ところで Swift では Array#map が使えます

map.rb

api_response = [
  {last_name: "Tanaka", first_name: "Taro"}, 
  {last_name: "Taguchi", first_name: "Ichiro"}, 
  {last_name: "Haraguchi", first_name: "Saburo"} 
]
full_names = api_response.map {|a| "#{a[:last_name]} #{a[:first_name]}"}

# Returns
# ["Tanaka Taro", "Taguchi Ichiro", "Haraguchi Saburo"]

map.swift

let api_response = [
  ["last_name": "Tanaka", "first_name": "Taro"],
  ["last_name": "Taguchi", "first_name": "Ichiro"],
  ["last_name": "Haraguchi", "first_name": "Saburo"]
]
let full_names = api_response.map {(a) in "\(a['last_name']) \(a['first_name'])"}

# Returns
#  ["Tanaka Taro", "Taguchi Ichiro", "Haraguchi Saburo"]

まさに神
エセC言語で痛み付けられた心が洗われますね