Why is an empty else-if statement bad style, and how should I rewrite it?

Question

The program that automatically grades my code is docking me "style-points" for an else-if that doesn't execute any code. It says that it may cause an error, but I don't think it could.

I'm not sure how to change it so that it still works but doesn't break the rule. Why is doing this bad form? I think any other way I write it will be harder for a reader to understand. How should it be written instead?

if (! seesWater(LEFT))
{
    turn(LEFT);
}
else if (! seesWater(AHEAD));
else if (! seesWater(RIGHT))
{
    turn(RIGHT);
}
else
{
    turn180();
}

The reason the else-if is there but does nothing is because of the priority in which I want the code to act:

if (! seesWater(AHEAD)), then I don't want the rest of the conditions to run at all because they don't matter.

Answer 1

Who says it's "bad style"?

The relevant question to ask is, is this clearer than alternatives? In your specific case, I'd say it is. The code clearly expresses a choice between 4 options, of which one is "do nothing".

The only change I'd make is to replace that rather insignificant semicolon by an empty pair of braces, possibly with a comment to make it clear it's not a mistake.

if (! seesWater(LEFT)) {
    turn(LEFT);
}
else if (! seesWater(AHEAD)) {
    // nothing required
}
else if (! seesWater(RIGHT)) {
    turn(RIGHT);
}
else {
    turn180();
}

This is not endorsing 'empty clauses' as a generally-acceptable style; merely that cases should be argued on their merits, not on the basis of some Rule That Must Be Obeyed. It is a matter of developing good taste, and the judgement of taste is for humans, not mindless automata.

Answer 2

If this is the logic you want, there is nothing wrong with your code. In terms of code styling wise, I agree with the other users. Something like this would be clearer in my opinion:

if (! seesWater(LEFT))
{
    turn(LEFT);
}
else if (! seesWater(AHEAD))
{
    //Do nothing
}
else if (! seesWater(RIGHT))
{
    turn(RIGHT);
}
else
{
    turn180();
}

But by having priority of making a left turn over moving ahead (by doing nothing), the movement may end up in circles:

If you want the movement to "do nothing" but avoid moving into waters like this:

You may want to change your logic to something like this:

if (! seesWater(AHEAD))
{
    //Do nothing. Keep moving
}
else if (! seesWater(LEFT))
{
    turn(LEFT);
}
else if (! seesWater(RIGHT))
{
    turn(RIGHT);
}
else
{
    turn180();
}

Answer 3

You can invert the ! seesWater(AHEAD) condition. Then you can move the code in its else clause to its if clause:

if (! seesWater(LEFT))
{
    turn(LEFT);
}
else if (seesWater(AHEAD)) 
{
    if (! seesWater(RIGHT))
    {
        turn(RIGHT);
    }
    else
    {
        turn180();
    }
}