Ruby - set instance variable inside class method from string

Question

I have a class with a method register(key, val). I am trying to add key as a instance variable of the class and set it equal to val. Right now I'm trying to use self.instance_variable_set(':@' + key, val) but I'm getting this error:

in `instance_variable_set': `:@table' is not allowed as an instance variable name (NameError)

I am calling register('table', {'key' => 'value'})

Any idea how to do this properly? Thanks!

@DaveNewton "it's not a class method if you're referencing self" this makes very little sense to me. self can reference a Class just as it can reference an instance of a Class and if we are being technical about this all methods are instance_methods because everything is an instance. — engineersmnky
– engineersmnky, Commented Jun 14, 2017 at 16:39
Just out of curiosity: is the error message really that hard to understand? Can you give any hints as to how it could be improved for future readers? — Jörg W Mittag
– Jörg W Mittag, Commented Jun 15, 2017 at 0:16
I'm really new to Ruby, so I didn't know what the solution was. In hindsight, the error message made perfect sense. — user2309843
– user2309843, Commented Jun 15, 2017 at 14:00

yzalavin · Accepted Answer · 2017-06-14 14:02:20Z

6

Remove : from your method.

self.instance_variable_set('@' + key, val)

Moreover, self can be redundant here. Try instance_variable_set('@' + key, val).

And prefer to use interpolation over concatenation. instance_variable_set("@#{key}", val)

edited Jun 14, 2017 at 14:02

answered Jun 14, 2017 at 13:57

yzalavin

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3 Comments

user2309843 Over a year ago

I tried the interpolation method and it works with double quotes but not single quotes, do you know why that is?

Stefan Over a year ago

@achang27 it's because, well ... string interpolation works within double quotes but not within single quotes. That's the way string literals work in Ruby.

Stefan Over a year ago

Regarding : – if you want to pass the instance variable's name as a symbol (as shown in the docs), the syntax is :"@#{key}". With key = 'table', it returns the symbol :@table

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