How can I ouput leading zeros in Ruby?

Question

I'm outputting a set of numbered files from a Ruby script. The numbers come from incrementing a counter, but to make them sort nicely in the directory, I'd like to use leading zeros in the filenames. In other words,

file_001...

instead of

file_1

Is there a simple way to add leading zeros when converting a number to a string? (I know I can do "if less than 10.... if less than 100")

Answer 1

Use the % operator with a string:

irb(main):001:0> "%03d" % 5
=> "005"

The left-hand-side is a printf format string, and the right-hand side can be a list of values, so you could do something like:

irb(main):002:0> filename = "%s/%s.%04d.txt" % ["dirname", "filename", 23]
=> "dirname/filename.0023.txt"

Here's a printf format cheat sheet you might find useful in forming your format string. The printf format is originally from the C function printf, but similar formating functions are available in perl, ruby, python, java, php, etc.

Answer 2

Cant you just use string formatting of the value before you concat the filename?

"%03d" % number

Answer 3

If the maximum number of digits in the counter is known (e.g., n = 3 for counters 1..876), you can do

str = "file_" + i.to_s.rjust(n, "0")

Answer 4

I had a similar need i.e., generate several files with incrementing filenames such that i have file_001 instead of file_1. What i did was used a string for the counter and used "next". e.g.,

n = "001"
filename = "file_" + n
n = n.next