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

Answer 1

Yes exactly, you're gaining runtime, it won't seem much for one operation, but you have to keep in mind that operations will get repeated millions of times

Why perform two evaluations when one is enough is the logic

Answer 2

At runtime if(a||b) will test a first, if a is true it will not waste time testing b therefor the compiler will be 1 execution ahead. Therefore if a is more likely to be true than b this test is also likely to cut 1 line. The total number of lines not executed is tiny on a single line but it’s huge if the statement is nested in a loop of some sort(for,while ,recession or database related queries ). Eg per say we have 1million mins to test data in a database at 1 minute per record (30sec for condition A and 30 sec for condition B). Let A have 80% chances to be true and B have 20% chances to be true. The total time needed if you put A first is 600-000hrs yet it’s 900-000hrs if you put B first.if A is tested first[(0,8*1millions hours)*0,5mins+(0,2*1million hours)*1min]===6000-000hrs : if B is tested first [(0,2*1million hours)*0,5mins+(0,2*1million hours)*1min]===9000-000hrs. However you will notice the difference is less significant if the probability of A becoming true is closer to that of B.

Answer 3

In many cases there is no practical difference apart from a tiny performance improvement. Where this becomes useful is if your checks are very expensive function calls (unlikely) or you need to check things in order. Say for example you want to check a property on something and to check if that something is nil first, you might do something like:

If (a != nil && a.attribute == valid) {}