Understanding the “||” OR operator in If conditionals in Ruby

Question

Just briefly, why are the following three lines not identical in their impact?

if @controller.controller_name == \"projects\" || @controller.controller_name          


        

Answer 1

Basically, == doesn't distribute over other operators. The reason 3 * (2+1) is the same as 3 * 2 + 3 * 1 is that multiplication distributes over addition.

The value of a || expression will be one of its arguments. Thus the 2nd statement is equivalent to:

if @controller.controller_name == "projects"

|| is of lower precedence than ==, so the 3rd statement is equivalent to:

if (@controller.controller_name == "projects") || "ports"

Answer 2

The simple way to get a non verbose solution is

if ["a", "b", "c"].include? x

This actually has nothing to do with ||, but rather what values are considered to be true in ruby. Everything save false and nil is true.

Answer 3

|| is also a null coalescing operator, so

"projects" || "parts"

will return the first string that is not null (in this case "projects"), meaning that in the second two examples, you'll always be evaluating:

if @controller.controller_name == "projects"

Firing up irb, you can check that this is happening:

a = "projects"
b = "parts"

a || b

returns projects

Answer 4

The logical or operator || works on boolean expressions, so using it on strings doesn't do what you want.

There are several ways to achieve what you want that are less verbose and more readable.

Using Array#include? and a simple if-statement:

if ["projects", "parts"].include? @controller.controller_name
  do_something
else
  do_something_else
end

Using a case-statement:

case @controller.controller_name
when "projects", "parts" then
  do_something
else
  do_something_else
end

Answer 5

The difference is the order of what's happening. Also the || isn't doing what you think it does in the 2 and 3.

You can also do

if ['projects','parts'].include?(@controller.controller_name)

to reduce code in the future if you need to add more matches.

Answer 6

the exact semantics of || are:

• if first expression is not nil or false, return it
• if first expression is nil or false, return the second expression

so what your first expression works out to is, if @controller.controller_name == "projects", then the expression short-circuits and returns true. if not, it checks the second expression. the second and third variants are essentially if @controller.controller_name == "projects", since "projects" || "parts" equals "projects". you can try this in irb:

>> "projects" || "parts"
=> "projects"

what you want to do is

if ["projects", "parts"].include? @controller.controller_name