The Elements of Style in Ruby #2: Favor sprintf(format) over String#%

Today’s topic is the following rule from the Ruby Style Guide:

Favor the use of sprintf and its alias format over the fairly </br> cryptic String#% method.

Kernel#sprintf and String#% basically do the same thing - the main difference is that sprintf is generally used as a command(it does not operate on its receiver) and String#% is obviously an instance method of the class String. Here’s the two of them in action:

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

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

So, considering they both do the same thing why should you opt to use sprintf instead of %? Here’s a few reasons:

• % takes either a single element or an array of elements as its sole argument; sprintf consistently takes a variable number of arguments.

'%d' % 20
# => '20'

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

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

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

Personally, I dislike such inconsistencies a lot.

• It’s not always clear what % means without additional context. Take a look at this short snippet:

# a and b are variables
a % b

Without some knowledge of a and b we cannot know if we’re dealing with a modulo operation (if a and b are fixnums), String#% or if a is an instance of some other class which has implemented the % method.

With sprintf it’s always crystal-clear what’s going on.

• % does not carry much of a semantic value in it.

Sure, it was named so because of the % placeholders in the target string, but people not familiar with that operator will probably be confused by such odd-looking syntax. sprintf on the other hand is old as time (so people reading your code have probably encountered it somewhere before) and beside that it’s fairly easy to remember that it stands for string print formatted (or something similar). On a related note - the use of Kernel%sprintf’s alias format yields even better readability, since format is obviously less cryptic name than sprintf and the same name is employed in many programming languages (most notably Java and many Lisp dialects).

There’s one thing about sprintf/format that I dislike, though. It doesn’t make that much sense to have such an operation as command in a OO language like Ruby. Alas, those things are not up to me - I guess the authors had something in mind when they made that particular decision about the standard library.

In an ideal world the standard library would have included a String#format method, that took variable number of arguments. For some reason (unknown to me) - that has not happened (and maybe never will). For now the use of Kernel#sprintf (and Kernel#format) yields the best results when it comes down to code clarity and consistency. I encourage you to use them (format in particular)!

As usual I’m looking forward to hearing your thoughts here and on Twitter!

Update

String#format is part of the Powerpack library.