CODEWITHSUNDEEP
ruby
jinleileiking
jinleileiking

Reputation: 247

ruby inject problem, the result not same

here is the code: when calculate, something error:

    arr = [0.054954087385762455, 0.5093998626512432, 5.880466289995431, 0.8639141517665208, 0.2152490168404071]


      sum = arr.inject {|sum, item| sum + item}

      tmp = 0
      arr.each do |pwr|
        tmp = tmp + (pwr / sum - 0.2) ** 2
      end
      puts tmp #   0.42948006253339877



pp ( arr.inject  { |result, item| result + (item / sum - 0.2) ** 2} ) # 0.4473023458029664

the result:

0.42948006253339877 != 0.4473023458029664

why ??

Upvotes: 1

Views: 141

Answers (3)

DigitalRoss
DigitalRoss

Reputation: 146053

Because in the second case you don't do any operations on the first element but immediately put it into the result.

In the first computation, the explicit loop, you begin with an external sum variable initialized to zero.

To be an equivalent of the later (inject) code, the coded loop would need to look like this:

tmp = arr[0]
arr[1..-1].each do |pwr|
...

Upvotes: 4

hammar
hammar

Reputation: 139840

You're not specifying a starting value for inject, so it uses the first value as the accumulator instead of 0 in your explicit code.

Do this instead:

arr.inject(0) { ... }

Upvotes: 4

mrb_bk
mrb_bk

Reputation: 726

What you want is:

arr.inject(0){ |result, item| result + (item / sum - 0.2) ** 2}

Upvotes: 3

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