Each with previous?

Ruby
1

I’m going through lots of data where the result of the current is
affected by the previous element. So what I would need is an
“each_with_previous {|current, prev|}”
…to make it a bit more readable. Is there any built-in Ruby method
that I might have overlooked, or should I build my own?

Best regards,

Jari W.

2

On Dec 14, 2007 4:28 PM, Jari W.
[email protected] wrote:

I’m going through lots of data where the result of the current is
affected by the previous element. So what I would need is an
“each_with_previous {|current, prev|}”
…to make it a bit more readable. Is there any built-in Ruby method
that I might have overlooked, or should I build my own?

Enumerable.each_cons(2) { |current, prev| … }

Eivind.

3

On Sat, Dec 15, 2007 at 12:28:51AM +0900, Jari W. wrote:

I’m going through lots of data where the result of the current is affected
by the previous element. So what I would need is an
“each_with_previous {|current, prev|}”
…to make it a bit more readable. Is there any built-in Ruby method that I
might have overlooked, or should I build my own?

You can fake it pretty simply with inject. For example:

[ 1,2,3,4,5 ].inject { |prev,cur| puts “#{prev}, #{cur}”; cur }
1, 2
2, 3
3, 4
4, 5

Note that you do need the block to “return” (i.e. evaluate to) the
current
element so that it gets passed into the next iteration.

Best regards,
Jari W.
–Greg

4

On Dec 14, 8:28 am, Jari W.
[email protected] wrote:

I’m going through lots of data where the result of the current is
affected by the previous element. So what I would need is an
“each_with_previous {|current, prev|}”
…to make it a bit more readable. Is there any built-in Ruby method
that I might have overlooked, or should I build my own?

Take your pick:

irb(main):001:0> a = %w| a b c d e f g h |
=> [“a”, “b”, “c”, “d”, “e”, “f”, “g”, “h”]

irb(main):002:0> require ‘enumerator’

irb(main):003:0> a.each_cons(2){ |x,y| puts “#{x}-#{y}” }
a-b
b-c
c-d
d-e
e-f
f-g
g-h

irb(main):004:0> a.each_slice(2){ |x,y| puts “#{x}-#{y}” }
a-b
c-d
e-f
g-h

5

On Dec 14, 2007, at 10:28 AM, Jari W. wrote:

I’m going through lots of data where the result of the current is
affected by the previous element. So what I would need is an
“each_with_previous {|current, prev|}”
…to make it a bit more readable. Is there any built-in Ruby method
that I might have overlooked, or should I build my own?

Best regards,

Jari W.

require ‘enumerator’

[first, second, third].each_slice(2) do |prev,curr|

do stuff

end

Except that you don’t get a [nil,first] pair to start, you get
[first,second].

You could also do something like:

[first, second, third].inject(nil) do |prev, curr|

do stuff

curr
end

And curr is assigned to prev in the next iteration and the first
iteration gets [nil,first]

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

6

On 14.12.2007 16:39, Gregory S. wrote:

1, 2
2, 3
3, 4
4, 5

Note that you do need the block to “return” (i.e. evaluate to) the current
element so that it gets passed into the next iteration.

There is a subtlety: with your code there will be no previous for the
first element. Depending on what the OP needs you can as well do

irb(main):001:0> (1…5).inject(nil) {|prev,curr| p [prev,curr];curr}
[nil, 1]
[1, 2]
[2, 3]
[3, 4]
[4, 5]
=> 5

Jari, what kind of calculation do you do?

Cheers

robert

7

Robert K. wrote:

Jari, what kind of calculation do you do?

I’ll use different approaches for different situations. Starting inject
with nil or first item will both become handy. Thanks!

The tasks are for analyzing musical data. The current note is most often
dependent of previous context.

Best regards,

Jari W.