Why does this code snippet output "true" when it should return "false"?

Question

ALLOWED_TARGETS = ["dresden", "paris", "vienna"]

def missile_launch_allowed(target, secret_key)
  allowed = true
  аllowed = false if secret_key != 1234
  allowed = false unless ALLOWED_TARGETS.include?(target)
  allowed
end

puts(missile_launch_allowed("dresden", 9999))

Found this code snippet in a blog. Tracking the code by hand gives me false, but why does this output true when run?

The part I am not seeing is just not crossing my mind at the moment. Please help me understand Ruby a bit better.

"Found this code snippet in a blog" – in the blog post, was the bug introduced deliberately as a way to circumvent the secret key check? :-) — Stefan
– Stefan, Commented Oct 29, 2019 at 10:14
@Viktor thanks for the link. Seems like the OP missed the explanation below the code. — Stefan
– Stefan, Commented Oct 29, 2019 at 10:25
"was the bug introduced deliberately as a way to circumvent the secret key check?" Interesting idea though — Viktor
– Viktor, Commented Oct 29, 2019 at 10:33
@Stefan yes, I got intrigued and tried before reading the part where this was explained. Actually, good that I didn't read that in full. Got to learn a few things here in the answer and the links that you posted. Also, realizing that "The warnings come after the spells" :) — Syed Aslam
– Syed Aslam, Commented Oct 29, 2019 at 10:45

Amadan · Accepted Answer · 2019-10-29 09:52:25Z

8

allowed is not аllowed; you have two different variables. Specifically, the first letter is different: the first variable has 'LATIN SMALL LETTER A' (U+0061), the second has 'CYRILLIC SMALL LETTER A' (U+0430). The glyphs are either similar or identical in most (all?) fonts. Your code is thus equivalent to:

ALLOWED_TARGETS = ["dresden", "paris", "vienna"]

def missile_launch_allowed(target, secret_key)
  first = true
  second = false if secret_key != 1234
  first = false unless ALLOWED_TARGETS.include?(target)
  first
end

puts(missile_launch_allowed("dresden", 9999))

With variables renamed somewhat more sensibly, it should be obvious why you are getting the result you are.

answered Oct 29, 2019 at 9:52

Amadan

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7 Comments

Sergio Tulentsev Over a year ago

Ah, I hate these problems :)

Syed Aslam Over a year ago

Ah, that makes sense. Thanks. Pity the people who get asked these kinda questions in interviews :)

Amadan Over a year ago

On one hand, I want to strangle people who pose trick questions in interviews. On the other hand, I remember helping a fellow student (years ago, in FORTRAN) find the cause of a segfault they couldn't find for an hour; it was something like X = A[O] (where A[0] was intended). Glyph equivalence is more of a potential problem today than in ASCII (though I'd say not in actual practice), you can still see it in the wild, and should know to debug it.

Stefan Over a year ago

The proper term seems to be homoglyph

Stefan Over a year ago

@SyedAslam "What exactly are you testing with such questions in a programming language interview?" – how you approach debugging a strange problem maybe? Or to see if you can think "outside the box".

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