A Swift implementation of https://www.fluentcpp.com/2017/09/25/expressive-cpp17-coding-challenge/

csvchallenge.swift

import Foundation
import Darwin

if CommandLine.argc < 2 {
    print("input file missing")
    exit(1)
}
if CommandLine.argc < 3 {
    print("column name missing")
    exit(1)
}
if CommandLine.argc < 4 {
    print("replacement value missing")
    exit(1)
}
if CommandLine.argc < 5 {
    print("output file missing")
    exit(1)
}

let inputFileName = CommandLine.arguments[1]
let columnName = CommandLine.arguments[2]
let replacementValue = CommandLine.arguments[3]
let outputFileName = CommandLine.arguments[4]

let inputFileContent = try! String(contentsOfFile: inputFileName, encoding: .utf8)
    .split(separator: "\r\n")
let columnNames = inputFileContent[0].split(separator: ",").map { String($0) }

guard let columnNameIndex = columnNames.index(of: columnName) else {
    print("column name doesn’t exists in the input file")
    exit(1)
}

func replaceValues(in rows: ArraySlice<Substring>, with replacement: String, forColumnAt columnIndex: Int) -> [String] {
    return rows.map { replaceValue(in: String($0), with: replacement, forColumnAt: columnIndex) }
}

func replaceValue(in row: String, with replacement: String, forColumnAt columnIndex: Int) -> String {
    var values = row.split(separator: ",").map { String($0) }
    values[columnIndex] = replacementValue
    return values.joined(separator: ",")
}

let header = String(inputFileContent[0]) + "\r\n"
let body = replaceValues(
    in: inputFileContent[1...],
    with: replacementValue,
    forColumnAt: columnNameIndex
).joined(separator: "\r\n")

try! (header + body).write(toFile: outputFileName, atomically: true, encoding: .utf8)