creating default vars (objects) in objective c - ios

Question

dumb question: lets say I'm assigning a var in a conditional statement. I don't know if the condition will be satisfied and i still want the var to be defined.. whats the correct way of writing this
example:
NSDecimalNumber *number = [[NSDecimalNumber alloc]init]; // this is pointless right?
if(x == z){
number = [whatevernum1 decimalNumberByMultiplyingBy: whatevernum2];
} else {
number = [whatevernum2 decimalNumberByDividingBy: whatevernum3];
}
// do something with number variable.

Answer 1

There is no need to initialize number since it will be set. Just do this:
NSDecimalNumber *number;
if(x == z){
number = [whatevernum1 decimalNumberByMultiplying: whatevernum2];
} else {
number = [whatevernum2 decimalNumberByDividing: whatevernum3];
}
// do something with number variable.
In your case number will be assigned a value one way or another. But you might have a situation like this:
if (someCondition) {
// set number to value A
} else if (anotherCondition) {
// set number to value B
}
Here, it is possible that neither condition is met. In this case you need to deal with this properly by initializing number to nil.
NSDecimalNumber *number = nil;
if (someCondition) {
// set number to value A
} else if (anotherCondition) {
// set number to value B
}
if (number) {
// process result
}

Answer 2

You need to declare the variable but not assign it, like this:
NSDecimalNumber *number;
if(x == z){
number = [whatevernum1 decimalNumberByMultiplying: whatevernum2];
} else {
number = [whatevernum2 decimalNumberByDividing: whatevernum3];
}
This tells the compiler that you want to use a variable named number, but don't have a value for it yet. In some cases, you may find it convenient to initialise the variable to nil rather than leaving it as a null pointer.

Answer 3

Normally, as others have pointed out, you would either not initialise (if you can guarantee that you will set a value, eg through an if/else pair), or you would initialise to nil.
In this simple case, a ternary statement would make your code much clearer:
NSDecimalNumber *number = x == z ? [whatevernum1 decimalNumberByMultiplyingBy:whatevernum2] : [whatevernum2 decimalNumberByDividingBy:whatevernum3];