printf("%02d", n)

irb(main):005:0> printf "%02d\n", 3
03
=> nil
irb(main):006:0> printf "%02d\n", 13
13
=> nil
irb(main):007:0> printf "%02d\n", 113
113
=> nil
Use printf to format numbers, filling in with zeros for 1 digit numbers. *
(This is Ruby but I'm marking this as C since that's where I'm most likely to end up using it.)