Update element in array

Question

How can we update element of an array

Method 1:-Working code:-

var numberWords = ["one", "two", "three"]
   for i in 0..<numberWords.count {
       if numberWords[i] == "two" {
           numberWords[i] = "2"
       }
   }

But I am looking for solution using Swift high order function

Method 2:

numberWords = numberWords.filter  {
       if $0 == "one" {
           $0 = "1"//cannot assign value $0 is immutable
       }
       return true
    }

Error thrown : Cannot assign value $0 is immutable

Is it possible or Method 1 is only way?

Don't use filter(), it doesn't make sense. Use map() instead: numberWords = numberWords.map({ return $0 == "two" ? "2" : $0 }) — Larme
– Larme, Commented Mar 27, 2018 at 9:15
as @Larme mentioned, you should map the array not filter it. The purpose of using filter is to determine (validate) which element should be exist in the collection, which could lead to decrease the number of element in a collection; map is just a transformation of the element, which probably what are you aiming to in your case... — Ahmad F
– Ahmad F, Commented Mar 27, 2018 at 9:27
Does this answer your question? Find an item and change value in custom object array - Swift — Jeff
– Jeff, Commented Jun 26, 2020 at 21:28

Larme · Accepted Answer · 2018-03-27 09:20:51Z

14

Do not use filter(), it doesn't make sense.
The point of filter is to iterate each elements and tells if you keep it (return true) or not (return false) according to your desires.

Instead, use map():

numberWords = numberWords.map({ return $0 == "one" ? "1" : $0 })

I used a ternary if and explicitly write a "return" (which is not necessary since it's already done "internally" (you need to returns something)), but you can do it more explicitly keeping your previous code:

numberWords = numberWords.map({
    if $0 == "one" {
        return "1"
    }
    return $0 })

map() is more suited for that. You use it if you want to iterate each items and modify it if needed which is what you want to do.

answered Mar 27, 2018 at 9:20

Larme

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1 Comment

Ramesh Over a year ago

Yes it's Totally fine. How can we change score.activeDate . in

self.monthlyActiveDayzModel.data?.responseMap?.resultsList?.scores = self.monthlyActiveDayzModel.data?.responseMap?.resultsList?.scores?.map {                     score in                     score.activeDate = "newValue"                     }

where monthlyActiveDayzModel is struct model.

JeremyP · Accepted Answer · 2018-03-27 10:01:08Z

5

Just as a bit of variety, you could write an in place map, which would probably be more efficient for large arrays (see also reduce(into:)).

extension Array 
{
    mutating func mapInPlace(transform: (inout Element) -> Void)
    {
        for i in 0 ..< self.count
        {
            transform(&self[i])
        }
    }
}

var a = ["one", "two", "three"]
a.mapInPlace{ if $0 == "two" { $0 = "2" } }

answered Mar 27, 2018 at 10:01

JeremyP

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2 Comments

Ramesh Over a year ago

Fab!!. How can we change score.activeDate . in

self.monthlyActiveDayzModel.data?.responseMap?.resultsList?.scores = self.monthlyActiveDayzModel.data?.responseMap?.resultsList?.scores?.map { score in score.activeDate = "newValue" }

where monthlyActiveDayzModel is struct model.

JeremyP Over a year ago

@Ramesh That's a different question. I'd ask a new SO question about that.

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