Leveling up our inject skills

inject_spec.rb

require 'rspec'

def sum(numbers)
  numbers.inject(0) { |sum, number| sum + number }
end

def product(numbers)
  numbers.inject(1) { |sum, number| sum * number }
end

# EASIER: Write product, a method that accepts an
# array of numbers, and multiplies them all together.


# HARDER: Write a method that accepts an array
# of strings.
# Return a hash where the keys are the strings
# and the values are the length of each string.
#
# hash_of_lengths(["foo", "ab"]) =>
#   { "foo" => 3, "ab" => 2 }

describe "#sum" do
  it "returns the sum of an array of numbers" do
    numbers = [1, 2, 3, 4]

    result = sum(numbers)

    expect(result).to eq 10
  end
end

def hash_of_lengths(strings)
  strings.inject({}) { |hsh, string| hsh[string] = string.length ; hsh }
end



describe "#hash_of_lengths" do
  it "returns a hash where keys are strings and values are their lengths" do
    strings = ["a", "bb", "ccc"]

    result = hash_of_lengths(strings)

    expect(result).to eq({ "a" => 1, "bb" => 2, "ccc" => 3 })
  end
end

# Harder: Write a method that takes an element and a collection
# and returns true if the collection contains only things
# that are == to that element.

# Easier: do hash_of_lengths
#
describe "#all_equal" do
  context "when all elements in collection == argument" do
    it "returns true" do
      result = all_equal("foo", ["foo", "foo"])

      expect(result).to be_truthy
    end
  end

  context "when all elements are not == to argument" do
    it "returns false" do
      result = all_equal("foo", ["foo", "bar"])

      expect(result).to be_falsey
    end
  end
end

def all_equal(argument, collection)
  collection.inject(true) { |result, element| result && element == argument }
end




count = 0
foos.each do |foo|
  count += 1 if foo.valid?
end
count

foos.inject(0) { |result, foo| result + 1 if foo.valid? ; result }
foos.count { |foo| foo.valid? }



results = []
foos.each do |foo|
  if whatever
    results << foo
  end
end
results

foos.inject([]) { |acc, element| acc << element if whatever ; acc }