struct is a value type . For value types, only methods explicitly marked as mutating can modify the properties of self, so this is not possible within a computed property. Structs are value types which means they are copied when they are passed around.So if you change a copy you are changing only that copy, not the original and not any other copies which might be around.If your struct is immutable then all automatic copies resulting from being passed by value will be the same.If you want to change it you have to consciously do it by creating a new instance of the struct with the modified data. (not a copy) If you change struct to be a class then your code compiles without problems.