如何在Swift中扩展变量的范围

Question

I'm writing a macOS Command Line Tool in Xcode. The variable "num1" keeps returning nil. I declared it in the beginning so it should be a global variable. How can I address this?

var num1: Int!

if userChoice == "add" {
    print("Enter first number")
    if let num1 = readLine() {
    }
}

print ("\(num1)")

Answer 1

As mentioned, you have two num1 variables. The 2nd one is scoped to just the block of the 2nd if statement.

You also have an issue that readLine returns an optional String , not an Int .

You probably want code more like this:

var num1: Int?

if userChoice == "add" {
    print("Enter first number")
    if let line = readLine() {
        num1 = Int(line)
    }
}

print ("\(num1)")

Of course you may now need to deal with num1 being nil .

One option is to properly unwrap the value:

if let num = num1 {
    print("\(num)")
} else {
    print("num1 was nil")
}

Answer 2

1. Call the second num1 something else.
2. Put something in the if.

 var num1: Int!

 if userChoice == "add" {
    print("Enter first number")
    if let num2 = readLine() { 
        // This if-let runs if num2 is not nill. 
        print ("\(num2)")
        num1 = Int(num2)
    }
}

print ("\(num1)")