ruby parallel assignment, step question

so, i'm trying to learn ruby by doing some project euler questions, and i've run into a couple things i can't explain, and the comma ?operator? is in the middle of both. i haven't been able to find good documentation for this, maybe i'm just not using the google as I should, but good ruby documentation seems a little sparse . . .

1: how do you describe how this is working? the first snippet is the ruby code i don't understand, the second is the code i wrote that does the same thing only after painstakingly tracing the first:

#what is this doing?
cur, nxt = nxt, cur + nxt

#this, apparently, but how to describe the above?
nxt = cur + nxt   
cur = nxt - cur

2: in the following example, how do you describe what the line with 'step' is doing? from what i can gather, the step command works like (range).step(step_size), but this seems to be doing (starting_point).step(ending_point, step_size). Am i right with this assumption? where do i find good doc of this?

#/usr/share/doc/ruby1.9.1-examples/examples/sieve.rb  
# sieve of Eratosthenes
max = Integer(ARGV.shift || 100)
sieve = []
for i in 2 .. max
  sieve[i] = i
end

for i in 2 .. Math.sqrt(max)
  next unless sieve[i]
  (i*i).step(max, i) do |j|
    sieve[j] = nil
  end
end
puts sieve.compact.join(", ")

Solution

1: It's called parallel assignment. Ruby cares to create temporal variables and not override your variables with incorrect values. So this example:

cur, nxt = nxt, cur + nxt

is the same as:

tmp = cur + nxt
cur = nxt
nxt = tmp

bur more compact, without place to make stupid mistake and so on.

2: There are 2 step methods in ruby core library. First is for Numeric class (every numbers), so you could write:

5.step(100, 2) {}

and it starts at 5 and takes every second number from it, stops when reaches 100.

Second step in Ruby is for Range:

(5..100).step(2) {}

and it takes range (which has start and end) and iterates through it taking every second element. It is different, because you could pass it not necessarily numeric range and it will take every nth element from it.

Take a look at http://ruby-doc.org/core-1.8.7/index.html