Performing Calculations from Slider Values in Swift 2

Question

I'm trying to calculate values from a UISlider in real time but am having errors in Swift 2. It's a simple temperature conversion App. Can someone have a look please?

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@IBOutlet var Fahrenheit: UILabel!
@IBOutlet var Celcius: UILabel!
@IBOutlet var sliCelcius: UISlider!
@IBAction func changeTemp(sender: UISlider) {
    let tempC = Int(sender.value)
    let tempF = tempC *9/5+32;
    Fahrenheit.text = "\(tempF)"
    Celcius.text = "\(tempC)"
}

because u use `Int` for that i think, try use `float` or `double` instead, and also, its easier to know if u include the error in the question too — Tj3n, Jan 08 '16 at 03:31
Thanks ZHZ, It builds but it doesn't convert the celcius and the fahrenheit at the same time as you drag the slider. — zacsprint, Jan 08 '16 at 03:48
oh right, its ur syntax wrong...should be `let tempF = tempC*9/5+32` — Tj3n, Jan 08 '16 at 03:48
Tj3n thanks, much appreciated. I tried it before and it didn't work (came up with different error), but tried it as suggested and now it works! Thanks everyone for your help. — zacsprint, Jan 08 '16 at 03:54

score 1 · Accepted Answer · answered Jan 08 '16 at 04:11

1

Space after operator '*' is missing, It should work

    let tempF = tempC * 9/5+32

answered Jan 08 '16 at 04:11

Aruna Mudnoor

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16

dfrib · Answer 2 · 2016-01-08T11:09:37.353

It might be of interest as to why Swift yields an error here (not only how to avoid it).

As many error messages in Swift, the actual error of interest is obfuscated by other error messages, in this case the somewhat standard 'consecutive statments ...' message. The core error here is, however, that there exists no unary (prefix) operator *. The follow-up error, given the case where a unary prefix operator * were to exist, is that the expression 'tempC *9/5+32' would reduce to (assuming high precedence for this * unary operator) 'tempC IL/5+32' (IL integer literal), which is naturally erroneous, much like an expression 'let a: Int = 1 1' would be.

Now, unary prefix operators exists for e.g. + and -:

/* '+' and '-' unary (prefix) operators in action: OK */
let a : Int = 1 + -1              //  0, OK
let b : Int = +(-1)              // -1, OK
let c : Int = -(-(-(-(-(-(1)))))) //  1, OK

let d : Int = *1 /* error: '*' is not a prefix unary operator */

Swift infers an operator to be unary if it appears immediately before its target and if no other operand appears immediately before itself. If it finds another operand immediately before itself, it infers the operator as binary. Take the following examples:

let e: Int = 1 + 1 // binary, OK
let f: Int = 1+1   // binary, OK
let g: Int = 1 +1  /* unary prefix: resulting expression 'let g: Int = 1 1' erroneous */
let h: Int = 1+    /* + not a postfix unary operator, erroneous */

See the Swift Language Guide - Basic Operators for details:

Unary operators operate on a single target (such as -a). Unary prefix operators appear immediately before their target (such as !b), and unary postfix operators appear immediately after their target (such as i++).

The lesson here is to keep a keen eye out for expressions where operators are used in a unary prefix (or postfix) fashion unintendently, to avoid getting confused by obfuscated error messages from Swift.

Finally, note that we may naturally define our own prefix unary operators in case we find them to be missing

prefix operator * {}
prefix func * (rhs: Int) -> Int {
    return rhs*rhs
}
let i : Int = *2 // 4, OK

Performing Calculations from Slider Values in Swift 2

2 Answers2