Question

I just want to know what the difference between all the conditional statements in objective-c and which one is faster and lighter.

Answer 1

The language-agnostic version (mostly, obviously this doesn’t count for declarative languages or other weird ones):

When I was taught programming (quite a while ago, I’ll freely admit), a language consisted of three ways of executing instructions:

• sequence (doing things in order).
• selection (doing one of many things).
• iteration (doing something zero or more times).

The if and case statements are both variants on selection. If is used to select one of two different options based on a condition (using pseudo-code):

if condition:
    do option 1
else:
    do option 2

keeping in mind that the else may not be needed in which case it’s effectively else do nothing. Also remember that option 1 or 2 may also consist of any of the statement types, including more if statements (called nesting).

Case is slightly different – it’s generally meant for more than two choices like when you want to do different things based on a character:

select ch:
    case 'a','e','i','o','u':
        print "is a vowel"
    case 'y':
        print "never quite sure"
    default:
        print "is a consonant"

Note that you can use case for two options (or even one) but it’s a bit like killing a fly with a thermonuclear warhead.

While is not a selection variant but an iteration one. It belongs with the likes of for, repeat, until and a host of other possibilities.

As to which is fastest, it doesn’t matter in the vast majority of cases. The compiler writers know far more than we mortal folk how to get the last bit of performance out of their code. You either trust them to do their job right or you hand-code it in assembly yourself (I’d prefer the former).

You’ll get far more performance by concentrating on the macro view rather than the minor things. That includes selection of appropriate algorithms, profiling, and targeting of hot spots. It does little good to find something that take five minutes each month and get that running in two minutes. Better to get a smaller improvement in something happening every minute.

The language constructs like if, while, case and so on will already be as fast as they can be since they’re used heavily and are relative simple. You should be first writing your code for readability and only worrying about performance when it becomes an issue (see YAGNI).

Even if you found that using if/goto combinations instead of case allowed you to run a bit faster, the resulting morass of source code would be harder to maintain down the track.