From: Brian Candler <b.candler@...>
Date: 2009-07-22T01:45:39+09:00
Subject: Re: Mean method

Älphä Blüë wrote:
> From my understanding of inject, using above..
> 
> a = first instance of array
> b = subsequent instances of array

No, not exactly. You are using the new 1.9 usage (possibly also 1.8.7):

  foo.inject { |a,b| ... }

a is the first element of the array in the first iteration only. For 
subsequent iterations, a is the value of the previous block evaluation.

It may be clearer if you stick to the old usage:

  foo.inject(init) { |a,b| ... }

In the first invocation, a is init and b is the first element of the 
array. For the next invocation, a is the previous block value and b is 
the next element of the array. And so on.

Looking at your initial code:

@arr.each do |a|
  n += ((a - mean)**2)
  std = Math.sqrt(n / @size)
end

This is strange. You're calculating a value in every iteration and 
assigning it to std, but then throwing it away apart from the last one. 
Shouldn't this go outside the loop?

n = 0
@arr.each do |a|
  n += ((a - mean)**2)
end
std = Math.sqrt(n / @size)

If that's correct, then the solution becomes obvious:

n = @arr.inject(0) { |accum,elem| accum + (elem-mean)**2 }
std = Math.sqrt(n / @size)
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