Creating an array based on the percentage difference of each values of a previous element

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I have an array that is the following:

var array: [Double] = [4, 5, 2, 4, 3, 6]

What I would like to do is create a new array from this array that contains the percentage difference of each value compared to its previous one. So my above array would look like:

var newArray = [0, 25, -60, 100, -25, 100]

If that makes sense? Any help is greatly appreciated, many thanks!!

This can be conveniently done with zip() to enumerate over pairs of adjacent array elements:

let array: [Double] = [4, 5, 2, 4, 3, 6]

let percentages = [0] + zip(array, array.dropFirst()).map {
    (old, new) in
    return 100.0 * (new - old) / old

print(percentages) // [0.0, 25.0, -60.0, 100.0, -25.0, 100.0]

(It is assumed that none of the array elements is zero.)

Or with shorthand syntax:

let percentages = [0] + zip(array, array.dropFirst()).map {
    100.0 * ($1 - $0) / $0

How do you calculate percentage change between sequential , Creating an array based on the percentage difference of each values of a previous element. python list difference between consecutive elements numpy setdiff  Often, the elements of an array represent a series of values to be used in a calculation. For example, if the elements of an array represent exam grades, a professor may wish to total the elements of the array and use that sum to calculate the class average for the exam. The program sums the values contained in the 10-element integer array a

You can use a var to keep a reference of the last element and use defer to update it inside map:

extension Collection where Element: FloatingPoint {
    var percentageDifference: [Element] {
        guard var last = first else { return [] }
        return map { element in
            defer { last = element }
            return (element - last) / last * 100

let numbers: [Double] = [4, 5, 2, 4, 3, 6]
let percentageDifference = numbers.percentageDifference
print(percentageDifference)   // "[0.0, 25.0, -60.0, 100.0, -25.0, 100.0]\n"

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You can do it this way:

func percentageDifference(num1: Double, num2: Double) -> Double {
    return (num2 - num1) / num1 * 100

var array: [Double] = [4, 5, 2, 4, 3, 6]
var resultArray: [Double] = [0]

for i in (1..<array.count) {
    resultArray.append(percentageDifference(num1: array[i-1], num2: array[i]))

Combining and Modifying SAS Data Sets: Examples, (b) Using Big-O analysis, calculate the value of each function. (c) Determine the percent difference between the values calculated in parts (a) and (b) for each algorithm. A 1000 element array is used to store integers in ascending order. The array is to be State the Java code to declare an array of 100 integers named ages. Currently I need to find all values in an array that are over a defined value, if the last 6 preceding values are under 0. At the moment finding values over the defined value is no problem but I can't figure out how to check for the preceding value.

One-liner: you could iterate over the indices and compute the result:

let array: [Double] = [4, 5, 2, 4, 3, 6]
let newArray = { i in (array[i] - array[max(0,i-1)]) / array[max(0, i-1)] * 100.0 }
print(newArray) // [0.0, 25.0, -60.0, 100.0, -25.0, 100.0]

Has a minor code duplication, however it does it job in a performant manner, I'd say :)

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