Question

I’m trying to evaluate the expression (a=10) || (rr=20) while the rr variable is not defined

so typing rr in the ruby console before evaluating the previous expression returns

rr
NameError: undefined local variable or method `rr' for main:Object
from (irb):1
from :0

When I write the expression (a=10) || (rr=20) it returns 10, and when I write rr afterwards it says nil

(a=10) || (rr=20)
rr  # => nil

so, why is this happening? Shouldn’t rr be defined only if the second argument of the || operator is evaluated, which should be never based on the documentation?

Answer 1

This happens because the ruby interpreter defines a variable when it sees an assignment to it (but before it executes the actual line of code). You can read more about it in this answer.

Boolean OR (||) expression will evaluate to the value of left hand expression if it is not nil and not false, else || will evaluate to the value of right hand expression.

In your example the ruby interpreter sees an assignment to a and rr (but it doesn’t execute this line yet), and initializes (defines, creates) a and rr with nil. Then it executes the || expression. In this || expression, a is assigned to 10 and 10 is returned. r=20 is not evaluated, and rr is not changed (it is still nil). This is why in the next line rr is nil.