Swift subclassing – how to override Init()

In addition to Chuck’s answer, you also have to initialize your new introduced property before calling super.init

A designated initializer must ensure that all of the properties
introduced by its class are initialized before it delegates up to a
superclass initializer. (The Swift Programming Language -> Language Guide -> Initialization)

Thus, to make it work:

init(nm: String, ad: String) {
    numberPriorVisits = 0  
    super.init(nm: nm, ad: ad) 
}

This simple initialization to zero could have been done by setting the property’s default value to zero too. It’s also encouraged to do so:

var numberPriorVisits: Int = 0

If you don’t want such a default value it would make sense to extend your initializer to also set a new value for the new property:

init(name: String, ads: String, numberPriorVisits: Int) {
    self.numberPriorVisits = numberPriorVisits
    super.init(nm: name, ad: ads)
}