Getting an accurate known percent difference

Background

I am creating an array of Double values, hours, that are computed around a central value, price. For x iteration, percent is used to calculate values at that percentage above and then below the price. The list is then formatted as,

• Above values (+ percent for iteration)
• Central value
• Below values (- percent for iteration)

I realize the way I am doing this is kind of hacky and I suspect my issue lies within this code.

let price : Double = earliestExpiryLeg.object(forKey: "underlying") as! Double    var i = 1.00  var x = price  var a : Array<Double> = [] as Array  while i <= self.iteration {        let y = x * (1 + self.percent)      a.append((round (y*100)/100))      x = y        i = i + 1.00  }    var w = 1.00  var s = price  var r : Array<Double> = [] as Array  while w <= self.iteration {        let y = s * (1 - self.percent)      r.append((round (y*100)/100))      s = y        w = w + 1.00  }    a.reverse()  a.append(price)  a.append(contentsOf: r)    self.hours = a  

What I want to do

I now want to compare a value from hours to the original price value and get the percent difference. I am using the following function to accomplish this.

func pctDiff(x1: CGFloat, x2: CGFloat) -> Double {      let diff = (x2 - x1) / x1      let doub = Double((100 * (diff * 100) / 100))      return doub  }    self.pctDiff(x1: CGFloat(earliestExpiryLeg.object(forKey: "underlying") as? Double ?? 0.00), x2: CGFloat(hours![indexPath.row - 2]))  

What's going wrong

The percent change values returned are not exact, even though percent is exact. For example, if percent is 10.00%, I would expect the percentage change for instance to be 30.00%-even for a value three iteration above the central price value, but instead I get 33.10%.

https://stackoverflow.com/questions/66631790/getting-an-accurate-known-percent-difference March 15, 2021 at 10:52AM