Ruby: Multiply All Elements of an Array

Ruby: Multiply all elements of an array

This is the textbook case for inject (also called reduce)

[1, 2, 3, 4, 5].inject(:*)

As suggested below, to avoid a zero,

[1, 2, 3, 4, 5].reject(&:zero?).inject(:*)

How to multiply each number in array by 2 in ruby

You could use map and over each element in the array check if is Numeric, and if so, then multiply it by 2, then you can compact your result for nil values:

p ['a', 2, 2.5].map{|e| e * 2 if e.is_a? Numeric}.compact
# => [4, 5.0]

If you want to leave the elements which won't be applied the *2 operation, then:

p ['a', 2, 2.5].map{|e| e.is_a?(Numeric) ? e * 2 : e}

Also you could use grep to simplify the checking, and then map your only-numeric values:

p ['a', 2, 2.5].grep(Numeric).map{|e| e*2}
# => [4, 5.0]

I don't know the side effects of doing this, but looks good (of course if the output won't be only Numeric objects):

class Numeric
  def duplicate
    self * 2
  end
end

p ['a', 2, 2.5].grep(Numeric).map(&:duplicate)

Or also:

p ['a', 2, 2.5].grep(Numeric).map(&2.method(:*))
# [4, 5.0]

How can I multiply two arrays in Ruby?

a.zip(b).map{|x, y| x * y}

This works because zip combines the two arrays into a single array of two element arrays. i.e.:

a = [3, 2, 1]
b = [1, 2, 3]
a.zip(b)
#=> [[3, 1], [2, 2], [1, 3]]

Then you use map to multiply the elements together. This is done by iterating through each two element array and multiplying one element by the other, with map returning the resulting array.

a.zip(b).map{|x, y| x * y}
#=> [3, 4, 3]

Multiply every element of an n-dimensional array by a number in Ruby

The long-form equivalent of this is:

[ 1, 2, 3, 4, 5 ].collect { |n| n * 2 }

It's not really that complicated. You could always make your multiply_by method:

class Array
  def multiply_by(x)
    collect { |n| n * x }
  end
end

If you want it to multiply recursively, you'll need to handle that as a special case:

class Array
  def multiply_by(x)
    collect do |v|
      case(v)
      when Array
        # If this item in the Array is an Array,
        # then apply the same method to it.
        v.multiply_by(x)
      else
        v * x
      end
    end
  end
end

Ruby: how to repeat/multiply elements in array?

Try this one

array.flat_map { |item| Array.new(5, item) }
[
    [ 0] "A",
    [ 1] "A",
    [ 2] "A",
    [ 3] "A",
    [ 4] "A",
    [ 5] "B",
    [ 6] "B",
    [ 7] "B",
    [ 8] "B",
    [ 9] "B",
    [10] "C",
    [11] "C",
    [12] "C",
    [13] "C",
    [14] "C"
]

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