Struct mutating func calling a func in another instance of the same struct

I was learning about structures and classes in swift today and decided to try and use my newly found knowledge to streamline a combat calculator I had built previously. Now I understand that the properties called in a func are local variable by nature and even more so ‘let’ constants. So I know why the below code has an error but what I can’t figure out is how to accomplish my goal without adding a whole lot more complexity to my code using the nil coalescing operator (??).

Any advice would be very much appreciated.

import Foundation
import Glibc


struct Unit {
   enum UnitType: String {
       case sniper
       case shocktrooper
       case infantry
       case support
   }
   let name: String
   let type: UnitType 
   var hitPoints: Int
   let attackStrength: Int

   //attack another unit
   mutating func attack(target: Unit) {
       print("\(self.name) is attacking \(target.name)...")
       if self.attackStrength > target.attackStrength {
           print("\(self.name) hit \(target.name) for
\(self.attackStrength) points of damage!")
           target.hit(target, self.attackStrength) /*error: cannont use
muatating member on imutable value: 'target is a 'let' constant */
       } else {
           self.repelled(by: target.attackStrength)
       }
   }
   //take damage from defender
   mutating func repelled(by damage: Int) {
       self.hitPoints -= damage
       print("\(name) was repelled and took \(damage) points of damage!")
   }
   //take damage from attack
   mutating func hit(for damage: Int) {
       self.hitPoints -= damage
   }
}

//declaring two seperate units
var player1 = Unit(name: "Player 1", type: .sniper, hitPoints: 10,
attackStrength: 3)
var player2 = Unit(name: "Player 2", type: .shocktrooper, hitPoints: 15,
attackStrength: 2)


func score() {
   print("The current hitpoints are: \(player1.name): \(player1.hitPoints)
& \(player2.name): \(player2.hitPoints)")
}

player1.attack(target: player2)
score()

You just need to use inout keyword to be able to mutate your player2:

struct Unit {
    enum Kind: String {
        case sniper, shocktrooper, infantry, support
    }
    let name: String
    let kind: Kind
    var hitPoints: Int
    let attackStrength: Int
    mutating func attack(target: inout Unit) {
        print("\(name) is attacking \(target.name)...")
        if attackStrength > target.attackStrength {
            print("\(name) hit \(target.name) for \(attackStrength) points of damage!")
            target.hit(for: attackStrength)
        } else {
            repelled(by: target.attackStrength)
        }
    }
    mutating func repelled(by damage: Int) {
        hitPoints -= damage
        print("\(name) was repelled and took \(damage) points of damage!")
    }
    mutating func hit(for damage: Int) {
        hitPoints -= damage
    }
}

Playground testing

var player1 = Unit(name: "Player 1", kind: .sniper, hitPoints: 10,  attackStrength: 3)
var player2 = Unit(name: "Player 2", kind: .shocktrooper, hitPoints: 15, attackStrength: 2)
func score() {
    print("The current hitpoints are: \(player1.name): \(player1.hitPoints) & \(player2.name): \(player2.hitPoints)")
}
player1.attack(target: &player2)
score()

This will print

Player 1 is attacking Player 2...

Player 1 hit Player 2 for 3 points of damage!

The current hitpoints are: Player 1: 10 & Player 2: 12

Another option you have is to use a class instead of a struct. You would need to provide a custom initializer to your class and remove the mutating keyword from your methods

class Unit {
    enum Kind: String {
        case sniper, shocktrooper, infantry, support
    }
    let name: String
    let kind: Kind
    var hitPoints: Int
    let attackStrength: Int
    init(name: String, kind: Kind, hitPoints: Int,  attackStrength: Int){
        self.name = name
        self.kind = kind
        self.hitPoints = hitPoints
        self.attackStrength = attackStrength
    }
    func attack(target: Unit) {
        print("\(name) is attacking \(target.name)...")
        if attackStrength > target.attackStrength {
            print("\(name) hit \(target.name) for \(attackStrength) points of damage!")
            target.hit(for: attackStrength)
        } else {
            repelled(by: target.attackStrength)
        }
    }
    func repelled(by damage: Int) {
        hitPoints -= damage
        print("\(name) was repelled and took \(damage) points of damage!")
    }
    func hit(for damage: Int) {
        hitPoints -= damage
    }
}
let player1 = Unit(name: "Player 1", kind: .sniper, hitPoints: 10,  attackStrength: 3)
let player2 = Unit(name: "Player 2", kind: .shocktrooper, hitPoints: 15, attackStrength: 2)
func score() {
    print("The current hitpoints are: \(player1.name): \(player1.hitPoints) & \(player2.name): \(player2.hitPoints)")
}
player1.attack(target: player2)
score()