How to convert a String into a Function call in Swift

Question

I am writing a little app which I would like to use as a calculator. For the operators of the calculator I started with a switch statement to e.g. multiply two values. Therefore I created a dictionary and it worked fine.

I now found it would be quite useful and code saving if I'd write something like

number1 operationButton.currentTitle number2

where operationButton.currentTitle would represent +, -, * or /.

My question now is how can I convert the string of operationButton.currentTitle into a function call for these operators?

You should not rely on the button titles to have certain values (such as "+"). What if the title is changed to "add" later? – Better connect different actions to each button. — Martin R
– Martin R, Commented Apr 18, 2015 at 10:48

Airspeed Velocity · Accepted Answer · 2015-04-18 10:48:58Z

8

In Swift, you can treat functions like variables - for example, you can store them in dictionaries. Also, operators in Swift are just functions with non-alphabetic names (and that can be used infix etc.).

This means you can store binary operations in a dictionary:

let operators: [String:(Double,Double)->Double] = [
    "plus":     (+),
    "minus":    (-),
    "divide":   (/),
    "multiply": (*),
]

And then look up those operations and use whichever one you get, like so:

if let op = operators["divide"] {
    op(3,4)  // returns 0.75
}

edited Apr 18, 2015 at 10:48

answered Apr 18, 2015 at 10:41

Airspeed Velocity

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2 Comments

Martin R Over a year ago

That's correct and nice – but in this concrete case, relying on the button titles (which may be different in different localizations) in order to determine which operation to perform seems a bad idea to me.

Airspeed Velocity Over a year ago

Then again, some hybrid of the two is possibly appropriate if you want to be able to configure what the buttons do at runtime

Martin R · Accepted Answer · 2015-04-18 13:49:35Z

4

You should not rely on the button titles to have certain values in order to decide which operation to perform. What if + is changed to add later? And then localized to a different language? Or the button has no title at all, only a fancy image?

Better connect different actions to each button, such as

@IBAction func addAction(sender: UIButton) {
    // add operands ...
}

@IBAction func subtractAction(sender: UIButton) {
    // subtract operands ...
}

Note also that different operators require may require different error handling, e.g. the divideAction would check if the denominator is zero.

edited Apr 18, 2015 at 13:49

answered Apr 18, 2015 at 11:13

Martin R

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4 Comments

Vatsal Over a year ago

This is not an answer —a suggestion at best.

Martin R Over a year ago

@VatsalManot: If you think so ... I think it shows a way towards a better solution of the problem.

Martin R Over a year ago

@VatsalManot: It could well be that the question is a XY Problem, and if you read it as "how do I translate a button click into the math operation" then it is an answer.

Vatsal Over a year ago

Wow, had no idea what an XY problem was! Could you add a dummy edit? Because I can't change my vote.

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