Adding two arrays in Ruby when the array length will always be the same

Question

So, I need to add two arrays together to populate a third. EG

a = [1,2,3,4]
b = [3,4,5,6]

so that:

c = [4,6,8,10]

I read the answer given here: https://stackoverflow.com/questions/12584585/adding-two-ruby-arrays

but I'm using the codecademy labs ruby editor and it's not working there, plus the lengths of my arrays are ALWAYS going to be equal. Also, I don't have any idea what the method ".with_index" is or does and I don't understand why it's necessary to use ".to_i" when the value is already an integer.

It seems like this should be really simple?

Cary Swoveland · Accepted Answer · 2014-01-17 05:15:33Z

3

a = [1,2,3,4]
b = [3,4,5,6]
a.zip(b).map { |i,j| i+j } # => [4, 6, 8, 10]

Here

a.zip(b) # => [[1, 3], [2, 4], [3, 5], [4, 6]]

and map converts each 2-tuple to the sum of its elements.

answered Jan 17, 2014 at 5:15

Cary Swoveland

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4 Comments

Nathan_U Over a year ago

What does the zip method do?

Cary Swoveland Over a year ago

Nathan_U, I anticipated your question :-) Let me know if that's not clear.

Nathan_U Over a year ago

That makes sense, thanks! So, zip puts both values together to make a 2d array, and map changes the values to the sum with { |i,j| i+j }. What I'm trying to do is create a one-time pad program as I'm learning ruby. So my question was to enable me to XOR the message by the key. All I need to do now is learn how to turn the characters into binary and back again and I'll be 70% there!

Cary Swoveland Over a year ago

Nathan_U, have a look at Fixnum#to_s(base) and String#to_i(base).

zeantsoi · Accepted Answer · 2014-01-17 06:16:18Z

3

OPTION 1:

For a pure Ruby solution, try the transpose method:

a = [1,2,3,4]
b = [3,4,5,6]
c = [a, b].transpose.map{|x, y| x + y} 
#=> [4,6,8,10]

OPTION 2:

If you're in a Rails environment, you can utilize Rails' sum method:

[a, b].transpose.map{|x| x.sum} 
#=> [4,6,8,10]

EXPLANATION:

transpose works perfectly for your scenario, since it raises an IndexError if the sub-arrays don't have the same length. From the docs:

Assumes that self is an array of arrays and transposes the rows and columns. If the length of the subarrays don’t match, an IndexError is raised.

edited Jan 17, 2014 at 6:16

answered Jan 17, 2014 at 5:14

zeantsoi

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2 Comments

Nathan_U Over a year ago

What exactly does the transpose method do?

zeantsoi Over a year ago

I've updated the answer with an explanation of both the Ruby transpose method and the Rails sum method.

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