# Multiplying array

HI,

Does anyone know of a function that multiplies the contents of an array.
For example:

one = [1,2,3]
two = [[2],[3],[4]]

output = [[2],[6],[12]]

I've written a simple function that does this, however I'm sure there is
a better way, instead of casting the item to a float.

def multiplyArray(arr1,arr2)
newArr = []
i=0
arr1.each do |x|
x = x.to_s.to_f
item = arr2[i].to_s.to_f
ele = x * item
newArr.push(ele)
i = i + 1
end
puts newArr
end

Thanks,

···

--
Posted via http://www.ruby-forum.com/.

Why do you cast to float? You can multiply integers directly.

First with a straightforward array "two":

irb(main):001:0> require 'enumerator'
=> true
irb(main):002:0> one = [1,2,3]
=> [1, 2, 3]
irb(main):003:0> two = [2,3,4]
=> [2, 3, 4]
irb(main):004:0> one.to_enum(:zip, two).map {|a,b| a*b}
=> [2, 6, 12]

irb(main):005:0> two.map! {|i| [i]}
=> [[2], [3], [4]]
irb(main):006:0> one.to_enum(:zip, two).map {|a,b| a * b[0]}
=> [2, 6, 12]

Kind regards

robert

···

On 23.01.2007 16:05, WKC CCC wrote:

Does anyone know of a function that multiplies the contents of an array.
For example:

one = [1,2,3]
two = [[2],[3],[4]]

output = [[2],[6],[12]]

I've written a simple function that does this, however I'm sure there is
a better way, instead of casting the item to a float.

if you are doing lots of this then check out narray:

harp:~ > cat a.rb
require 'narray'

a = NArray.to_na [1,2,3]
b = NArray.to_na [2,3,4]

p( a * b )

harp:~ > ruby a.rb
NArray.int(3):
[ 2, 6, 12 ]

regards.

-a

···

On Wed, 24 Jan 2007, WKC CCC wrote:

HI,

Does anyone know of a function that multiplies the contents of an array.
For example:

one = [1,2,3]
two = [[2],[3],[4]]

output = [[2],[6],[12]]

I've written a simple function that does this, however I'm sure there is
a better way, instead of casting the item to a float.

def multiplyArray(arr1,arr2)
newArr = []
i=0
arr1.each do |x|
x = x.to_s.to_f
item = arr2[i].to_s.to_f
ele = x * item
newArr.push(ele)
i = i + 1
end
puts newArr
end

Thanks,

--
we can deny everything, except that we have the possibility of being better.
simply reflect on that.
- the dalai lama

WKC CCC wrote:

Does anyone know of a function that multiplies the contents of an array.

module Enumerable
def product
inject{ |piece, prod| prod*piece }
end
def sum
inject(0){ |piece, total| total+piece }
end
end

a = (1..10)
p a.sum
#=> 55
p a.product
#=> 3628800

WKC CCC wrote:

HI,

Does anyone know of a function that multiplies the contents of an array.
For example:

one = [1,2,3]
two = [[2],[3],[4]]

output = [[2],[6],[12]]

I've written a simple function that does this, however I'm sure there is
a better way, instead of casting the item to a float.

def multiplyArray(arr1,arr2)
newArr = []
i=0
arr1.each do |x|
x = x.to_s.to_f
item = arr2[i].to_s.to_f
ele = x * item
newArr.push(ele)
i = i + 1
end
puts newArr
end

Thanks,

--
Posted via http://www.ruby-forum.com/.

[1,2,3].zip([2,3,4]).map{|a,b| a*b}
==>[2, 6, 12]

module Enumerable
def product
inject(1) { |prod, piece| prod*piece }
end
def sum
inject(0) { |total, piece| total+piece }
end
end

True, except the block parameters were reversed. In these cases, the final result wasn't affected because the operations are commutative.

I also added the argument to the inject within product for symmetry. You could equally remove the 0 argument from the sum method's use of inject.

-Rob

Rob Biedenharn http://agileconsultingllc.com
Rob@AgileConsultingLLC.com

···

On Jan 23, 2007, at 11:25 AM, Phrogz wrote:

WKC CCC wrote:

Does anyone know of a function that multiplies the contents of an array.

module Enumerable
def product
inject{ |piece, prod| prod*piece }
end
def sum
inject(0){ |piece, total| total+piece }
end
end

a = (1..10)
p a.sum
#=> 55
p a.product
#=> 3628800

Rob Biedenharn wrote:

module Enumerable
def product
inject(1) { |prod, piece| prod*piece }
end
def sum
inject(0) { |total, piece| total+piece }
end
end

True, except the block parameters were reversed. In these cases, the
final result wasn't affected because the operations are commutative.

Man, I swear that every time I use #inject I get it backwards, and
think that I've fixed the answer in my mind for the next time. Thanks
for the correction. (Anyone got a good _why-like mnemonic they use for
remembering the order?)

I also added the argument to the inject within product for symmetry.
You could equally remove the 0 argument from the sum method's use of
inject.

I thought about that, but in my mind, the sum of an empty array is
zero, but the product of an empty array is nil (or 0?), not 1. Of
course, totally up the the OP as to how s/he wants to handle this edge
case.

How can the inject function work on 2 arrays?

···

--
Posted via http://www.ruby-forum.com/.

[inject confusion]

(Anyone got a good _why-like mnemonic they use for

remembering the order?)

not good, but it helps me:

"memo" comes first (alphabetically), so it is

inject(memo, obj)

P.

···

--
Posted via http://www.ruby-forum.com/.

WKC CCC wrote:

How can the inject function work on 2 arrays?

It can't; I only read the first line of your post (which described a
single array) and rushed off my answer. As penance, I give you another
solution:

a = [ 1, 2, [3], [4], 5]
b = [ 6, [7], [8], 9, 10 ]

p a.flatten.zip(b.flatten).map{ |a,b| a*b }
#=> [6, 14, 24, 36, 50]

Hi,

···

Am Mittwoch, 24. Jan 2007, 01:57:36 +0900 schrieb Patrick Gundlach:

[inject confusion]

> (Anyone got a good _why-like mnemonic they use for
> remembering the order?)

not good, but it helps me:

"memo" comes first (alphabetically), so it is

inject(memo, obj)

I'm trying to help myself with the order invariance in

a == a.inject([]) { |r,e| r+[e] }
a == a.inject([]) { |r,e| r << e }

Bertram

--
Bertram Scharpf
Stuttgart, Deutschland/Germany
http://www.bertram-scharpf.de