Speed of switch/case vs hash table for parsing XML

  • Thread starter Thread starter _DS
  • Start date Start date
D

_DS

I'm currently using a switch with about 50 case statements in a
stretch of code that's parsing XML attributes. Each case is a string.
I'm told that switch statements will actually use hash tables when
number of cases is around 10 or more, so I haven't changed the code.
Just wondering if anyone knows about how that's structured internally.
It does seem a bit slow.
 
_DS said:
I'm currently using a switch with about 50 case statements in a
stretch of code that's parsing XML attributes. Each case is a string.
I'm told that switch statements will actually use hash tables when
number of cases is around 10 or more, so I haven't changed the code.
Just wondering if anyone knows about how that's structured internally.
It does seem a bit slow.
Click to expand...

What exactly do you mean by the last sentence? Do you mean you've
implemented some code and it's working slowly, or that it sounds like a
slow way of doing things?

I suspect if you're really seeing something working slowly, it won't be
due to the switch statement. I very much doubt that that will be the
bottleneck.
 
I'm almost certain that the C# compiler will not implement the select
construct with a Hashtable. You can easily check by running ildasm on
the assembly. Either way 50 case statements is an awful lot. If
nothing else I recommend refactoring the code to gain readability.

Brian
 
Brian Gideon said:
I'm almost certain that the C# compiler will not implement the select
construct with a Hashtable. You can easily check by running ildasm on
the assembly. Either way 50 case statements is an awful lot. If
nothing else I recommend refactoring the code to gain readability.
Click to expand...

It's easy to verify that at least in the case of strings, the C#
compiler does indeed use a hash table. Compile the following and then
disassemble it:

class Test
{
static void Main (string[] args)
{
int i=0;
switch (args[0])
{
case "1":
case "2":
case "3":
case "4":
case "5":
i=1;
break;
case "6":
case "7":
case "8":
case "9":
case "10":
i=2;
break;
}
i++;
}
}
 
Jon,

That's actually pretty cool. I suppose if it does it for strings it
would do it for others as well. I never would have thought...

Brian
Brian Gideon said:
I'm almost certain that the C# compiler will not implement the select
construct with a Hashtable. You can easily check by running ildasm on
the assembly. Either way 50 case statements is an awful lot. If
nothing else I recommend refactoring the code to gain readability.
Click to expand...

It's easy to verify that at least in the case of strings, the C#
compiler does indeed use a hash table. Compile the following and then
disassemble it:

class Test
{
static void Main (string[] args)
{
int i=0;
switch (args[0])
{
case "1":
case "2":
case "3":
case "4":
case "5":
i=1;
break;
case "6":
case "7":
case "8":
case "9":
case "10":
i=2;
break;
}
i++;
}
}
Click to expand...
 
Brian Gideon said:
That's actually pretty cool. I suppose if it does it for strings it
would do it for others as well. I never would have thought...
Click to expand...

It doesn't actually need to do it for other types. The thing is that
the IL switch instruction only supports integral types, so the C#
compiler needs to "fake it" for string.
 
Back
Top