nested if-s vs. multiple if-s

Question

Which one if better, and why? Eg.

if (conditionA && conditionB) {
   //do something 
} else if (conditionA) {
   //do something else
} else if (condition C) {
   ...
}

or

if (conditionA) {
     if (conditionB) {
       //do something 
    } 
 //do something else
} else if (conditionC) {
   ...
}

Answer 1

Given your simple example, I strongly prefer the first option. It makes it obvious that at most one block of code is executed and also specifies the precedence in which the alternatives are considered.

As originally written, your second example doesn't even do the same thing as your first because when both conditionA and conditionB are true it executes both //do something and then //do something else.

I suspect you intended your second option to be:

if (conditionA) {
    if (conditionB) {
        //do something
    } else {
        //do something else
    }
} else if (conditionC) {
    ...
}

Answer 2

Here's an alternative, one which I prefer under many circumstances.

if (conditionA && conditionB) {
   //do something 
}

if (conditionA && !conditionB) {
   //do something else
} 

if (!conditionA && condition C) {
   ...
}

For me, else complicates matters. It means I can't reorder or extract clauses as easily. It implies complexity and imposes sequence that are not really required by the problem I'm trying to solve. So if I can eliminate it, I try to. It's useful; it has its place - but I try not to overuse else.