Why is order of expressions in if statement important

Question

Suppose I have an IF condition :

if (A || B) 
    ∧
    |
    |
   left
{ 
   // do something  
}

Now suppose that A is more likely to receive a true value then B , why do I care which one is on the left ?

If I put both of them in the IF brackets , then I know (as the programmer of the code) that both parties are needed .

The thing is , that my professor wrote on his lecture notes that I should put the "more likely variable to receive a true" on the left .

Can someone please explain the benefit ? okay , I put it on the left ... what am I gaining ? run time ?

Answer 1

Its not just about choosing the most likely condition on the left. You can also have a safe guard on the left meaning you can only have one order. Consider

if (s == null || s.length() == 0) // if the String is null or empty.

You can't swap the order here as the first condition protects the second from throwing an NPE.

Similarly you can have

if (s != null && s.length() > 0) // if the String is not empty

The reason for choosing the most likely to be true for || or false for && is a micro-optimisation, to avoid the cost of evaluated in the second expression. Whether this translates to a measurable performance difference is debatable.

Answer 2

I put it on the left ... what am I gaining ? run time ?

Because || operator in C++ uses short-circuit evaluation.
i.e: B is evaulated only if A is evaluated to a false.

However, note that in C++ short-circuit evaluation is guaranteed for "built in" data types and not custom data types.