 1 << 0,
TYPE_B = 1 << 1,
TYPE_C = 1 << 2,
TYPE_D = 1 << 3,
};
int main( int argc, char **argv )
{
Type type = TYPE_B;
Test test;
memset( &test.a, 0x0, sizeof( Test ) );
switch ( type )
{
case TYPE_A: test.a += 1; break;
case TYPE_B: test.b -= 1; break;
case TYPE_C: test.c[ 0 ] = 1; break;
case TYPE_D: test.d += 2; break;
defaullt: ;
}
}
Basically I would like to avoid the switch statetement. What I am
trying to do in the code is add the result to the appropriate element
of the variable "test" of type Test based on the type of the variable
"type". Do you guys see a super efficient way of doing this ;-)
Reorder your struct so that the values you're interested in are consecutive.
Represent them as an array, then use indexing.
That said, this smells very much like a kind of Evil(TM) premature optimization.
The cost of catering to this micro-efficiency can be quite high. Conversely,
much work can be saved by disregardning this level of efficiency completely,
which is what people do when they program in languages like C#, Java or Python.
Cheers & hth.
- Alf
'
Thanks a lot Alf... However saddly I don't understand what you
mean ;-) Could you please show me an example. I am happy to fix my
code but I just can't make sense of what you are saying...
You are right about the optimization but I have to do this switch
millions of time per process (and this program is runned thousands and
thousands of time ;-) which is the reason I want to be sure this is
fast ...
Oh well.
Off the cuff:
class Whatever
{
private:
float myItems[6];
public:
enum Kind{ kind_a, kind_b, kind_d, kind_c };
Whatever(): myItems() {} // Zero the items
float& a() { return myItems[0]; }
float& b() { return myItems[1]; }
float& d() { return myItems[2]; }
float& c() { return myItems[3]; }
float* c_arr() { return myItems + 3; }
float& operator[]( Kind const i ) { return myItems[i]; }
};
int main()
{
static const delta[] = { 1, 1, 2, 1 }
Whatever test;
Whatever::Kind const i = Whatever::kind_c;
test[i] += delta[i];
}
Cheers & hth.,
- Alf