Update JSON Array in Swift 5

Question

I have a JSON array fetching from url and convert using SwiftyJSON. Now i want to update some value of JSON Array for some functionality.

My JSON is like

[
    {
      "id" : 48845152,
      "studentPhotoTakenCount" : 0,
      "updatedAt" : null,
      "isAttendedToday: false
    },
 {     "id" : 48845153,
      "studentPhotoTakenCount" : 0,
      "updatedAt" : null,
      "isAttendedToday: false
    },
  
  ]

After some action i want to update my JSON Array by filtering id. Like if i have id = 48845152 then only update

{
      "id" : 48845152,
      "studentPhotoTakenCount" : 0,
      "updatedAt" : null,
      "isAttendedToday: false
    }

and finally merge with my JSON Array. So the final result should be

[
    {
      "id" : 48845152,
      "studentPhotoTakenCount" : 0,
      "updatedAt" : null,
      "isAttendedToday: false
    },
 {     "id" : 48845153,
      "studentPhotoTakenCount" : 0,
      "updatedAt" : null,
      "isAttendedToday: false
    },

  ]

My code is like that.

self.studentList.forEach {
                        if let id = $0["id"].int {
                            if id == _studentId {
                                $0["isAttendedToday"] =  true
                                self.filteredStudentList.append($0)
                            }
                            else {
                                self.filteredStudentList.append($0)
                            }
                        }
                    }

Here self.studentList is my JSON. But i get the error saying that

Cannot assign through subscript: '$0' is immutable

Please someOne help me to find out what getting wrong here.

$0["isAttendedToday"] = true, this line causing the issue

vijeesh
– vijeesh

2020-12-24 16:06:16 +00:00
Commented Dec 24, 2020 at 16:06 — vijeesh
– vijeesh, Commented Dec 24, 2020 at 16:06

vadian · Accepted Answer · 2020-12-24 16:34:26Z

2

With this syntax you cannot modify the source array, but you could modify the destination array

self.studentList.forEach {
    self.filteredStudentList.append($0)
    if let id = $0["id"].int, id == _studentId {
       let lastIndex = self.filteredStudentList.count - 1
       self.filteredStudentList[lastIndex]["isAttendedToday"] = true
    }
}

If the source array is supposed to be modified you have to use this

for (index, item) in self.studentList.enumerated() {
    self.filteredStudentList.append(item)
    if let id = item["id"].int, id == _studentId {
       self.studentList[index]["isAttendedToday"] = true
    }
}

In both cases due to value type semantics you have to modify the arrays directly.

PS:

In Swift 5 there is no reason anymore to use SwiftyJSON. Codable is more versatile, more efficient and built-in.

edited Dec 24, 2020 at 16:34

answered Dec 24, 2020 at 16:06

vadian

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5 Comments

M Mahmud Hasan Over a year ago

your first solution got some error saying 'Cannot assign to property: 'last' is a get-only property'. But second method is working. thank you

vadian Over a year ago

Please see the edit and be aware that both suggestions do different things.

M Mahmud Hasan Over a year ago

thank you very much but can you describe why 'Cannot assign through subscript: '$0' is immutable' error occured??

vadian Over a year ago

$0 represents the current item in the loop. It's a copy and treated as let constant, therefore immutable.

M Mahmud Hasan Over a year ago

Thanks again. Actually i am new to swift and learning with working

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