Question

In Rails, I have a date saved in an instance variable. I need to grab the beginning of the decade before it. If @date.year= 1968 then I need to return 1960. How would I do that?

Answer 1

You can do this several ways. As suggested, you can always use integer division which divides the number and truncates the remainder. So 1968/10 returns 196 and if you multiply it by 10, it will give you 1960. Or simply,

@date.year = @date.year/10 * 10
@date.year 
=> 1960

I prefer the method of using modular arithmetic. If you do @date.year % 10 it will return the remainder if you divide by 10 which you can then subtract from the year like so:

@date.year = @date.year - (@date.year % 10)
@date.year 
=> 1960

The reason I prefer the latter is because integer division truncating the remainder may not be some thing that is obvious to everyone looking at your code. However, modular arithmetic works generally the same in all programming languages.

Keep in mind if you’re trying to change the date, you need to use the appropriate method.

@date.change(:year => 1960)