Role of underscore "_" in conditional statement (if else) in Swift-CodePudding

Can somebody please explain the role of "_" in Swift code below.

var guessWasMade: Bool {
    if let _ = game.guesses[currentQuestion] {
        return true
    } else {
        return false
    }
}

I understand how it is usually used as unnamed parameter in for-loops and functions. But could not find any explanation for if else statement.

Tutorial explains it as guessWasMade checks game.guesses for a value. If a value is found we know the user has made a guess at the question.

game.guesses is an array of integers.

This is part of class declaration. Full code looks like this

class GameViewModel: ObservableObject {

  // MARK: - Published properties
  // 2
  @Published private var game = Game()
 
  // MARK: - Internal properties
  // 3
  var currentQuestion: Question {
     game.currentQuestion
  }
  // 4
  var questionProgressText: String {
    "\(game.currentQuestionIndex   1) / \(game.numberOfQuestions)"
  }

// 1
  var guessWasMade: Bool {
       if let _ = game.guesses[currentQuestion] {
           return true
       } else {
           return false
       }
   }

   // MARK: - Internal Methods
   // 2
   func makeGuess(atIndex index: Int) {
       game.makeGuessForCurrentQuestion(atIndex: index)
   }
   // 3
   func displayNextScreen() {
       game.updateGameStatus()
   }
}

CodePudding user response：

If let statements check to see if the value is nil. If it is, then the statement is false. If it isn't, then the value is assigned to a variable.

var optionalValue: Int? = 42

if let nonOptionalValue = optionalValue {
    print(nonOptionalValue) // -> 42
} else {
    print("optionalValue is nil")
}

So, when you do if let _ = game.guesses[...], you are checking to see if game.guesses[...] is nil. If it isn't, then you are ignoring the value with a wildcard pattern (_), which will match anything.

Because you are ignoring the value, it is the same as saying

if game.guesses[...] != nil {
    return true
} else {
    return false
}

Then, because you are simply returning the value of the condition, you can just return the condition itself:

var guessWasMade: Bool {
   game.guesses[currentQuestion] != nil
}