Ruby on Rails: How do you add add zeros in front of a number if it's under 10?

A lot of people using sprintf (which is the right thing to do), and I think if you want to do this for a string it's best to keep in mind the rjust and ljust methods:

"4".rjust(2, '0')

This will make the "4" right justified by ensuring it's at least 2 characters long and pad it with '0'. ljust does the opposite.

Did you mean sprintf '%02d', n?

irb(main):003:0> sprintf '%02d', 1
=> "01"
irb(main):004:0> sprintf '%02d', 10
=> "10"

You might want to reference the format table for sprintf in the future, but for this particular example '%02d' means to print an integer (d) taking up at least 2 characters (2) and left-padding with zeros instead of spaces (0).

how about "%02d" % 9? see http://www.ruby-doc.org/core-2.0/String.html#method-i-25 and http://www.ruby-doc.org/core-2.0/Kernel.html#method-i-sprintf .