How do I simplify this overly long Swift switch statement?

I have six of these code blocks and am wanting to shorten all of them while still having them work effectively. This code works the way that I intended it to but is really bulky.

    let strStatResult = strStat
    strengthModLabel.text = String(strStatResult)
    
    switch strStatResult {
    case let strStatResult where strStatResult == 1:
        strengthModLabel.text = "-5"
    case let strStatResult where strStatResult == 2 || strStatResult == 3:
        strengthModLabel.text = "-4"
    case let strStatResult where strStatResult == 4 || strStatResult == 5:
        strengthModLabel.text = "-3"
    case let strStatResult where strStatResult == 6 || strStatResult == 7:
        strengthModLabel.text = "-2"
    case let strStatResult where strStatResult == 8 || strStatResult == 9:
        strengthModLabel.text = "-1"
    case let strStatResult where strStatResult == 10 || strStatResult == 11:
        strengthModLabel.text = "0"
    case let strStatResult where strStatResult == 12 || strStatResult == 13:
        strengthModLabel.text = "+1"
    case let strStatResult where strStatResult == 14 || strStatResult == 15:
        strengthModLabel.text = "+2"
    case let strStatResult where strStatResult == 16 || strStatResult == 17:
        strengthModLabel.text = "+3"
    case let strStatResult where strStatResult == 18 || strStatResult == 19:
        strengthModLabel.text = "+4"
    case let strStatResult where strStatResult == 20 || strStatResult == 21:
        strengthModLabel.text = "+5"
    case let strStatResult where strStatResult == 22 || strStatResult == 23:
        strengthModLabel.text = "+6"
    case let strStatResult where strStatResult == 24 || strStatResult == 25:
        strengthModLabel.text = "+7"
    case let strStatResult where strStatResult == 26 || strStatResult == 27:
        strengthModLabel.text = "+8"
    case let strStatResult where strStatResult == 28 || strStatResult == 29:
        strengthModLabel.text = "+9"
    case let strStatResult where strStatResult == 30:
        strengthModLabel.text = "+10"
    default:
        strengthModLabel.text = "Error"
    
    }

I've thought of using for loops or while loops to shorten it but I am not sure how to implement that all the while shortening the code.

The result seems to be strStatResult / 2 - 5. This works with integer division. Both 10 / 2 and 11 / 2 give 5. Subtract 5 and you get 0, just like your giant switch statement.

Assuming strStatResult is an Int (or other integer type), all you need is:

if strStatResult < 1 || strStatResult > 30 {
    strengthModLabel.text = "Error"
} else {
    let num = strStatResult / 2 - 5
    strengthModLabel.text = num > 0 ? "+\(num)" : "\(num)"
}