Converting an integer to a hexadecimal string in Ruby

To summarize:

p 10.to_s(16) #=> "a"
p "%x" % 10 #=> "a"
p "%02X" % 10 #=> "0A"
p sprintf("%02X", 10) #=> "0A"
p "#%02X%02X%02X" % [255, 0, 10] #=> "#FF000A"

Here's another approach:

sprintf("%02x", 10).upcase

see the documentation for sprintf here: http://www.ruby-doc.org/core/classes/Kernel.html#method-i-sprintf

How about using %/sprintf:

i = 20
"%x" % i  #=> "14"

You can give to_s a base other than 10:

10.to_s(16)  #=> "a"

Note that in ruby 2.4 FixNum and BigNum were unified in the Integer class. If you are using an older ruby check the documentation of FixNum#to_s and BigNum#to_s