The Elements of Style in Ruby #4: Array#join vs Array#*

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

Favor the use of Array#join over the fairly cryptic Array#* with a string argument.

Array#join and Array#* (with a string argument) behave exactly the same:

%w(Bruce Wayne).join(' ')
# => "Bruce Wayne"

%w(Bruce Wayne) * ' '
# => "Bruce Wayne"

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

  • * behaves totally differently when passed an integer argument:
[1 2] * 3
# => [1 2 1 2 1 2]

Personally, I’d expect this to be only behavior of such an operator method and find the alternative one (with a string argument) to be pretty much counter-intuitive.

  • 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 be certain what this code is going to do. It’s hard even to speculate what the code is going to do. Obviously better variable names would certainly help, but the point still stands.


While we still cannot be absolutely certain, it’s highly likely that a is an Array and b is a String.

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

Unlike String#%, Array#* with a string argument carries pretty much no meaning. It’s absolutely beyond me how this came into existence. On the other hand the behavior of Array#* with an integer argument is pretty reasonable. Here we see a classic example of the notion that too much operator overloading can be a bad thing, leading to some pretty unreadable code. An operator should be employed only when it’s use would add clarity to the code, not take clarity away.

Array#* has one thing going for it, however - the fact that few people know about its use as a substitute for Array#join. I hope they realize that some unknown features are unknown for a reason - because it’s bad idea to make use of them.

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