Ruby’s format Method

To format a String in Ruby we can use the Kernel#format or Kernel#sprintf method. They work by returning a copy of the String passed in, with its format sequence replaced by the additional arguments passed to it. This is easier with an example:

sprintf('%d %d', 20, 10)  # => "20 10"

The first argument passed in method sprintf is a String object containing two format sequences.

A format sequence is a parcent sign followed by flags, width and precision indicators, then terminated with a field type character.


In the example above, the d in %d is the field type character. It converts the passed argument as a decimal number.

The [width] controls the size of the returned String object, while [.precision] mainly specify the number of digits after the decimal points.

A common use for format or sprintf is to format the result of Float objects.

format('%f', 1.0/3.0)    # => "0.333333"
format('%.2f', 1.0/3.0)  # => "0.33"

Note the use of the dot . before the precision number.

String#% Method

Rubocop advises against using the String#% method:

Prefer the use of sprintf and its alias format over the fairly cryptic String#% method.

The documentation does not give any reason why. Kernel#sprintf and String#% basically do the same thing. However, String#% is an instance method of the String class, while the former is a command.

On his blog, Rubocop’s author gives several answer:

One reason to prefer the Kernel methods over the String method is the inconsistencies of the % method which takes either a single element or an array of elements. Another reason is its cryptic meaning: % can either be a modulo or the String#% method, which, without context is hard to parse:

a % b

Even with hardcoded values, the repetition of the % symbol is difficult to parse:

'%d %d' % [20, 10]  # => "20 10"