best style for setting items in a collection

I've only recently started learning Ruby and notice in Thomas'
Programming Ruby 1.9 that it seems to be stylistically preferable (and
presumably also faster fwiw) not to use a for loop but rather to use the
each method as often as possible. But I'm having trouble using combining
that with re-assigning values inside an array.

More specifically, consider the following problem: We have an array of
integers and want the result to be the same as the following for loop:

for i in 1...arr.length
   arr[i] += arr[i - 1]
end

Is it stylistically better to use the for loop here or do something like
this:

1.upto(arr.length - 1) {|i| arr[i] += arr[i - 1]}

Or is there another way of doing it that is better than both of these? I
feel like I should be able to do this using the each method but can't
figure out how to make the assignment work that way, and the upto
version is pretty short and sweet.

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