json - Prevent Nil Values in Transformed Array

Removing nils is quite simple, however, I'd like to know:

1) What am I doing wrong, why does my array result below include nils?

2) How I can PREVENT the nils from being added to my array, instead of removing them after the fact.

@cars = Array.new
plucked_array = [
    [8, "Chevy", "Camaro", 20],
    [9, "Ford", "Mustang", 55],
    [9, "Ford", "Fusion", 150]

]
plucked_array.
    each { |id, make, model, model_count|
      @cars[id] ||= {name: make, id: make, data: []}
      @cars[id][:data].push([model, model_count])
    }
puts @cars.inspect
#=>[nil, nil, nil, nil, nil, nil, nil, nil, {:name=>"Chevy", :id=>"Chevy", :data=>[["Camaro", 20]]}, {:name=>"Ford", :id=>"Ford", :data=>[["Mustang", 55], ["Fusion", 150]]}]

puts @cars.compact.inspect

#=>[{:name=>"Chevy", :id=>"Chevy", :data=>[["Camaro", 20]]}, {:name=>"Ford", :id=>"Ford", :data=>[["Mustang", 55], ["Fusion", 150]]}]
# This gives the result I'm looking for, 
# just wondering how to best get transformed array without the post-cleanup.

I also attempted @theTinMan's recommendation to select first, then map, but I had the same results:

plucked_array.select { |id, make, model, model_count|
  @cars[id] = {'name' => make, 'id' => make, 'data' => []}
}.map { |id, make, model, model_count|

  @cars[id]['data'].push([model, model_count])
}
puts @cars.inspect
#=>[nil, nil, nil, nil, nil, nil, nil, nil, {:name=>"Chevy", :id=>"Chevy", :data=>[["Camaro", 20]]}, {:name=>"Ford", :id=>"Ford", :data=>[["Mustang", 55], ["Fusion", 150]]}]

I've tried, with partial success, to use Hash instead of an array for @cars. This prevented nils, but my end goal is to build "drilldown: series[]" below, which is an array of hashes:

// Create the chart
Highcharts.chart('container', {
    chart: {
        type: 'column'
    },
    title: {
        text: 'Imaginary Car Stats'
    },
    subtitle: {
        text: 'Click the columns to view models.'

    },
    xAxis: {
        type: 'category'
    },
    yAxis: {
        title: {
            text: 'Total car count'
        }
    },
    legend: {

        enabled: false
    },
    plotOptions: {
        series: {
            borderWidth: 0,
            dataLabels: {
                enabled: true,
                format: '{point.y}'
            }
        }

    },
    tooltip: {
        headerFormat: '{series.name}
',
        pointFormat: '{point.name}: {point.y:.2f}% of total
'
    },
    /*I have separate `pluck` query for this top-level series:*/
    "series": [
        {
            "name": "Cars",
            "colorByPoint": true,

            "data": [
                {
                    "name": "Ford",
                    "y": 205,
                    "drilldown": "Ford"
                },
                {
                    "name": "Chevy",
                    "y": 20,
                    "drilldown": "Chevy"

                },
                {
                    "name": "Other",
                    "y": 16,
                    "drilldown": null
                }
            ]
        }
    ],
    "drilldown": {

        /*This is the array of hashes I'm attempting to create:*/
        "series": [
            {
                "name": "Ford",
                "id": "Ford",
                "data": [
                    [
                        "Fusion",
                        150
                    ],

                    [
                        "Mustang",
                        55
                    ]
                ]
            },
            {
                "name": "Chevy",
                "id": "Chevy",
                "data": [

                    [
                        "Camaro",
                        20
                    ]
                ]
            }
        ]
    }
});

Answer

The reason your code is behaving the way it does is because array[8] = x inserts x at position 8 in the array, and ruby fills the spaces up to 8 with nils.

a = []
a[7] = 4
a == [nil, nil, nil, nil, nil, nil, nil, 4]

You need @cars to be a hash - not an array

I think this will do what you are trying to do:

plucked_array = [

    [8, "Chevy", "Camaro", 20],
    [9, "Ford", "Mustang", 55],
    [9, "Ford", "Fusion", 150]
]

cars = plucked_array.each_with_object({}) do |(id, make, model, count), cars|
  cars[id] ||= {id: id, make: make, data: []}
  cars[id][:data] << [model, count]
end


p cars.values

Which actually is almost identical to @Austio's solution.