Finding and combining duplicate entries in an array - ios

Question

Im new here so correct me if I've formatted this incorrectly (also new to swift) but basically what I'm trying to do is take an array of dates and numbers and if any of the dates are the same combine the numbers into one entry.
so this
//This is how i format the data after i pull it from core data
var dateAndEntry = [(String, Double)]
//i've split them into two seperate arrays for sorting, but I feel like theres a better way i don't know
var dates = ["01/01/2016", "02/01/2016", "02/01/2016", "04/01/2016", "05/01/2016", "06/01/2016","07/01/2016","07/01/2016"]
var entries = [-14,2,8,9,-1,8,25,6]
becomes this
var dates = ["01/01/2016", "02/01/2016", "04/01/2016", "05/01/2016", "06/01/2016","07/01/2016"]
var entries = [-14,10,9,-1,8,19]
I've tried doing this but i can only get it so that it makes a new array that only contains the new values rather than allowing me to get the duplicated values, combine, insert at index then delete original entries in the two arrays.
func combineDuplicates(dates: [String]) -> [String]{
var output: [String] = []
var checked = Set<String>()
for date in dates {
if !checked.contains(date) {
checked.insert(date)
output.append(date)
}
}
print(output)
return output
}
let sorted = combineDuplicates(dates)
print(sorted)
and yes i have looked on google and here for answers but turned up nothing.
Any solutions, explanations, help, pointers or references to other questions or sources I may have missed would all be greatly appreciated.

Answer 1

This will get you a dictionary with keys the dates and values the sum of entries for each date:
dates.enumerate().reduce([String:Int]()) { dict, item in
var dict = dict
dict[item.1] = (dict[item.1] ?? 0) + entries[item.0]
return dict
}
// ["06/01/2016": 8, "04/01/2016": 9, "01/01/2016": -14, "05/01/2016": -1, "07/01/2016": 31, "02/01/2016": 10]