What does "If let result = (some function here)" do?

Question

import UIKit
func isDivisible(numberOne: Int, numberTwo: Int) -> Bool? {
    if numberOne % numberTwo == 0 {
        return true
    }
    else {
        return nil
}
}
if let result = isDivisible(4, 2) {
    println("Divisible")
}
else {
    println("Not Divisible")
}
So here's the code that was the correct answer for this exercise. I'm confused on what this line does: if let result = isDivisible(4, 2) {
Does this call the function isDivisible with the parameters and store it in result? If so, then how come it will print Not Divisible when the numbers in the parameter don't divide? What part in the isDivisible function occurs to cause the Not Divisible to be printed? Does the nil thing make it goto the else in the second if else?

Di Su · Accepted Answer

Hi Roland,
if let result = isDivisible(4, 2) is used to check if the optional does not equal nil. 
If the optional does not equal nil, then it must equal true, therefore println("Divisible")
If the optional does not equal true, then it must equal nil, therefore println("Not Divisible")
So for example, we could do: 
swift
if isDivisible(firstNumber: 4, secondNumber: 2) != nil {
    println("Divisible")
} else {
    println("Not Divisible")
}

where != translates to 'does not equal'.
This works because our goal is to check to see if a value has been returned from our function. The actual value does not matter. 
So for example, we could do: 
```swift
func isDivisible(#firstNumber: Int, #secondNumber: Int) -> Int? {
    if firstNumber % secondNumber == 0 {
        return 5
    }
    return nil
}
if isDivisible(firstNumber: 4, secondNumber: 2) != nil {
    println("Divisible")
} else {
    println("Not Divisible")
}
```
This will println("Divisible") because our function has returned a value and does not equal nil.

Dylan Shine · Answer

Swift uses a thing called Optionals, which in laymen's terms allow Swift to gracefully handle an absence of a value (aka nil). When you write an if let statement, you are checking to make sure that your value isn't nil.
Example:
```swift
let favFood = ["Pizza", "Ice Cream"]
if let result = favFood[1] {
   println("Yum")
} // this will execute
if let result = favFood[2] {
   println("Yum")
} //This will not execute, favFood[2] doesn't exist

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Roland Shen

Roland Shen

What does "If let result = (some function here)" do?

2 Answers

Di Su

Di Su

Dylan Shine

Dylan Shine

Roland Shen

Roland Shen

Dylan Shine

Dylan Shine