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Standard Deviation in Swift
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| func standardDeviation(arr : [Double]) -> Double | |
| { | |
| let length = Double(arr.count) | |
| let avg = arr.reduce(0, {$0 + $1}) / length | |
| let sumOfSquaredAvgDiff = arr.map { pow($0 - avg, 2.0)}.reduce(0, {$0 + $1}) | |
| return sqrt(sumOfSquaredAvgDiff / length) | |
| } | |
| let responseTimes = [ 18.0, 21.0, 41.0, 42.0, 48.0, 50.0, 55.0, 90.0 ] | |
| standardDeviation(responseTimes) // 20.8742514835862 | |
| standardDeviation([2,4,4,4,5,5,7,9]) // 2.0 |
also, I think .reduce(0, {$0 + $1}) can be simplified to .reduce(0, +).
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stddev([ 18.0, 21.0, 41.0, 42.0, 48.0, 50.0, 55.0, 90.0 ]) => 22.315514
stddev([2,4,4,4,5,5,7,9]) => 2.1380899
It should be
sqrt(sumOfSquaredAvgDiff / (length - 1))