Wayne Conrad's Blog

When a Ruby program needs to embed the value of an expression in a string, string interpolation is the go-to tool:

name = "Fred"
puts "Hello, #{name}"
#=> "Hello, Fred"

But sometimes the string can get a little long:

puts "#{blamee(longest_line.location)}: #{longest_line.location} is #{longest_line.length} bytes long"

This can be fixed by continuing the string on another line:

puts "#{blamee(longest_line.location)}: #{longest_line.location} "\
  "is #{longest_line.length} bytes long"

But that’s ugly. Much prettier is to use the String#% method:

puts '%s: %s is %d bytes long' % [
  blamee(longest_line.location),
  longest_line.location,
  longest_line.length,
]

This takes care of the long line. It also puts each expression on a line by itself, making it easier to read and edit.

String#% is a wrapper for Kernel#sprintf, which has many useful ways to format things. This print statement left-justifies the version string with a width of 9, and right-justifies the use count with a width of 7:

puts "version %-9s used %7d times from %s through %s" % [
  api_version.version,
  api_version.use_count,
  format_time(api_version.min_time),
  format_time(api_version.max_time),
]
# => version v3.1.0    used    2453 times from 2015-07-20 through 2015-07-22
# => version v3.0.0    used  138893 times from 2015-07-19 through 2015-07-23

And it does so while looking good.

So the next time you’re interpolating and the string gets too long or hard to read, consider the virtues of String#%.