Equation solver/parser - ios

Question

I have an app that solves physics problems based on given knowns and unknowns .. but it does it the wrong way..
let's say we have this equation ( d = v * t )
We have here three variables .. the way I did it is like this
let v = 10
let t = 5
let d = nil
let result = Double()
if d == nil && v != nil && t != nil {
result = v * t
}if v == nil && d != nil && t != nil {
result = d / t
}if t == nil && v != nil && d != nil {
result = d / v
}else{
result = nil
}
print(result) // "50"
obviously it's a headache .. this is only a small equation with three vars.. what if I had 20 equations each has 5 vars.. it would be very exhausting to type.. So I'm wondering if there is a way to just type the equation and call .solve or something .. something like this
let equation = "d = v * t"
let knowns = [d:20,v:50]
let result = equation.solve(usingKnowns: knowns)
print(result) // "Optional(t = 0.4)"
Thanks in advance,
Any help would be really great

Answer 1

The problem here is to see what variable is unknown, supose you have X variables and you always get X-1 variables known and 1 unknown you don't really need to check if the knowns variables are != nil.
Other possibility is the one I recommend you:
func resolve(a: Double?, b: Double?, c: Double?)-> Double{
//Default value
var result: Double = 0.0
//a missing
if let b = b, let c = c{
result = b*c
}
//b missing
if let a = a, let b = b{
result = a/b
}
//c missing
if let a = a, let c = c{
result = a/c
}
return result
}
In this case you have a single call with the 3 parameters, all of them optionals, so if you try a "if let" with 2 o them simultaneously and it works, it means the other one is the unknow variable.
This is the "cleanest" solution I can think of.