Modify particular items in array while traversing in reverse order in Ruby

Question

Is there a way to modify particular array elements (based on some condition) while traversing it in reverse order in Ruby?

To be more clear lets say,

problem is replace even numbers in [1,2,3,4,5] with x

output should be [1,x,3,x,5] (same array) but replace should happen from right to left..traversing from 5 to 1.

Thanks in Advance!

This works: (arr.length -1).downto(0) { |x| do something with arr[x] }

yes exactly, i have noticed that map, collect work from left to right because they in turn depend on each but i want to modify from right to left..i just want to know is there any existing way to do it instead of writing custom code. to traverse we have reverse_each but couldn't find anything to modify :( — mssrivatsa
– mssrivatsa, Commented Sep 3, 2012 at 11:18
is there a reason you cant just use my_array.reverse.map{} ? if you want to put it back in order again just add another .reverse to the end — TomDunning
– TomDunning, Commented Sep 3, 2012 at 11:24
but you don't want the output reserved, right? only the traverse order must be in reverse? and you want the updates always in-place? — tokland
– tokland, Commented Sep 3, 2012 at 11:55
@Isotope reverse is an additional performance overhead which i want to avoid. — mssrivatsa
– mssrivatsa, Commented Sep 3, 2012 at 11:57

steenslag · Accepted Answer · 2012-09-03 11:43:45Z

3

p [1,2,3,4,5].reverse_each.map{|e| e.odd? ? e : e/2} #[5, 2, 3, 1, 1]

answered Sep 3, 2012 at 11:43

steenslag

80.2k16 gold badges144 silver badges174 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

tokland Over a year ago

but this reverses the array and it's not inplace, I think that's what the OP wanted.

tokland · Accepted Answer · 2012-09-03 12:36:30Z

1

I understand you want to traverse in reverse order, not get the output also reversed. Maybe this:

xs = [1, 2, 3]
xs.reverse_each.with_index { |x, idx| xs[xs.size-1-idx] = x.to_s if x == 2 }
xs #=> [1, "2", 3]

edited Sep 3, 2012 at 12:36

answered Sep 3, 2012 at 11:45

tokland

68.2k13 gold badges151 silver badges174 bronze badges

5 Comments

Michael Kohl Over a year ago

xs won't look like you show when using map instead of map!.

tokland Over a year ago

@Michael: oops, yes, it's a enumerator... uhhmm, maybe with each.

mssrivatsa Over a year ago

@tokland Thanks! It works :) I couldn't think of it because i wanted to avoid index based access..Looks like it cant be avoided.

mssrivatsa Over a year ago

@tokland But should be careful while using this because the index starts from 0 lets say your x is 3 but if u try replacing using xs[idx] 1 is modified because ur idx for x = 3 is 0 in this case..please correct me if i am wrong :)

tokland Over a year ago

indeed, fixed, the index must be also "reversed"

marzhaev · Accepted Answer · 2017-04-06 18:46:44Z

0

I appreciate and love Ruby's humane syntax, but you may want to consider more verbose options such as:

ary = [1,2,3,4,5]
i = ary.count - 1
while i >= 0 do
    ary[i] = "x" if ary[i] % 2 == 0
    i -= 1
end
puts ary.join(",")

answered Apr 6, 2017 at 18:46

marzhaev

4874 silver badges4 bronze badges

Collectives™ on Stack Overflow

Modify particular items in array while traversing in reverse order in Ruby

3 Answers 3

1 Comment

5 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related