Add Empty Column to DataFrame of Type Double

I am learning about data frames. I have a set of functions that will read CSV files into data frames, allow me to format a string into a date and then sort by date in ascending order. What I would like to do now is add a column to the data frame then populate it with values calculated from values already in the data frame. I am unable to get past the first step of adding an empty column to the data frame. Any assistance will be appreciated.

import TabularData
import SwiftUI

func GetAlphaVantageDF(fileName: String) -> DataFrame {
    var alphaVantageDF = ReadAlphaVantageCSV(fileName: fileName)
    TransformStringToDateAV(dataFrame: &alphaVantageDF)
    SortDataFrameByDate(dataFrame: &alphaVantageDF, sortOn: "timestamp")
    alphaVantageDF.append(column: Column<T>) // do not know what to put in place of Column<T> or if this is even the right path to take
    let alphaVanDF1Year: DataFrame.Slice = OneYearSlice(dataFrame: alphaVantageDF, filterOn: "timestamp")
    return alphaVantageDF

func ReadAlphaVantageCSV(fileName: String) -> DataFrame {
    guard let pathURL = FileManager.default.urls(for: .downloadsDirectory, in: .userDomainMask).first else { return DataFrame() }
    let fileURL = pathURL.appendingPathComponent((fileName + ".csv"))
    let options = CSVReadingOptions(hasHeaderRow: true, ignoresEmptyLines: true, delimiter: ",")
    let dataFrame = try! DataFrame(contentsOfCSVFile: fileURL, columns: ["timestamp","close"], types: ["timestamp": .string , "close": .double], options: options)
    return dataFrame

func TransformStringToDateAV(dataFrame: inout DataFrame) {
    dataFrame.transformColumn("timestamp") { (theDate: String) -> Date in
        return theDate)!

func SortDataFrameByDate(dataFrame: inout DataFrame, sortOn: String) {
    dataFrame.sort(on: sortOn, order: .ascending)

func OneYearSlice(dataFrame: DataFrame, filterOn: String) -> DataFrame.Slice {
    let lastRowNum = dataFrame.shape.rows - 1
    let lastDataFrameRow = dataFrame[row: lastRowNum]
    let latestDate = lastDataFrameRow[0]
    let oneYearAgo = .year, value: -1, to: latestDate as! Date)
    let dataFrame1Year: DataFrame.Slice = dataFrame.filter(on: filterOn, Date.self, { $0! > oneYearAgo! })
    return dataFrame1Year

let alphaVantageDateFormatter: DateFormatter = {
    let result = DateFormatter()
    result.dateFormat = "yyyy-MM-dd"
    result.locale = Locale(identifier: "en_US_POSIX")
    result.timeZone = TimeZone(secondsFromGMT: 0)
    return result

After some additional research I was able to resolve my problem. In the function "GetAlphaVantageDF" I replaced the line "alphaVantageDF.append(column: Column) with the code below. The solution was to create a new column, populate with the same number of rows as the data frame then append the column to the data frame.

    let numRows = alphaVantageDF.shape.rows
    var deltaCol = Column<Double>.init(name: "delta", capacity: numRows)
    let initVal: Double = 1.0
    for _ in  0...numRows - 1 {
    alphaVantageDF.append(column: deltaCol)