Advantage of switch over if-else statement


Question

What's the best practice for using a switch statement vs using an if statement for 30 unsigned enumerations where about 10 have an expected action (that presently is the same action). Performance and space need to be considered but are not critical. I've abstracted the snippet so don't hate me for the naming conventions.

switch statement:

// numError is an error enumeration type, with 0 being the non-error case
// fire_special_event() is a stub method for the shared processing

switch (numError)
{  
  case ERROR_01 :  // intentional fall-through
  case ERROR_07 :  // intentional fall-through
  case ERROR_0A :  // intentional fall-through
  case ERROR_10 :  // intentional fall-through
  case ERROR_15 :  // intentional fall-through
  case ERROR_16 :  // intentional fall-through
  case ERROR_20 :
  {
     fire_special_event();
  }
  break;

  default:
  {
    // error codes that require no additional action
  }
  break;       
}

if statement:

if ((ERROR_01 == numError)  ||
    (ERROR_07 == numError)  ||
    (ERROR_0A == numError)  || 
    (ERROR_10 == numError)  ||
    (ERROR_15 == numError)  ||
    (ERROR_16 == numError)  ||
    (ERROR_20 == numError))
{
  fire_special_event();
}
1
159
3/8/2019 8:48:59 PM

Accepted Answer

Use switch.

In the worst case the compiler will generate the same code as a if-else chain, so you don't lose anything. If in doubt put the most common cases first into the switch statement.

In the best case the optimizer may find a better way to generate the code. Common things a compiler does is to build a binary decision tree (saves compares and jumps in the average case) or simply build a jump-table (works without compares at all).

152
4/25/2013 3:09:03 PM

For the special case that you've provided in your example, the clearest code is probably:

if (RequiresSpecialEvent(numError))
    fire_special_event();

Obviously this just moves the problem to a different area of the code, but now you have the opportunity to reuse this test. You also have more options for how to solve it. You could use std::set, for example:

bool RequiresSpecialEvent(int numError)
{
    return specialSet.find(numError) != specialSet.end();
}

I'm not suggesting that this is the best implementation of RequiresSpecialEvent, just that it's an option. You can still use a switch or if-else chain, or a lookup table, or some bit-manipulation on the value, whatever. The more obscure your decision process becomes, the more value you'll derive from having it in an isolated function.


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