Use a named format with Ruby's format method

Ruby has several methods for formatting strings according to what is called format specification.

#All"hoge = 1 + 2"Is evaluated as
sprintf('hoge = %s + %s', 1, 2)
format('hoge = %s + %s', 1, 2)
'hoge = %s + %s' % [1, 2]

Both format and sprintf are Kernel methods and are aliases with different names.

The third String #% has a slightly different coat color, but it seems that you can do virtually the same thing just by calling format by regarding the self side as a format-specified character string.

What is the recommended writing style?

At the time of this article, the default behavior of RuboCop is "Allow only format methods". I will be corrected.

hoge.rb:3:18: C: Style/FormatString: Favor format over String#%.
'hoge = %s + %s' % [1, 2]

In addition, the traditional style like % s is angry that it is not recommended.

hoge.rb:1:16: C: Style/FormatStringToken: Prefer annotated tokens (like %<foo>s) over unannotated tokens (like %s).
format('hoge = %s + %s', 1, 2)

How to write after all

It seems that it is recommended to add annotation _ \ <name > _ to the format specification and pass the value you want to fit in Hash.

format('hoge = %<x>d + %<y>d', x: 1, y: 2)

By the way, there is also _% {name} _ in a similar notation, but it seems that it is not recommended. You should specify the specifier.

hoge.rb:4:16: C: Style/FormatStringToken: Prefer annotated tokens (like %<foo>s) over template tokens (like %{foo}).
format('hoge = %{x} + %{y}', x: 1, y: 2)

Why that style is recommended

RuboCop seems to be intended to unify the style to one, with a default of format.

The original issue seems to be ↓, but it seems to be a rough process like "Isn't it nice to have it?" Or "Like!" Rather than being decided after a difficult discussion. Maybe default is more than a matter of decision?

Cop idea: Enforce preferred style for format string sequences #3438

The claim that _% \ <name > _ is better than _% {name} _ seems to be based on The Ruby Style Guide.

When using named format string tokens, favor %s over %{name} because it encodes information about the type of the value.


In Ruby, so-called printf-like string formatting methods are provided in several styles.

If you use Ruby for modern, especially team development, you will often want to introduce RuboCop. Here is a brief summary of what styles are specified and what the rationale is.

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