Question

How to find the first power of 2 whose leading digits are 12 in Swift

Answer 1

import Foundation

// return true if 2^n starts with prefix, else false
func startsWithPrefix(_ n: Int64, _ prefix: Int) -> Bool {
    let log2v: Double = log10(2.0)

    let x: Double = Double(n) * log2v
    let frac: Double = x - floor(x)

    // count digits in prefix
    let buf: String = String(prefix)
    let digits: Int = buf.count

    // compute leading digits
    let leading: Int = Int(floor(pow(10.0, frac + Double(digits) - 1.0)))

    return leading == prefix
}

func main() {
    let prefix: Int = 12

    var n: Int64 = 1
    while true {
        if startsWithPrefix(n, prefix) {
            print("First n = \(n)")
            print("2 ^ \(n) = \(Int(pow(2.0, Double(n))))")
            break
        }
        n += 1
    }
}

main()



/*
run:

First n = 7
2 ^ 7 = 128

*/

 

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