Ruby Newbie: Confused About Boolean Logic

Question

I have an array, if I find a value in it, I want to execute a block of code. Also, if the array is nil, I want to execute that block. So the code I tried is:

if !array.respond_to? :index || array.index(str) #some code

So if it's nil it's true, or if str is somewhere in the array, it's true, right? But if it finds the item at index 0, it doesn't enter the block. Also, according to irb false || 0 evalueates to 0. WTF?? I thought that everything was true except false and nil. I guess || does something odd that I'm not expecting??

My questions are: What's going on? What's a nice way to write a conditional that does what I want?

You just said it yourself: "I thought that everything was true except false and nil." So why are you surprised that false || 0 is 0? — Uri
– Uri, Commented Nov 2, 2011 at 19:57
I'm surprised that false || 0 != true. 0 isn't false or nil, so that should be false || true = true. Why isn't it? — pseudopeach
– pseudopeach, Commented Nov 2, 2011 at 20:07
@pseudopeach Because 0 is truthy, and that's how Ruby works. — Dave Newton
– Dave Newton, Commented Nov 2, 2011 at 20:24
@Dave Newton "Truthy" is obviously ill-defined for the poster. Perhaps you would do better to define and explain, instead of saying "that's how Ruby works." — Andy_Vulhop
– Andy_Vulhop, Commented Nov 3, 2011 at 14:25

Sahil Muthoo · Accepted Answer · 2011-11-02 19:59:20Z

2

Using nil? and include? with an inline if seems most idiomatic to me.

#your code if arr.nil? || arr.include?(str)

answered Nov 2, 2011 at 19:59

Sahil Muthoo

12.6k2 gold badges32 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

pseudopeach Over a year ago

I like your code better, but I still don't get why false or 0 is 0.

Dave Newton Over a year ago

@pseudopeach A Ruby "or" will return the first truthy value it finds, in this case, 0.

Sahil Muthoo Over a year ago

A 'truthy' expression is true in a boolean context.

Marnen Laibow-Koser Over a year ago

Specifically, in Ruby, everything but false and nil is truthy.

pseudopeach Over a year ago

So if (false || 0) is truthy, why didn't my IF block execute?

Dave Newton · Accepted Answer · 2011-11-02 20:49:52Z

if array.nil? || array.member?(s) # ...

false || 0 evaluates to 0 because it's an or. False isn't truthy (obviously ;) but 0 is, so the expression is truthy.

robotcookies · Accepted Answer · 2011-11-02 19:56:26Z

Are you checking for a nil array or an empty one? If you've already declared the array it won't be nil even if it's empty. I'd write it like:

if array.empty? || array.include(str)

or if you really want to check for a nil array:

if array.nil? || array.include(str)

I'd use .include rather than .index to avoid getting a 0.

Andy_Vulhop · Accepted Answer · 2011-11-02 19:57:06Z

if array.nil? || array.member?(str) #code block end

The || operator almost reminds you of a coalesce.

Given a = false, b = :bacon

return a || b #returns :bacon

Collectives™ on Stack Overflow

Ruby Newbie: Confused About Boolean Logic

4 Answers 4

5 Comments

Comments

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

5 Comments

Comments

Comments

Comments

Related