Ruby Array default value?

Question

I have hundreds of arrays that am normalizing for a CSV.

[
  ["foo", "tom", nil,     1,   4, "cheese"],
  ["foo", "tom", "fluffy",nil, 4],
  ["foo", "tom", "fluffy",1, nil],
  ...
]

Currently to make them all equal length i am finding the max length and setting to a value.

rows.each { |row| row[max_index] ||= nil }

this is cool because it makes the array length equal to the new length.

Instead of appending a bunch of nils at the end I needed to append COLUMN_N where N is the index (1-based).

table_rows.each do |row|
  last_index = row.length - 1
  (last_index..max_index).to_a.each { |index| row[index] ||= "COLUMN_#{index+1}" }
end

Which seemed like an awkward way to have a default value that is a function of the index.

We'd like to see your attempt at solving this. Without that it looks like you want us to write code for you, which isn't what SO is for. Please read "How to Ask" and the linked pages, "minimal reproducible example" and meta.stackoverflow.com/q/261592/128421, which will help explain what are expected. — the Tin Man
– the Tin Man, Commented Feb 13, 2017 at 0:38

Ursus · Accepted Answer · 2017-02-12 23:50:08Z

2

You can't choose a default value for filling elements with []= method. But you can easily do something like this if there aren't other nils that you don't want to replace.

row.each_with_index.map { |item, index| item.nil? ? "column_#{index}": item }

edited Feb 12, 2017 at 23:50

answered Feb 12, 2017 at 23:21

Ursus

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Community · Accepted Answer · 2017-05-23 10:29:32Z

2

To get a default value instead of nil you can use fetch:

row = ["foo", "tom", "fluffy", 1, 4]
row.fetch(7) { |i| "COLUMN_#{i + 1}" }
=> "COLUMN_8"

But it won't fill the array for you.

Also see: Can I create an array in Ruby with default values?

edited May 23, 2017 at 10:29

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answered Feb 12, 2017 at 23:18

Coolness

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Comments

mccalljt · Accepted Answer · 2017-02-12 23:31:27Z

0

This seems like it could work for you.

class Array
  def push_with_default(item, index, &block)
    new_arr = Array.new([self.size + 1, index].max, &block)
    self[index] = item
    self.map!.with_index { |n, i| n.nil? ? new_arr[i] : n }
  end
end

>> array = [1,2,5,9]
[
    [0] 1,
    [1] 2,
    [2] 5,
    [3] 9
]
>> array.push_with_default(2, 10) { |i| "column_#{i}" }
[
    [ 0] 1,
    [ 1] 2,
    [ 2] 5,
    [ 3] 9,
    [ 4] "column_4",
    [ 5] "column_5",
    [ 6] "column_6",
    [ 7] "column_7",
    [ 8] "column_8",
    [ 9] "column_9",
    [10] 2
]

I don't believe a method like this exists on Array already though.

answered Feb 12, 2017 at 23:31

mccalljt

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