Run A and then B or run C

问题

I'm looking for a control structure to deal with potentially multiple blocks of code or a NONE block. The most basic case is when there are only two conditions, A and B:

+-------+-------+------------+
|   A   |   B   | Blocks Run |
+-------+-------+------------+
| false | false | C          |
| false | true  | B          |
| true  | false | A          |
| true  | true  | A + B      |
+-------+-------+------------+

The best thing I have so far is:

if( A )
{
    // Run Block A
}

if( B )
{
    //Run Block B
}
else if( !A )
{
    //Run Block C
}

回答1:

For clarity, I'm going to rename your conditions P and Q (as is the convention in propositional logic)

if( P || Q)
{
    P && A();
    Q && B();
}
else
{
    C();
}

Edit:

adding more checks is rather easy:

if( P || Q || R || S)
{
    P && A();
    Q && B();
    R && D();
    S && E();
}
else
{
    C();
}

Edit, after comment:

Well how about this.

int runC = 1;
P && (runC=0, A());
Q && (runC=0, B());
R && (runC=0, D());
S && (runC=0, E());
runC && C();

回答2:

If it's all about minimising comparsions using a table is the only way to go.

switch ((B << 1) | A)
{
  case 0:
    // C
    break;

  case 0b01: // case 1:
    // A
    break;

  case 0b10: // case 2:
    // B
    break;

  case 0b11: // case 3:
    // A
    // B
    break;

  default:
    // Shouldn't arrive here.
    break;
}

回答3:

If you just want to evaluate each condition once:

if (a) {
  A();
  if (b) {
    B();
  }
} else if (b) {
  B();
} else {
  C()
}

I don't think it's much better than your code, though. While it does evaluate the condition only once, the code blocks to execute are now duplicated. As far as I can see, you'll either have to evaluate a condition twice or a write a code block twice. In that case, I'd prefer evaluating the conditions and saving them to variables, then go with your example: duplicating blocks of code is worse than reading a variable twice (most of the time, it's more important to have code that you can understand and maintain instead of trying to "optimize" it when you haven't measured that it's a bottleneck).

回答4:

You want to run block A when condition A, followed by block B when condition B and so on. With one exception: When no criteria is met then execute the NONE block. Right? Then:

if( !A && !B && !C && ...)
{
  run NONE-BLOCK;
}
else
{
  if( A ) run A-BLOCK;
  if( B ) run B-BLOCK;
  if( C ) run C-BLOCK;
  ...
}

来源：https://stackoverflow.com/questions/21627707/run-a-and-then-b-or-run-c

标签

control-structure