var and let

Hi amir, I'm still learning swift myself, so please take my answer with a pinch of salt, as i may be wrong.

Firstly, from what i understand, let is preferred over var, because using a constant allows the compiler to optimise code and memory allocation.

But I think you're also asking why Let is being used in this specific example, as the function will clearly be called more than once. So in effect, you're asking - how can we call the function more than once with a Let value that appears to be fixed forever as a constant??!! The reason for this is that the constant "greeting" is local to the function - ie "greeting" is not recognised as a variable outside of the function. Greeting only exists within the function. The return value from the call is recognised and received - but you will have to assign this to a variable explicitly (see my example below).

Thus, because the constant Greeting is local to the function, it is only ever assigned once, so a let assignment is perfectly acceptable. Within the scope of the sayHello function, its value never changes. The next time you call the function, you are creating a completely new instance of the variable, so again, a new let assignment makes perfect sense (but the new instance of greeting has no relation to the old instance). Within the scope of the sayHello function, the constant never changes its value (and once the function returns its value, the memory allocated to that instance is deleted entirely, presumably). ie every time you call the function, you're creating a completely new instance of the variable greeting.

To backup my thinking.... If you add the following line to your code (outside of your function definition!!), then you will get an error: println("Greeting is: (greeting)") Error - use of unresolved identifier greeting. This proves that greeting does not exist as a variable outside of the function.

In contrast, if i now instead define a new variable outside of the function definition, then it works without an error. func sayHello(personName: String) -> String { let greeting = "Hello, " + personName + "!" return greeting }

var greeting2 = sayHello("Tom") println("this is the greeting (greeting2)")

var greeting3 = sayHello("Amir") println("this is the greeting (greeting3)")

hope this helps,

cheers, martin.