What is an optional variable? - ios

Question

I have the following code to work out if a number is prime;
#IBAction func isPrimeBtn(sender: AnyObject) {
if(isPrimeTxt.text.isEmpty){
isPrimeResultLbl.text = "Please Enter A Value";
}else{
var value = isPrimeTxt.text.toInt();
println(value);
if(value == 0){
println("value was 0");
isPrimeResultLbl.text = "Its Not";
}else{
for(var i = 3; i * i < value; i += 2){
println("value was 0");
if(value! % i == 0){
isPrimeResultLbl.text = "Its Not";
}
}
isPrimeResultLbl.text = "Its Prime!!";
}
}
}
When i print the value using println(value) I get Optional 22, if I check the variable is 0 it is true.
What am I doing incorrectly?
Thanks

Answer 1

There's a few problems here. Firstly, you correctly use value! in your modulus test, but in the first test and in the for loop you use value. I suggest you use an if let block instead, so value is not an optional ever:
if let value = isPrimeTxt.text.toInt() {
// now you know it's an integer
// ... do your test
} else {
// some error message
}
However, even if you do this, your logic is wrong on three counts:
1) isPrimeResultLbl.text = "Its Prime!!" is always executed after your for loop
2) it's incorrect for everything less than 10 because your for (var i = 3; i * i < value; i += 2) { will never execute if 3 * 3 >= value.
3) you need to start checking divisors at 2, not 3. 16 will appear prime using your algorithm (though 4 & 8 won't, as they're under 9!)

Answer 2

What happens if prime.text is "abc"? What happens is that the conversion toInt() will fail. Because it can fail, toInt() doesn't return an integer, it returns an optional integer. The result is either an integer or nothing. When you call println, you don't print an integer, you print an optional integer. And that's what println prints - it says "optional 22". It might have said something like "empty".
If you print value! instead of value, you tell Swift that you are 100% sure that the optional integer value is actually there. If it isn't, the expression value! will crash. If it is there, println will print 22. So the exclamation mark turns an optional either into a proper value, or into a crash.