Why Treat 0 as True in Ruby?

Question

I'm reading through the excellent Ruby on Rails Tutorial and have encountered the following code.

if 0 true else false end

The above returns true and illustrates how unlike many languages (C being the obvious example), Ruby treats 0 as true. Rather than dismiss the behavior as idiosyncratic, I assume there is a good reason for this significant departure from convention. Python, for instance, treats 0 as False, just as one would expect.

In short, what is the rationale in designing Ruby to treat 0 as true?

maerics · Accepted Answer · 2012-04-30 18:08:08Z

66

I'm guessing that Matz wanted conceptual simplicity of "truthiness" as such - the only "false" values are false and nil. Period.

Using just false would be the cleanest but there is understandable need for including nil. To include the integer zero as a special case might open the mental floodgates of questioning truthiness of other types. What about strings, is "" false? And arrays, is [] false? And hashes, is {} false? Ad insanitum (see JavaScript)...

edited Apr 30, 2012 at 18:08

answered Apr 30, 2012 at 17:02

maerics

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5 Comments

enocom Over a year ago

What's interesting, especially since I'm coming to ruby from python, is that all the examples you name ([], {}, and "") are false in python. In my view, this only enhances the quirkiness of ruby.

maerics Over a year ago

And I'm sure people learning Python after knowing Ruby, Perl, JavaScript, Lua (etc.) find it quirky that those values aren't true!

allenhwkim Over a year ago

Rails introduced #blank?, #present?, #empty?, and maybe more because, i think, 0, [], {}, and "" are not false. That make code bigger against ruby's less code principle.

rubyprince Over a year ago

in ReactJS (Javascript), I wrote baseAmount && <MyComponent> and suddenly my component disappeared when baseAmount was 0 as 0 is falsey in Javascript.

vlsd Over a year ago

all your examples can be easily categorized as falsy without any danger, since they're pretty much the only ones; where it might get a little hairy is if you wanted to categorize stuff like [""] or {[]} as falsy but I'm not aware of any languages that do that

allenhwkim · Accepted Answer · 2012-04-30 17:09:56Z

15

In ruby, if exists, it's true. If not, it's false.

so, with Ruby null(no address assigned) and false are only false.

All others are true because it has address assigned to it.

I think of this way; "Does it exist?"

answered Apr 30, 2012 at 17:09

allenhwkim

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3 Comments

jackdoe Over a year ago

what you said is not true: nil is an object, so is false, and both of them exist :), they are instances of NilClass and FalseClass. the only false values are false and nil.

DigitalRoss Over a year ago

...i.e., "except for" false

allenhwkim Over a year ago

ok, yes everything is object in ruby. nil is the one and only instance of the NilClass class.I don't think it really matters because it's pointing to a instance that has null pointer. I regard this as 'not exist'.

wjandrea · Accepted Answer · 2019-06-18 15:05:23Z

In Common Lisp, 0 is also treated as true. For example, the following code returns true.

(if 0 'true 'false)

No doubt, Ruby is following the same design decision made in Lisp. In Lisp, only an empty list (represented by nil) is false.

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