From: Mark Hubbart <discord@...>
Date: 2004-03-13T05:44:39+09:00
Subject: Re: iterating with multiple values


On Mar 12, 2004, at 10:09 AM, gabriele renzi wrote:

> I thought this should work:
> [1,2,3,4,5,6].inject do |pre2,cur|
>   print pre2[0],pre2[1],cur,"\n"
>   [pre2[1],cur]
> end
>
> but it gives strange results in the first loops.
> 102
> 023
> 234
> 345
> 456

to get an idea of why it doesn't work, try this:

[1,2,3,4,5,6].inject do |pre2,cur|
   print pre2[0],pre2[1],cur," :: ",pre2.inspect,cur.inspect,"\n"
   [pre2[1],cur]
end

output:
102 :: 12
023 :: [0, 2]3
234 :: [2, 3]4
345 :: [3, 4]5
456 :: [4, 5]6

pre2 is not an array on the first pass. Also, unless you pass an 
argument to #inject, on the first loop, it passes in the first two 
values. Try this:

   [1,2,3,4].inject([nil]*2) do |pre,cur|
     puts "%s,%s,%s" % (pre + [cur])
     pre.slice(1..-1) + [cur]
   end

,,1
,1,2
1,2,3
2,3,4

If you pass an array of nils, it works the way you might expect. Also, 
this is scalable; you can multiply [nil] by a larger number to capture 
more previous variables:

   (1..8).to_a.inject([nil]*4) do |pre,cur|
     puts "%s,%s,%s,%s,%s" % (pre + [cur])
     pre.slice(1..-1) + [cur]
   end

,,,,1
,,,1,2
,,1,2,3
,1,2,3,4
1,2,3,4,5
2,3,4,5,6
3,4,5,6,7
4,5,6,7,8

--Mark