cities = [
    {name: , population: 21}, 
    {name: , population: 15.2}, 
    {name: , population: 11.3}, 
    {name: , population: 7.55}, 
    {name: , population: 5.85}, 
    {name: , population: 4.98}, 
    {name: , population: 4.7}, 
    {name: , population: 4.58}, 
    {name: , population: 4.4}, 
    {name: , population: 3.98},
]

puts cities.index{|city| city[:name] == }      
puts cities.find {|city| city[:population] < 5.0}[:name]      
puts cities.find {|city| city[:name][0] == }[:population]