How to change a variable stored property in a structure in Swift?

Question

On apple website, https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html it states that we can change a variable property in a structure. I am trying to change the person's last name. However I recieve an error at that line of code. Thank you for helping me.

import UIKit


struct people {
    let firstName : String
    var lastName : String
}

let person = people(firstName: "InitialFirstName", lastName: "InitialLastName")
person.lastName = "ChangedLastName" //<-- error

println(person.firstName)
println(person.lastName)

Error:

Playground execution failed: TestPlayground2.playground:12:17: error: cannot assign to 'lastName' in 'person' person.lastName = "ChangedLastName" ~~~~~~~~~~~~~~~ ^

score 10 · Accepted Answer · answered Aug 29 '15 at 21:19

You can only modify a property of a struct if that struct is declared as var, and not let. Even though your property is var, because the struct is let it is immutable.

var person = people(firstName: "InitialFirstName", lastName: "InitialLastName")
person.lastName = "ChangedLastName" //No problem!

Note that this is different behavior from classes. A class's mutable properties can be changed even if if the instance of the class is a constant.

I forgot that my person was a "let". Thanks! – Entitize Aug 29 '15 at 21:34 — Entitize, Aug 29 '15 at 21:34

score 5 · Answer 2 · answered Aug 29 '15 at 21:20

5

Unlike class which is reference type struct is value type.
You have to declare the instance as mutable

var person = people(firstName: "InitialFirstName", lastName: "InitialLastName")

answered Aug 29 '15 at 21:20

vadian

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score 0 · Answer 3 · edited Dec 29 '19 at 11:22

0

By using the 'let' syntax you have declared 'person' as a constant, which means that it's properties can't be changed. Just change 'let' to 'var' and the error will disappear.

Here's more info about the difference between let and var in swift.

edited Dec 29 '19 at 11:22

Peter

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answered Aug 29 '15 at 21:24

aaaschmitt

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How to change a variable stored property in a structure in Swift?

3 Answers3