Help please. I want to make an expression analysis calculator using the reverse Polish Notation (Reverse Polish NOTATION). I need to do so that if the user has entered the wrong expression to inform about it. I do not need to catch mistakes and just print “Error”
I made 2 stack (because it is a stack calculator)

struct doublestack {
  Var Items = [Double] ()
  Mutating Func Push (_ Item: Double) {
    Items.APPEND (Item)
  }
  Mutating Func Pop () - & gt; Double {
    Return Items.Removelast ()
  }
  Mutating Func Empty () - & gt; BOOL {
    If Items.Count == 0 {
      Return True.
    } else {
      Return False.
    }
  }
  Mutating Func Peek () - & gt; Double {
    RETURN ITEMS.LAST!
  }
}
Struct CharactersStack {
  Var Items = [Character] ()
  Mutating Func Push (_ Item: Character) {
    Items.APPEND (Item)
  }
  Mutating Func Pop () - & gt; Character {
    Return Items.Removelast ()
  }
  Mutating Func Empty () - & gt; BOOL {
    If Items.Count == 0 {
      Return True.
    } else {
      Return False
    }
  }
  Mutating Func Peek () - & gt; Character {
    RETURN ITEMS.LAST!
  }
}

My expressional expression parsing features and rpntoanswer.

Func Isoper (C: Character) - & gt; BOOL {
  IF C == "+" || C == "-" || C == "*" || C == "/" || C == "^" || C == "×" || C == "÷" || C == "%" {
    Return True.
  }
  Return False.
}
FUNC GETP (Token: Character) - & gt; Int8 {
  If token == "+" || Token == "-" {
    Return 1.
  } ELSE If TKen == "*" || token == "/" || token == "×" || token == "÷" || token == "%" {
    Return 2.
  } ELSE if token == "^" {
    Return 3.
  }
  Return 0.
}
FUNC ExpressionTorpn (EXPR: [Character]) Throws - & GT; [Character] {
  Var Current = [Character] ()
  Var Stack = CharactersStack ()
  VAR i: int = 0
  While I & LT; EXPR.COUNT {
    IF Expr [i] == "-" {
      IF i == 0 || EXPR [I - 1] == "(" {
        CURRENT.APPEND ("(")
        CURRENT.APPEND (EXPR [I])
        i = i + 1
        While Expr [i] .isnumber || EXPR [i] == "." {
          CURRENT.APPEND (EXPR [I])
          I + = 1
        }
        current.append (")")
        Current.append ("")
      }
    }
    If EXPR [i] .ISNumber {
      While (EXPR [i] .ISNumber || Expr [i] == ".") {
        CURRENT.APPEND (EXPR [I])
        I + = 1
      }
      Current.append ("")
    }
    IF Expr [i] == "(" {
      Stack.push (EXPR [I])
    }
    IF Expr [i] == ")" {
      While! stack.empty () & amp; & amp; stack.peek ()! = "(" {
        current.append (stack.pop ())
        Current.append ("")
      }
      stack.pop ()
    }
    IF Isoper (C: EXPR [I]) {
      While! stack.empty () & amp; & amp; GetP (Token: EXPR [i]) & lt; = getp (token: stack.peek ()) & amp; & amp; stack.peek ()! = "(" {
        current.append (stack.pop ())
        Current.append ("")
      }
      Stack.push (EXPR [I])
    }
    I + = 1
  }
  While (! stack.empty ()) {
    current.append (stack.pop ())
    Current.append ("")
  }
  Return Current
}
FUNC RPNToANSWER (RPN: [Character]) Throws - & gt; Double {
  VAR Operand = [Character] ()
  Var Stack = DoubleStack ()
  VAR i: int = 0
  While I & LT; RPN.COUNT {
    Operand = []
    IF RPN [i] == "" {
      I + = 1
      Continue.
    }
    IF RPN [i] .ISNumber || RPN [i] == "." {
      While RPN [i] .isnumber || RPN [i] == "." {
        Operand.APPEND (RPN [i])
        I + = 1
      }
      Stack.Push (Double (String (Operand))!)
      Continue.
    }
    IF GetP (Token: RPN [i]) & gt; 0 {
      Let A: Double = stack.pop ()
      Let B: Double = Stack.pop ()
      IF RPN [i] == "+" {
        Stack.push (A + B) 
} ELSE IF RPN [I] == "-" {
        Stack.push (B - a)
      } ELSE IF RPN [i] == "*" || rpn [i] == "×" {
        Stack.push (b * a)
      } ELSE IF RPN [I] == "/" || RPN [i] == "÷" {
        Stack.push (b / a)
      } ELSE IF RPN [I] == "%" {
        Stack.push (A * B / 100)
      } ELSE IF RPN [i] == "^" {
        Stack.push (POW (B, A))
      }
    }
    I + = 1
  }
  Return Stack.pop ()
}

And finally, what I want to withdraw

var r: string = "(11 + 222-111 +)"
var str: [character] = r.compactmap {$ 0}
do {
  Let EXPRTORPN = Try ExpressionTorpn (EXPR: STR)
  Let Answer = Try RPNToANSWER (RPN: Exprtorpn)
  PRINT (ANSWER)
} Catch {
  Print ("Error !!!!!!!")
}

I get an error

Thread 1: Fatal Error: CAN’t Remove Last Element from An Empty Collection

In the Return Items.Removelast ()

line

mutating func pop () - & gt; Double {
  Return Items.Removelast ()
}

How can I get out to the “Error” to the Console if the code collapses?
Thank you in advance !!!

Answer 1, Authority 100%

Use the documentation. And this is not sarcasm, this is the only actual source of truth for the developer. To begin with, simply select the removelast () (CMD-Click) method and see what is written:

Collection should not be empty. Accordingly, if you want to use a do-try-catch, you need to make it so that the error is thrown away.

// describe the errors that are planned
ENUM CALCULATIONERROR: ERROR {
Case emptycollection
}
// rewrite your method so that it discardes the error:
FUNC POP () Throws - & gt; Double {
  Guard! Items.isempty else {
    Throw CalculationError.EmptyCollection
  }
  Return Items.Removelast ()
}

and use it already try ...
Similarly with the rest.