How to add amounts from an array of arrays

Question

I have an array of arrays like so...

a1 = [[9, -1811.4], [8, 959.86], [7, -385], [6, -1731.39], [5, 806.78], [4, 2191.65]]

I need to get the average of the 2nd items(the amounts) from the total array.

So add -1811.4,959.86,-385,-1731.39,806.78 divided by the count (6)

I have tried...

a1.inject{ |month, amount| amount }.to_f / a1.size

This is not right and I cant see what I need to do

Andrey Deineko · Accepted Answer · 2016-10-07 23:45:43Z

3

a1.map(&:last).inject(:+) / a1.size.to_f
#=> 5.0833333333332575

Steps:

# 1. select last elements
a1.map(&:last)
#=> [-1811.4, 959.86, -385, -1731.39, 806.78, 2191.65]
# 2. sum them up
a1.map(&:last).inject(:+)
#=> 30.499999999999545
# 3. divide by the size of a1
a1.map(&:last).inject(:+) / a1.size.to_f
#5.0833333333332575

edited Oct 7, 2016 at 23:45

answered Oct 7, 2016 at 21:23

Andrey Deineko

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3 Comments

SupremeA Over a year ago

Does this method turn negatives to positives when adding?

Andrey Deineko Over a year ago

@SupremeA it does not. It just sums it. Do you need to sum absolute values?

SupremeA Over a year ago

yep I see now Thanks!

Cary Swoveland · Accepted Answer · 2016-10-07 23:25:43Z

2

One pass through a1 is sufficient.

a1.reduce(0) { |tot, (_,b)| tot + b }/a1.size.to_f
  #=> 5.0833333333332575

.to_f allows a1 to contain only integer values.

The steps:

tot = a1.reduce(0) { |tot, (_,b)| tot + b }
  #=> 30.499999999999545 
n = a1.size.to_f
  #=> 6.0 
tot/n
  #=> 5.0833333333332575

edited Oct 7, 2016 at 23:25

answered Oct 7, 2016 at 23:19

Cary Swoveland

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3 Comments

Sagar Pandya Over a year ago

Just another option, we can also use .fdiv(a1.size) instead of /a1.size.to_f.

Cary Swoveland Over a year ago

@sagarpandya82, I'd never heard of Numeric#fdiv. Thanks for the tip.

Andrey Deineko Over a year ago

Was also considering adding a inject option, but it took more letters to type then my original one, so i dropped it :)

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