why these lines are valid in swift

var a = 3.3
a > 3

and these are not

var a = 3.3
var b = 3
a > b

is that because swift treat 3 as double

In the first case - yes, and in the second case - no.

In 'a > 3', the '3' is a constant number literal, and the compiler will have methods to widen the type definition to match the lhs of the comparator, in this case to a 'double'

For 'a > b', 'a' and 'b' already have defined types of 'Double' and 'Int', and the compiler has no method defined to compare an 'Int' with a 'Double'.

You could overcome that limitation by either type casting 'b' to be a 'Double' or by writing your own extension for the '>' operator.

In this simple example type casting is the easiest solution, however, it would be a good opportunity to practice writing an extension for the '>' operator, as writing extensions to existing operators and functions is more likely when you have more complicated data types and structures.

@Greenamberred is correct. As Swift is type safe.

So this obviously will not work

var a = 3.3
var b = "3"
a > b

While writing an extension will work, it easier to make sure that the type match which is safer as you might make a mistake somewhere else,

You can either do var b = 3.0 which will imply that b a Double or do var b: Double = 3