Replacement for goto statement in Swift

Question

How can I transfer control to a specific line in Swift code?

In Objective-C I would do something like the following, using goto

if(a==b)
{
    goto i123;
}
else
{
    goto i456;
}
NSLog(@"the not reachable point");
i123:
NSLog(@"line 123 is here");
int j = 5;
int x = 2+j;
i456:
NSLog(@"line 456 is here");

The only control transfer statements in Swift I could find were continue, break, fallthrough, and return

continue and break only work with loops; return and fallthrough don't transfer control this way.

What can I use?

EDIT:-

Julien__'s answer didn't actually solve my problem but it could be the only available option right now. so i have accepted the answer by Julien__

Answer 1

Perhaps a switch statement ?

switch (a==b){
default:
    NSLog(@"the not reachable point");
    fallthrough­
case true:
    NSLog(@"line 123 is here");
    int j = 5;
    int x = 2+j;
    fallthrough­
case false:
    NSLog(@"line 456 is here");
}

EDIT : Here is how you could go backward.

let START = 0
let STOP  = -1
var label = START

while(label != STOP){
    switch (label){

    default:
        label = START­

    case START:
        NSLog(@"the not reachable point");
        if a==b {
            label = 123
        } else {
            label = 456
        }

    case 123:
        NSLog(@"line 123 is here");
        int j = 5;
        int x = 2+j;
        fallthrough­

    case 456:
        NSLog(@"line 456 is here");
        fallthrough

    case STOP:
        label = STOP
    }
}

Wrap your code in a giant (but well organized) switch statement. You could even create a function named goto in order to modify the value of the label var.

Answer 2

Ummm..... Swift actually support label which can be use as goto for control flow but less flexible. The following code is from Swift the programming language :

gameLoop: while square != finalSquare {
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }

    switch square + diceRoll {
    case finalSquare:
    // diceRoll will move us to the final square, so the game is over
        break gameLoop
    case let newSquare where newSquare > finalSquare:
    // diceRoll will move us beyond the final square, so roll again
    continue gameLoop
    default:
    // this is a valid move, so find out its effect
        square += diceRoll
        square += board[square]
    }
}
print("Game over!")

Answer 3

It seems as if Swift doesn't want anyone to use goto statements. Probably to avoid spaghetti code that would be too hard to follow in the future.

One possible alternative is to use functions. Functions have names which have meaning and are easier to understand than mere line numbers.

Answer 4

For the sake of posterity:

• The article goto in Swift clearly exemplifies how to implement goto style functionality, including warnings on why not to use them and a rationale for their absence in the language.

• The article author also made it available as a Swift package called Goto.swift on GitHub.

Answer 5

What about this?

var j : Int?
var x : Int?

if a == b {
    println("line 123 is here")
    j = 5
    x = 2 + j!
}
println("line 456 is here")

Answer 6

In languages where it is available I don't think GoTo is always a bad thing. I'd never use it to jump up, down and around within a function. That causes no end of confusion. But I do like to use GoTo to give me a common exit point. Here's an example of what I mean (pseudo code) something like this:

func SomeFunction() -> Bool
{
    var iReturnValue = false;

    // Do some processing

    if(SomeCondition == true)
    {
        // return true;
        // No. Instead of having a return statement here.
        // I'd rather goto a common exit point.

        iReturnValue = true;
        goto ExitPoint;
    }

    // Do Some More processing

    if(SomeOtherCondition == SomeResult)
    {
        iReturnValue = false;
        goto ExitPoint;
    }

    //
    // More processing
    //

ExitPoint:
    // By having a common exit point I can do any
    // cleanup here and I've got a single return point
    // which I think makes my code easier to read.

    return iResultValue;
}

I know I could achieve the same thing with a few well placed braces but I just find a well used Goto makes life much simpler.

Answer 7

Here is a closure ({...}()) approach:

let done = false
while !done {
    {
        for formantA in editedFormants {
            for formantB in editedFormants {
                if abs(formantA - formantB) < MIN_DISTANCE {
                    let newFormant = (formantA + formantB) / 2
                    editedFormants.removeAtIndex(editedFormants.indexOf(formantA)!)
                    editedFormants.removeAtIndex(editedFormants.indexOf(formantB)!)
                    editedFormants.append(newFormant)
                    editedFormants = editedFormants.sort()
                    return
                }
            }
        }
        done = true
    }()
}

Answer 8

I am sure your code could be refactored to avoid the use of goto. Alternatively, you can use functions within functions and still have access to outer parameters. For example,

func foobar(a: Int, b: Int) {

    func i123() {
        let j = 5
        let x = 2+j
        print("i123 x=\(x) b=\(b)")
    }

    func i456() {
        print("i456")
    }

    if a == b {
        i123()
    } else {
        i456()
    }
}