Example.swift

let passedEarnings = [
    [
"actual":"<null>",
        "code" : "AUTO.US",
        "date" : "2019-12-31",
        "difference" : "<null>",
        "estimate" : "-0.12",
        "percent" : "<null>",
    "report_date" : "2020-03-27"],
[
    "actual" : "<null>",
    "code" : "APTX.US",
    "date" : "2019-12-31",
    "difference" : "<null>",
    "estimate" : "-0.5600000000000001",
    "percent" : "<null>",
    "report_date" : "2020-03-30"
    ],
[
    "actual" : "<null>",
    "code" : "APTX.AUS",
    "date" : "2019-12-31",
    "difference" : "<null>",
    "estimate" : "-0.5600000000000001",
    "percent" : "<null>",
    "report_date" : "2020-03-30"
    ]
    ]

func createArray(from earnings:[[String:String]] ) -> [String:[[String:String]]] {
var arrayOfEarningsDicts = [String:[[String:String]]]()

for earning in earnings {
    guard let reportDate = earning["report_date"] else {
        break
    }
    arrayOfEarningsDicts[reportDate, default:[[String:String]]()].append(earning)
}
return arrayOfEarningsDicts
}

let array = createArray(from: passedEarnings)

for e in array.values {
    for a in e {
    print(a["report_date"]!)
  

Output.txt

2020-03-27
2020-03-30
2020-03-30