Different results with Set altough similar code? What is the excact problem in Swift?

Question

i have a small problem. I have two Arrays where i try to find the same content in it. So i decided to convert it to a Set and then using those nice functions with "subtract". However i get very different results. Can someone tell me why this happens? When i use "subtracting" instead of "subtract" i get no problems however this is very weird for me and i really have no clue why this happens.

   var objectIDsWhichExist = [ "kjugsJHL6JYoByOreUQ0wUefsbX2", "18ixZ21PJDXA1WzeJqZzctl7tTk2", "ZeQPYGfDvWMLSVykb4M5FQ6miGX2"]
    var helperObjectIDsWhichExistInAdded = [ "kjugsJHL6JYoByOreUQ0wUefsbX2", "18ixZ21PJDXA1WzeJqZzctl7tTk2"]
    


    var setA = Set(objectIDsWhichExist) /* Updated Data*/
    var setB = Set(helperObjectIDsWhichExistInAdded) /* Standard Data*/
    let different = setA.subtract(setB) // I GET HERE ()  
    print(different) // I GET THIS RESULT "()\n"

And this is the one sample which works, surprisingly. However i still dont know really why???

    var employees: Set = Set(objectIDsWhichExist)
    let neighbors: Set = Set(helperObjectIDsWhichExistInAdded)
    employees.subtract(neighbors)
    print(employees) // HERE IT DOES WORK because i get this -> ["ZeQPYGfDvWMLSVykb4M5FQ6miGX2"]\n"

score 2 · Accepted Answer · answered Mar 16 '21 at 04:25

subtract and subtracting both computes the difference of 2 sets, but subtract is mutating, whereas subtracting is not.

This means that x.subtract(y) changes the set x to the computed difference, whereas x.subtracting(y) doesn't change x at all, and instead returns the difference. On the other hand, subtract returns nothing (Void).

When you do

let different = setA.subtract(setB) // I GET HERE ()  
print(different)

you see () being printed, because that is what the string representation of Void looks like - an empty tuple.

This works

employees.subtract(neighbors)
print(employees)

because subtract changes employee.

This also works:

let different = setA.subtracting(setB)
print(different)

because the return value of subtracting - the set difference - is assigned to different. Note that this doesn't change setA.

This doesn't work:

employees.subtracting(neighbors)
print(employees) // still shows the original employees

Because subtracting doesn't change employees, and you are ignoring its return value.

There are many other pairs of such mutating vs non-mutating methods, like

Set.formUnion vs Set.union
String.append vs String.appending

Thank you very much for explaining this in detail. Thank you really very much for this that you took your time to give me an excellent answer like this! — esen, Mar 16 '21 at 06:06

Different results with Set altough similar code? What is the excact problem in Swift?

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