Ruby : What is the difference between " a = nil || 2008 " and " a = nil or 2008 "

Question

When I evaluate the expression a = nil || 2008 in Irb, I get the answer as 2008 and a is assigned the value 2008 which is correct as || operator return their first argument unless it is false or nil.

But when I do something like a = nil or 2008 on Irb console, a is assigned nil and the return value of the evaluation of the expression I get is still 2008.

Has it got something to do with operator precedence. I was just wondering why it works this way, can someone please shed some light on this ?

These operators have different precedence. In such expressions always use `||` (unless you know you *need* `or` there) — Sergio Tulentsev, Nov 25 '14 at 07:49

score 5 · Answer 1 · answered Nov 25 '14 at 07:52

5

|| and or have different precedences. This was meant to be used, e.g. for error handling:

foo = do_something_which_could_fail_and_return_nil_then(...) or deal_with_error(...)

Some people think it should be avoided, i.e. or and and should not be used at all.

answered Nov 25 '14 at 07:52

undur_gongor

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“Some people“ made my day. – Aleksei Matiushkin Nov 25 '14 at 08:02
Thanks undur_gongor, Link to ruby style guide was really helpful. – MegaChief Nov 25 '14 at 08:55

score 4 · Accepted Answer · answered Nov 25 '14 at 07:55

4

Order of precedence in your cases is following(from higher to lower):

||
=
or

So here a = nil || 2008

Operation 1: nil || 2008
Operation 2: a = Operation 1

and for a = nil or 2008

Operation 1: a = nil
Operation 2: Operation 1 or 2008 which returns 2008, because Operation 1 is nil

answered Nov 25 '14 at 07:55

Rustam Gasanov

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Thanks Rustam, the step by step explanation helped me understand it better. – MegaChief Nov 25 '14 at 08:54

score 3 · Answer 3 · answered Nov 25 '14 at 07:56

You answered it yourself. It is due to operator precedence. || has higher precedence then = and "or" has lower precedence then '='. Here is a precedence table for ruby operators

if you use

a = (nil or 2008)

then it will give same result as

a = nil || 2008

Ruby : What is the difference between " a = nil || 2008 " and " a = nil or 2008 "

3 Answers3