Loop through months (1-12)

Question

I currently have this monstrosity:

next_period = (((current_month + tax_period_months)-1) % 12) + 1 

I want to get the month that is tax_period_months after the current month.

E.g. if tax_period_months=3, and current_month=09 then next_period=12

This works, but has that horrible -1 +1 to stop month 12 becoming 0

Any alternatives?

• Dair: I would argue the -1 +1 is pretty slick once explained in a comment. Also, I honestly don't know what you would do to make it better. You need to map current_month + tax_period_months to $$\mathbb{Z}_{11}$$ and then increment everything by one. It would probably have to be a two step process that would require subtracting then adding 1.
• Martin R: @Dair: You probably meant \$ \mathbb{Z}_{12} \$.
• tokland: Is that a RoR project?
• Mirror318: @tokland yes it's Rails, @Dair I don't mind using -1+1, but my personal philosophy is that code that doesn't need comments is easier maintained and therefore more valuable than code that makes sense only once you read the comments.

Answer 1

If that really is all you need to calculate, then it's not too bad.

However, I suspect that you have other date calculations happening in your application. In that case, why not take advantage of Ruby's Date class? The >> operator adds months.

require 'date'
next_period = (Date.today >> tax_period_months).month

• 200_success: @tokland No, the + operator treats its second operand as the number of days to add, whereas the >> operator treats its second operand as the number of months.
• 200_success: @tokland Where do you get the .months extension method from? I get the error: NoMethodError: undefined method `months' for 2:Fixnum.
• Ram on Rails: ruby-doc.org/stdlib-2.3.1/libdoc/date/rdoc/…

Answer 2

The purpose of the +1/-1 is just to cope with the fact that december gives 0 as result. But it's not entirely apparent that it's it purpose.

On the other side there is a ruby native method Integer#nonzero? that returns the number itself when it's not zero, and nil when it's zero.

This way it's made much more evident that we are dealing with a "we didn't expect zero" edge condition - in a way that -1/+1 does not convey.

next_period = ((current_month + tax_period_months) % 12).nonzero? || 12

Note that this does not require any dependency, ActiveSupport or what else.

Answer 3

I ended up doing this

next_period = current_month + tax_period_months
next_period -= 12 if next_period > 12

Not the fanciest but easier to read.

I'll accept @200_success as the top answer because it takes advantage of the Date class, which will handle year changes, etc. (even though it didn't really work for my context).

• 200_success: I'm curious... why didn't it work?
• Mirror318: It was in the middle of a bunch of code dealing with periods (e.g. find the first day of the period which the calculated date lies in), and it was already constructed to use months separately to entire dates. I was adding a few changes, but the rest was already working ok.
• Wayne Conrad: This is much easier to read.